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PHYSICAL GEOGRAPHYGEOMORPHOLOGY

INDEX

1. GEOMAGNETISM.....................................................................................................1-62. ISOSTASY .................................................................................................................7-103. PLATE TECTONIC THEORY ................................................................................11-19

Sea Floor Spreading; Geomagnetic AnomaliesDeep Sea Trenches; Island Arcs; Hot Spots.

4. CHANNEL MORPHOLOGY ..................................................................................20-32Geomorphic Cycle

5. EROSION PLANATION SURFACE ......................................................................33-396. CYCLE OF EROSION ..........................................................................................40-477. SLOPE EVOLUTION ............................................................................................48-53

PREP-SUPPLEMENT: GEOMORPHOLOGY

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1. GEOMAGNETISMMagnetism is a force of attraction or repulsionthat acts at a distance. It is due to a magnetic field,which is caused by moving electrically chargedparticles. It is also inherent in magnetic objectssuch as a magnet. A magnet is an object thatexhibits a strong magnetic field and will attractmaterials like iron to it. Magnets have two poles,called the north (N) and south (S) poles. Twomagnets will be attracted by their opposite poles,and each will repel the like pole of the othermagnet. Magnetism has many uses in modern life.Magnetic Field: In a magnetic field, electriccurrents flowing in the same direction will bepulled towards each other and currents flowing indirections opposite to each other will experience arepulsive force. A magnetic field is found arounda magnetic body or a current carrying conductor.

The geomagnetic fieldThe Earth has four layers: the thin outermost layerof lighter rock, ‘crust’; the rocky ‘mantle’; a liquid-iron ‘outer core’ and the innermost layer, an iron‘inner core’. The ‘inner core’ of the Earth rotatesat a different rate as compared to the solid outerlayers. This feature, together with currents in themolten ‘outer core’, generates the Earth’s magneticfield.The magnetic field generally is similar to the fieldgenerated by a dipole magnet—a straight magnetthat has a north pole and a south pole—placed atthe Earth’s centre. But the magnetic field changesdepending on the time and location on the Earth.The axis of the dipole is approximately 11 degreesfrom the axis of rotation of the Earth which meansthat the geographical poles and the magnetic polesin the north and the south are not in the same place.Modern scientific study of the earth’s magneticfield is usually said to begin with the experimentsof Sir William Gilbert, physician to QueenElizabeth I. He showed that the field resemblesthat which would result from a giant bar magnetlocated near the centre of the earth and alignedapproximately along the axis of rotation.Extensions of the long axis of the magnet intersectthe surface of the globe at points known as thenorth and south magnetic poles. This dipole fieldat the surface of the earth may be described in termsof the magnitude and direction of the magnetic

force. The magnitude attains its maximum valueclose to the magnetic poles and its minimum valuearound the magnetic equator. The direction isusually specified in terms of inclination anddeclination . The inclination is the angle which afreely suspended magnetized needle makes withthe horizontal. In the case of a perfectly regulardipole field such a needle would stand verticallyat the magnetic poles and horizontally at themagnetic equator; between these two extremes theangle would vary as a function of the latitudinaldistance from the magnetic pole. The declinationis the angular difference between the geographicmeridian and the horizontal component of theearth’s magnetic field. Its value reaches theextremes of zero and 180° along the great circlepassing through both the geographic and magneticpoles.

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The observed geomagnetic field differs in a numberof important respects from the simple dipole fieldassumed so far. Measurements of magneticintensity show the expected general decline frompoles to equator, but superimposed on this arelongitudinal variations such that the intensity overAustralia, for instance, differs considerably fromthat over South America in the same latitude.Similarly inclination and declination show markeddivergences from the regular dipole pattern. Itshould be emphasized that these divergences arenot due to shallow crustal peculiarities such aslocal iron-ore bodies, but are part of a world-widepattern which appears to be independent of surfaceform and structure and to have its origin muchdeeper within the earth. In addition to these spatialcomplications there are also important temporalchanges. /Continuous recording stations widelydistributed over the globe reveal small dailyfluctuations in intensity, declination andinclination. These same properties also vary on alonger, secular time scale. Ever since theseventeenth century it has been known that thepositions of the magnetic poles slowly move. Whenfirst recorded the declination at London was about10°E; by 1800 it had reached over 20°W but isnow decreasing and in 1975 was about 7°W. Inthe same period the inclination has varied between75° and 67° and the intensity has graduallyweakened. Viewed on a global scale many of thedeviations from the simple dipole field migrateslowly westwards, moving at a rate of 0.18° oflongitude each year that is, making one revolutionin about 2000 years. On an even longer time scalethe most striking change is undoubtedly reversalof the earth’s polarity so that what is now the northpole becomes the south pole and vice versa. But,the pattern of polarity reversal and polar shiftingis unpredictable as evident from earlier records.It is from detailed studies of the modern magneticfield that the most acceptable hypotheses of originarise. These envisage that the outer core of theearth, being liquid and an electrical conductor, acts

as a ‘self-exciting dynamo’. The precisemechanism is still far from clear, but it seems thatsome form of energy in the earth’s interior isultimately converted into electric currents whichencircle the core and produce the dipole magneticfield. Most significantly, all the widely supportedhypo-theses demand a permanently dipolar fieldwith an alignment approximately parallel to theaxis of rotation.

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Remanent magnetism or Paleomagnetism.When an igneous rock containing iron-richminerals cools from the molten state it becomesmagnetized in accordance with the prevailinggeomagnetic field. Laboratory investigations showthat most of the magnetization is acquired quiteabruptly during cooling, the critical temperaturebeing designated the Curie point of the rock. Solong as the rock is not heated again to near its Curiepoint, which commonly lies between 400° and600°C, the acquired magnetization is remarkablystable and may be retained for hundreds of millionsof years. In other words, when an iron-rich lavacools through the Curie temperature a permanentrecord is frozen into the material. The record iscontained in the remanent magnetism of the rock,its strength varying according to the mineralogicalcomposition of the rock and the intensity of thegeomagnetic field at the time of cooling. At bestthe remanent magnetism is extremely weak, a factwhich undoubtedly delayed the widespreadapplication of palaeomagnetic principles. A majoradvance came with the development of the astaticmagnetometer by the distinguished Britishphysicist, Blackett. With this highly sensitiveinstrument it became possible to measure not onlythe remanent magnetism of igneous rocks but alsothe even weaker magnetism of certain sedimentaryrocks. As detrital particles settle through water,those fragments which have already beenmagnetized tend to align themselves in conformitywith the ambient magnetic field. In this way iron-bearing grains assume a preferred orientation thatconfers a weak remanent magnetism on thesediment and ultimately on the resulting rock. Thisis sometimes destroyed by chemical changes duringand after consolidation so that a sedimentary rockwith a weaker and less stable magnetizationpresents greater problems of interpretation than anigneous rock; nevertheless, with adequateprecautions in sampling and measurement it maystill yield much valuable information.The basis of palaeomagnetic reconstruction;; whena rock sample is selected for palaeomagnetic study,its orientation is first carefully measured and anynecessary allowance made to compensate fortectonic disturbance. Subsequent analysis of thesample in a magnetometer theoretically specifiesthe magnitude, declination and inclination of thelocal force at the time the remanent magnetism wasacquired. At this stage a major assumption needsto be made, namely that the total geomagnetic fieldwas dipolar in form. On that basis it is possible toreconstruct the contemporaneous positions of themagnetic poles. Of course there are several sourcesof potential error. The allowance for tectonic

disturbance is difficult to assess and the finalestimate may often be accurate to no more than±5°. Secular fluctuations distorting the pure dipolarfield place an additional constraint on the precisionof the method. In practice, the remanence of manysamples of the same age is measured, calculationsmade for each individual sample and then statisticalprocedures employed to define confidence limitsfor the positions of the poles.Most of the early palaeomagnetic investigationswere concerned with Cenozoic lavas in suchcountries as Japan, Italy and France. Then in the1950s came the dramatic studies initiated byBlackett on the Triassic sandstones of the BritishIsles. These revealed polar positions some 200 Maago widely differing from those of the present day.One obvious explanation, known as the polarwandering hypothesis, was that the magnetic poleshad slowly migrated over the earth’s surface. Thisled to attempts at drawing what are termed polarwandering curves. These involve plotting theapparent polar positions” at different geologicalperiods and joining the points by a smooth curve.When this is done for a single continent a consistentpattern emerges and seems to support the polarwandering hypothesis. However, when a polarwandering curve is calculated for a secondcontinent, it is found to differ from the first. It wasthis vital discovery which led to rejection of thepolar wandering hypothesis and revival forcontinental drift as the only satisfactoryexplanation for the palaeomagnetic findings. If thecontinents have shifted their relative positions, allcontemporaneous rocks from a single continentshould yield a unique polar position, whereas thosefrom different continents should yield differentpolar positions. In essence this is whatpalaeomagnetic investigators claim to have found.At first it might seem that an assumed identitybetween magnetic and geographic poles, justifiedon theoretical grounds, would permit detailedreconstruction of continental movement. However,brief reflection will show that this is not so.Whereas inclination defines palaeolatitude,declination does not define palaeolongitude; formerlongitudinal positions with respect to somearbitrary datum such as the Greenwich meridianmust remain indeterminate. Nevertheless, relativedisplacements of the continents can be evaluatedby drawing polar wandering curves. In theory, aslong as two continental blocks are fused together,or not moving with respect to each other, theyshould yield a curve of identical form. Once theystart drifting apart their individual curves shouldbegin to deviate. Employing this principle it isfeasible to check observed palaeomagnetic valuesagainst the predictions of any particular model of

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continental drift. For instance,/Wegener suggestedthat the major continental blocks once formed asupercontinent which he termed Pangaea (Fig. 3.2).He argued that this was in existence until at leastPermian times since evidence for Permo-Carboniferous glaciation in South America, Africa,India and Australia required contiguity of thesecontinents if the global ice cover were to be keptto acceptable proportions. The opening of theAtlantic Ocean as the Americas separated fromthe rest of Pangaea was believed to have begun inthe south in Jurassic times and gradually spreadnorthwards until Europe and North America finallyseparated in late Cenozoic times. The drifting apartof Africa, Asia, Australia and Antarctica was lessclosely dated but was believed to have begun inearly Jurassic times lf Wegener’s reconstructionof Pangaea is correct, all continents when fittedinto their respective positions should indicate asingle palaeomagnetic pole during late Palaeozoictimes. In broad outline this proves to be the caseand constitutes important support for Wegener’soriginal thesis. The polar wandering curves forindividual continents begin to split in late Triassictimes. This is clearly seen in a comparison of thecurves for Africa, South America and NorthAmerica. Australia and Antarctica appear to havemoved away as a separate unit at about the sameperiod. Palaeomagnetic data suggest that the finalsplitting of India from Africa and of Australia fromAntarctica occurred rather later, the latter possiblyas recently as early Cenozoic times.

Reversals of polarityHe inner core is composed of mostly solid iron,whereas the outer core is composed of liquid ormolten iron. The liquid iron provides the firstrequirement for producing a magnetic field,charged particles. The motion requirement isbelieved to be driven by a combination of thermalenergy, chemical energy, and the rotation of theEarth. This causes the molten core to rotate inmultiple spirals or corkscrews. The spin directionof these corkscrews determines the polarity of thegenerated magnetic field. Physicists know this asthe right-hand rule. If you curl the fingers of yourright hand in the direction of the current, yourthumb points in the direction of the magnetic northpole. Currently, the Earth’s magnetic north pole islocated in the direction of the Earth’s geographicsouth pole. This is referred to as normal polarity,the opposite being reversed polarity. The Earth’smagnetic poles have reversed multiple times overgeolgic history. This is evidenced by magneticminerals in the rock layers. Mid-Ocean ridgesprovide a common example of the reversals. Afterheated material is produced at the ridges, magneticminerals align themselves with the Earth’s

magnetic field. Once the rock cools, the mineralsare preserved in their aligned state. Samplesgathered from either side of ridges show parallelstrips normal and reversed polarities. Priorreversals appear to have no determined length orfrequency. The present normal polarity has lasted

for approximately 780,000 years.It was not long after the first systematic study ofremanent magnetism that instances of reversedpolarity were recorded. One of the earlydiscoveries was made by the French workerBrunhes in 1906, but for a long time there wasuncertainty about the correct interpretation of suchfindings. At one stage it was believed that theremight be a ‘self-reversal’ mechanism by which arock could become magnetized in the oppositesense to the ambient field. Laboratory experimentsin Japan showed that this could indeed happen,but that it required such exceptional circumstancesthat it could not be expected to be a commonoccurrence. Therefore when reversed remanencewas found to be almost as common as normalremanence it was obvious that the geomagnetic

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field must have changed. This was confirmed whendating was undertaken and it was shown that thefield reversals were synchronous all over the globe.Further research has disclosed well over 100polarity reversals during the Mesozoic andCenozoic eras. Detailed analysis of the transitionphases has shown that, prior to a reversal,geomagnetic intensity declines rapidly to aboutone-quarter of its normal value. This isaccompanied by growing instability of the polesuntil, quite abruptly, they move along a great circleroute to approximately antipodal positions. Thereafter the field gradually regains its normal strength.The whole process appears to take about 10 000years and the actual movement of the poles iscompleted in less than 5 000 years. At the presenttime the intensity of the field is declining quiterapidly, but as fluctuations are a normal feature ofthe field there is no proof that we are heading for apolar reversal; equally there is no proof that weare not.The upper Cenozoic era has been divided intointervals of either reversed or normal polarity, eachlasting about 1 Ma. These periods have beendesignated ‘epochs’ and accorded the names offamous research workers in the field ofgeomagnetism . During each epoch there weremuch shorter ‘events’ when the field wastemporarily changed; there may even have beenvery brief ‘flips’ if evidence accumulating for theend of the Brunhes epoch is finally substantiated.For reversals prior to the Gilbert epoch anumbering system has now been adopted. Owingto the frequency of reversals, isolated rock samplescan rarely be dated on the basis of their remanentmagnetism. More interest attaches to continuoussedimentary accumulations in which longsequences of polarity changes might be expected.Deep-sea cores fulfil these requirements andanalysis of many oceanic cores has demonstrateda succession of reversals directly comparable tothose inferred from the study of continental rocksthis not only provides a valuable way of correctingdifferent cores, but also permits dating by meansof the time scale that has been worked out for thepolarity changes

2012: Magnetic Pole Reversal HappensAll The (Geologic) Time—NASAScientists understand that Earth’s magnetic fieldhas flipped its polarity many times over themillennia. In other words, if you were alive about800,000 years ago, and facing what we call northwith a magnetic compass in your hand, the needlewould point to ‘south.’ This is because a magnetic

compass is calibrated based on Earth’s poles. TheN-S markings of a compass would be 180 degreeswrong if the polarity of today’s magnetic field werereversed. Many doomsday theorists have tried totake this natural geological occurrence and suggestit could lead to Earth’s destruction. But would therebe any dramatic effects? The answer, from thegeologic and fossil records we have from hundredsof past magnetic polarity reversals, seems to be‘no.’Reversals are the rule, not the exception. Earthhas settled in the last 20 million years into a patternof a pole reversal about every 200,000 to 300,000years, although it has been more than twice thatlong since the last reversal. A reversal happens overhundreds or thousands of years, and it is not exactlya clean back flip. Magnetic fields morph and pushand pull at one another, with multiple polesemerging at odd latitudes throughout the process.Scientists estimate reversals have happened at leasthundreds of times over the past three billion years.And while reversals have happened morefrequently in “recent” years, when dinosaurswalked Earth a reversal was more likely to happenonly about every one million years.Sediment cores taken from deep ocean floors cantell scientists about magnetic polarity shifts,providing a direct link between magnetic fieldactivity and the fossil record. The Earth’s magneticfield determines the magnetization of lava as it islaid down on the ocean floor on either side of theMid-Atlantic Rift where the North American andEuropean continental plates are spreading apart.As the lava solidifies, it creates a record of theorientation of past magnetic fields much like a tape

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recorder records sound. The last time that Earth’spoles flipped in a major reversal was about780,000 years ago, in what scientists call theBrunhes-Matuyama reversal. The fossil recordshows no drastic changes in plant or animal life.Deep ocean sediment cores from this period alsoindicate no changes in glacial activity, based onthe amount of oxygen isotopes in the cores. This isalso proof that a polarity reversal would not affectthe rotation axis of Earth, as the planet’s rotationaxis tilt has a significant effect on climate andglaciation and any change would be evident in theglacial record.Earth’s polarity is not a constant. Unlike a classicbar magnet, or the decorative magnets on yourrefrigerator, the matter governing Earth’s magneticfield moves around. Geophysicists are pretty surethat the reason Earth has a magnetic field isbecause its solid iron core is surrounded by a fluidocean of hot, liquid metal. This process can alsobe modeled with supercomputers. Ours is, withouthyperbole, a dynamic planet. The flow of liquidiron in Earth’s core creates electric currents, whichin turn create the magnetic field. So while parts ofEarth’s outer core are too deep for scientists tomeasure directly, we can infer movement in thecore by observing changes in the magnetic field.The magnetic north pole has been creepingnorthward – by more than 600 miles (1,100 km) –since the early 19th century, when explorers firstlocated it precisely. It is moving faster now,

actually, as scientists estimate the pole is migratingnorthward about 40 miles per year, as opposed toabout 10 miles per year in the early 20th century.Another doomsday hypothesis about a geomagneticflip plays up fears about incoming solar activity.This suggestion mistakenly assumes that a polereversal would momentarily leave Earth withoutthe magnetic field that protects us from solar flaresand coronal mass ejections from the sun. But, whileEarth’s magnetic field can indeed weaken andstrengthen over time, there is no indication that ithas ever disappeared completely. A weaker fieldwould certainly lead to a small increase in solarradiation on Earth – as well as a beautiful displayof aurora at lower latitudes - but nothing deadly.Moreover, even with a weakened magnetic field,Earth’s thick atmosphere also offers protectionagainst the sun’s incoming particles.The science shows that magnetic pole reversal is– in terms of geologic time scales – a commonoccurrence that happens gradually over millennia.While the conditions that cause polarity reversalsare not entirely predictable – the north pole’smovement could subtly change direction, forinstance – there is nothing in the millions of yearsof geologic record to suggest that any of the 2012doomsday scenarios connected to a pole reversalshould be taken seriously. A reversal might,however, be good business for magnetic compassmanufacturers.

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2. ISOSTASYIsostasy is the term used to describe a condition towhich the Earth's crust and mantle tend, in theabsence of disturbing forces. That is, it describes astate of equilibrium between the Earth's crust andthe underlying mantle. So, Isostasy is idealtheoretical balance of all large portions of Earth'slithosphere as though they were floating on thedenser underlying layer, the asthenosphere, asection of the upper mantle composed of weak,plastic rock that is about 110 km below the surface.Isostasy controls the regional elevations ofcontinents and ocean floors in accordance with thedensities of their underlying rocks. Imaginarycolumns of equal cross-sectional area that rise fromthe asthenosphere to the surface are assumed tohave equal weights everywhere on Earth, eventhough their constituents and the elevations of theirupper surfaces are significantly different. Thismeans that an excess of mass seen as materialabove sea level, as in a mountain system, is due toa deficit of mass, or low-density roots, below sealevel. Therefore, high mountains have low-densityroots that extend deep into the underlying mantle.Due to the dynamic nature of surface Earth, theequilibrium state is constantly shifting; mountainchains are formed and eroded, river deltas grow,ice sheets wax and wane, and volcanoes form anddisappear violently. The ideal isostatic state isdisturbed by these dynamic and continuouslychanging mass distributions on surface Earth.Seismic and gravity data, however, suggest that theEarth's outermost layers generally adjust to thesedisturbances. The concept of isostasy played animportant role in the development of the theory of

plate tectonics. In 1735, expeditions over the Andesled by Pierre Bouguer, a French photometrist andthe first to measure the horizontal gravitational pullof mountains, noted that the Andes could notrepresent a protuberance of rock sitting on a solidplatform. If it did, then a plumb-line should bedeflected from the true vertical by an amountproportional to the gravitational attraction of themountain range. The deflection was less than that

which was anticipated. About a century later,similar discrepancies were observed by Sir GeorgeEverest, surveyor general of India, in surveys southof the Himalayas, indicating a lack of compensatingmass beneath the visible mountain ranges.Early models of isostasy were proposed by Pratt(1855) and Airy (1855) to explain a positionaldiscrepancy of over 5" arc minutes between surveypositions determined using triangulation andastronomical methods in the Himalayas, northernIndia. Both Pratt (1855) and Airy (1855)recognised that the Himalayan Mountains causedthe error in the triangulation based measurementsdue to the deflection of the plumb-bob used to definethe vertical plane in this kind of survey. Pratt (1855)applied a model based on dividing the Himalayanrange into columns of varying densities above acompensation depth at which lithostatic pressurewas equal (Figure 1a). Pratt's theory failed tocorrectly predict the observed difference betweenthe astronomical and geodetic calculations.The Airy hypothesis says that Earth's crust is a morerigid shell floating on a more liquid substratum ofgreater density. Sir George Biddell Airy, an Englishmathematician and astronomer, assumed that thecrust has a uniform density throughout. Thethickness of the crustal layer is not uniform,however, and so this theory supposes that thethicker parts of the crust sink deeper into thesubstratum, while the thinner parts are buoyed upby it. According to this hypothesis, mountains haveroots below the surface that are much larger thantheir surface expression. This is analogous to aniceberg floating on water, in which the greater partof the iceberg is underwater.Figure 1. a) Pratt's model of isostatic compensation.Topography, h, is supported by density contrasts,above a depth of compensation, Dc, relative tostandard density crust. b) The Airy model ofisostasy. Topography of height h is supported by acrustal root, r, of lower density relative to mantledensity.The Pratt hypothesis, developed by John HenryPratt, English mathematician and Anglican

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missionary, supposes that Earth's crust has auniform thickness below sea level with its baseeverywhere supporting an equal weight per unitarea at a depth of compensation. In essence, thissays that areas of the Earth of lesser density, suchas mountain ranges, project higher above sea levelthan do those of greater density. The explanationfor this was that the mountains resulted from theupward expansion of locally heated crustalmaterial, which had a larger volume but a lowerdensity after it had cooled.

Following the failure of Pratt's theory of isostasyto correctly predict the difference between theastronomical and geodetic calculations in northernIndia, Airy (1855) presented an alternative model.Airy demonstrated that if the excess mass of themountains was supported at depth by a massdeficiency then the discrepancy could be accountedfor (Figure 1b). He proposed that similarly toicebergs in the ocean, a mountain range wascompensated at depth by a root of a relatively lowerdensity than the mantle (or "lava" as Airy referredto it) displaced by the root. Which was the moreappropriate and applicable model of isostaticcompensation remained the subject of ongoingdebate for over a century. Ultimately, Airy's theoryof varying crustal thickness is generally consideredcloser to a true representation of reality. The majorexception to this is the case of mid-ocean ridges,where Pratt's theory of varying density columns istypically more applicable.In the theory of isostasy, a mass above sea level issupported below sea level, and there is thus a certaindepth at which the total weight per unit area is equalall around the Earth; this is known as the depth ofcompensation or level of compensation. The depth

of compensation was taken to be 113 km accordingto the Hayford-Bowie concept, named for Americangeodesists John Fillmore Hayford and WilliamBowie. Owing to changing tectonic environments,however, perfect isostasy is approached but rarelyattained, and some regions, such as oceanic trenchesand high plateaus, are not isostaticallycompensated.The Heiskanen hypothesis, developed by Finnishgeodesist Weikko Aleksanteri Heiskanen, is anintermediate, or compromise, hypothesis between

Airy's and Pratt's. This hypothesis says thatapproximately two-thirds of the topography iscompensated by the root formation (the Airy model)and one-third by Earth's crust above the boundarybetween the crust and the substratum (the Prattmodel).Airy was mostly correct about what supports large(wide) mountains, but it took until the 1970's toprove this with seismic work that measured thethickness of the crust and lithosphere beneathmountains.Pratt was correct in that the differencebetween the low standing ocean basins and the highstanding continents is partially due to the fact thatoceans have dense gabbroic composition crustwhereas continents have lighter less dense'Andesitic' composition crust.Isostatic AnomaliesThe Pratt and Airy models of compensation arebased on contrasting assumptions; however, theyare similar in that they consider compensation tooccur on a purely local scale. That is, thecompensation of topography occurs directly belowthe topography. If it is assumed that the lithospherehas a finite strength or rigidity, then the

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compensation of topography or anomalous masscan occur over a greater area as it is supported bythe lateral strength of the plate; the load is supportedby flexure. This is the basic premise of regionalcompensation. The broader wavelength, but smallerpeak amplitude, of regional compensation relativeto local compensation is illustrated in Figure 2.Thus the regional compensation model bridges thetwo endmembers of a spectrum of 'crustal strength'.These endmembers are the Airy case whichassumes no flexural support, and the case assumedfor the Bouguer correction where the crust isinfinitely rigid and able to support all loads.

Figure 2. Schematic illustration of local andregional modes of compensation. ?c = crustaldensity, ?m = mantle density.An ideal setting to illustrate the concept of isostaticcompensation and flexure on gravity anomalymodelling is the passive rift margin. Rift marginsform the boundary between the two fundamentalgeological terranes of Surface Earth, the oceansand the continents, which requires a change incontinental crustal thickness (Figure 3). Riftmargins are also the locus of thick sedimentaccumulations due to their proximity to the elevatedcontinents, these sediments load the crust and the

system (i.e. the crust and mantle) isostaticallycompensates for this loading.Figure 3. Schematic representation of a non-volcanic rifted margin where a combination ofductile stretching and brittle deformation (i.e.faulting) accommodate massive thinning ofcontinental crust.At rift margins isostatic anomalies can becalculated by subtracting the isostatically modelledgravity field from the observed field. The modelledfield accounts for bathymetry, sediment loads andcrustal thinning; assumptions are requiredregarding:i. Mode of isostatic compensation.ii. A compensation depth where pressures are

assumed to be in equilibrium (see Figure 1).iii. Density contrasts for the water, sediments,

crust and mantle in the system. Often there areindependent constraints on these densitycontrasts from well bores, stacking velocities,and also ideally (but in reality less commonly)from wide-angle refraction velocity data.

This process aims to account for a greater numberof the known effects contributing to the observedfield to allow an interpreter to focus on anyremaining anomalies, which are likely to be ofgeological interest.If geometries and properties of a model are perfectlyconstrained, but isostatic anomalies remain, itindicates that a load is somehow"undercompensated" or "overcompensated" in thesense that there is a deficiency or excess of massinvolved in its compensation. If Airy isostaticanomalies remain it may indicate that loads aresupported by the flexural strength of the lithosphereso that compensating masses may be offset fromthe load, however, integration of such isostaticanomalies over the region of compensation shouldtend to zero. Two further cases where both Airyand flexural isostatic anomalies may remain are:i. Where a load may not have been present or

absent for a sufficient period to allow a regionto achieve isostatic equilibrium, accordinglyisostatic gravity anomalies will be measuredover the region. For example, areas subject toPleistocene glaciation not completely adjustedto the removal of ice loads .

ii. The presence of density anomalies within thelithosphere, even if compensated, can alsocause isostatic anomalies. A mass excesswithin the crust may be completely locallycompensated, however, if there is no associatedtopographic expression then neither the massnor its compensation is accounted for in an

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isostatic model. Hence, an isostatic anomalyexists despite perfect isostatic balance withinthe lithosphere.

This second case is of great interest where the desireis to characterize the subsurface density distributionto improve understanding of the geology. Theinterpretative uplift of carefully constructedisostatic anomaly maps relative to FAA and BAmaps is illustrated in Figure 4 where in these Gulfof Mexico examples a strong qualitative correlationbetween known features and depocentres anddiagnostic isostatic anomalies is observed (IGC,2002). Remaining energy in isostatic anomaly mapssuggest the presence of geological bodies ofanomalously high or low density relative to thesurrounding sediment and crust. These isostaticanomalies can be further investigated with moredetailed modelling.

Comparison:Pratt Airy

All blocks have equal depth/thickness with All blocks have varying depth/thickness withvarying density equal densityBased on law of compensation Based on law of floatationHe believed on contraction of earth in the He believed in upstanding portion was a resultprocess of origin of earth of expansion of earth's interior after getting

heated.His concept was based on local observation His concept was also based on local observation.Pratt model is used for mid oceanic ridges Airy model is used for continental topography,

especially mountain rangesPratt has not used word root, Height is proportional to root, higher the

elevation, thicker is the block and root.

Both of them have been criticized on the groundthat, crusts can't be composed of columns. If so, itinvites more problem than to resolve the isostaticequilibrium. Study of gravity anomalies raiseddoubts about it, e.g. in Canadian cordillera, a youngmountain range like Himalyas is isostaticallybalanced. Moreover, plate tectonics further deniedits acceptance as a theory of global isostaticadjustments. It may have relevance at local level,but unable to explain at regional level or globallevel. Also, Airy's proportional column would notbe intact,due to high temperature inside.

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3. PLATE TECTONIC THEORYDeveloped from the 1950s through the 1970s, platetectonics is the modern version of continental drift,a theory first proposed by scientist Alfred Wegenerin 1912. Wegener didn't have an explanation forhow continents could move around the planet, butresearchers do now. Plate tectonics is the unifyingtheory for explanation of geographic and geologicalfeatures. "Before plate tectonics, people had tocome up with explanations of the geologic featuresin their region that were unique to that particularregion," Van der Elst said. "Plate tectonics unifiedall these descriptions and said that you should beable to describe all geologic features as thoughdriven by the relative motion of these tectonicplates."Plate tectonics, possibly the most importantgeological theory ever developed, incorporated the

earlier theory of continental drift,espoused byGerman meteorologistand lecturer Alfred Wegenerin the early 20th century. Although the scientificcommunity of the time ridiculed Wegener and flatlyrejected his theory, current-day geologists,geophysicists, and oceanographers live by muchof what he had to say about our planet.According to the theory of continental drift, theworld was made up of a single continent throughmost of geologic time. That continent eventuallyseparated and drifted apart; forming into the sevencontinents we have today. The first comprehensivetheory of continental drift was suggested by theGerman meteorologist Alfred Wegener in 1912. Thehypothesis asserts that the continents consist oflighter rocks that rest on heavier crustal material-similar to the manner in which icebergs float onwater. Wegener contended that the relative positions

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of the continents are not rigidly fixed but are slowlymoving-at a rate of about one yard per century.The word tectonics derives from the Greektektonikos, meaning "pertaining to construction."In geology, tectonics concerns the formation andstructure of the earth's crust. The theory of platetectonics--formulated by American, Canadian, andBritish geophysicists--attributes earthquakes,volcanoes, the mountain-building process, andrelated geophysical phenomena to movement andinteraction of the rigid plates forming the earth'scrust.For a better understanding, we take a hard-boiledegg and crack its shell. Does the egg remind us ofanything? The Earth, perhaps? The egg could beseen as a tiny model of the Earth. The thin shellrepresents the Earth's crust, divided into plates;within the shell is the firm but slippery mantle.Move the pieces of shell around. Notice how theshell buckles in some places and exposes "mantle"in other places. The same thing happens on Earth,but on Earth, this activity results in the formationof mountains, earthquakes, and new ocean floor.Ingeologic terms, a plate is a large, rigid slab of solidrock. The word tectonics comes from the Greek

root "to build." Putting these two words together,we get the term plate tectonics, which refers to howthe Earth's surface is built of plates. A tectonic plate(also called lithospheric plate) is a massive,irregularly shaped slab of solid rock, generallycomposed of both continental and oceanic

lithosphere. Plate size can vary greatly, from a fewhundred to thousands of kilometers across; thePacific and Antarctic Plates are among the largest.Plate thickness also varies greatly, ranging fromless than 15 km for young oceanic lithosphere toabout 200 km or more for ancient continentallithosphere (for example, the interior parts of Northand South America).How do these massive slabs of solid rock floatdespite their tremendous weight? The answer liesin the composition of the rocks. Continental crustis composed of granitic rocks which are made upof relatively lightweight minerals such as quartzand feldspar. By contrast, oceanic crust is composedof basaltic rocks, which are much denser andheavier. The variations in plate thickness arenature's way of partly compensating for theimbalance in the weight and density of the two typesof crust. Because continental rocks are much lighter,the crust under the continents is much thicker (asmuch as 100 km) whereas the crust under theoceans is generally only about 5 km thick. Likeicebergs, only the tips of which are visible abovewater, continents have deep "roots" to support theirelevations.

Like many features on the Earth's surface, plateschange over time. Those composed partly orentirely of oceanic lithosphere can sink underanother plate, usually a lighter, mostly continentalplate, and eventually disappear completely. Thisprocess is happening now off the coast of Oregon

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and Washington. The small Juan de Fuca Plate, aremnant of the formerly much larger oceanicFarallon Plate, will someday be entirely consumedas it continues to sink beneath the North AmericanPlate. The theory of plate tectonics states that theEarth's outermost layer is fragmented into a dozenor more large and small plates that are movingrelative to one another as they ride atop hotter, moremobile material called asthenosphere.The expedition set to ease the Germany's economicplight through extracting gold from sea revealedthe extent of the sea floor's rugged terrain. Theexpedition also found that a continuous mountain-like ridge runs through the Atlantic to the southwestof Africa. Unfortunately it was not realized at thetime that this finding supported Alfred Wegener'stheory of continental drift.That ridge, it was later discovered, extendedthrough the major oceans of the world. It is nowcalled the Mid-Ocean Ridge. In 1953, Americanphysicists Maurice Ewing (1906-1974) and BruceHeezen (1924-1977) discovered that through thisunderwater mountain range ran a deep canyon. Insome places the canyon, called the Great GlobalRift, and came very close to land. The rift appearedto be breaks in the earth's crust, but perfectly fittedbreaks, like joints made by a carpenter. The riftoutlined chunks of the earth's crust, which werenamed tectonic (from a Greek word for "carpenter")plates. Six large and several smaller plates makeup the surface of the globe. Most of the world'searthquakes and volcanoes occur at the plates'edges. The large plate containing most of the PacificRim accounts for 80 percent of the earthquakeenergy of the planet. Ewing and Heezen's findingmarked an explosion in data from newly advancedtechnology that revolutionized geology. Harry Hesswas inspired by the findings to look back atsoundings he'd made during the war on a U.S.submarine. His evidence and the work of FrederickVine and Drummond Matthews brought the datatogether in the theory of sea-floor spreading.Sea floor spreadingHarry Hess was a geologist and Navy submarinecommander during World War II, had discoveredin 1946 that hundreds of flat-topped mountains,perhaps sunken islands, shape the Pacific floor. Thediscovery of the Great GlobalRift in the 1950sinspired him to look back at his data from yearsbefore. After much thought, he proposed in 1960that the movement of the continents was a result ofsea-floor spreading. In 1962, he added a geologicmechanism to account for Wegener's movingcontinents. It was possible, he said, that moltenmagma from beneath the earth's crust could oozeup between the plates in the Great Global Rift. As

this hot magma cooled in the ocean water, it wouldexpand and push the plates on either side of it --North and South America to the west and Eurasiaand Africa to the east. This way, the Atlantic Oceanwould get wider but the coastlines of the landmasseswould not change dramatically. Hess provedWegener's basic idea right and clarified themechanism that broke the once-joined continentsinto the seven with which we are familiar. Thecontinents are attached to the plates and do notmove independently of them. But the platesthemselves shift and change shape, carrying thecontinents along. Running along the top of thischain of mountains is a deep crack, called a riftvalley. It is here that new ocean floor is continuouslycreated.

As the two sides of the mountain move away fromeach other, magma wells up from the Earth'sinterior. It then solidifies into rock as it is cooledby the sea, creating new ocean floor.The speed at which new ocean floor is createdvaries from one location on the ocean ridge toanother. Between North America and Europe, therate is about 2.2 inches (3.6 cm) per year. At theEast Pacific rise, which is pushing a plate into thewest coast of South America, the rate is 12.6 inches(32.2 cm) per year.Around 1930, Holmes suggested a mechanism thatcould explain AlfredWegener's theory ofcontinental drift: the power of convection. Currentsof heat and thermal expansion in the Earth's mantle,he suggested, could force the continents toward oraway from one another, creating new ocean floorand building mountain ranges (a theory laterclarified by HarryHess).Magnetic bands provide evidence of sea-floorspreading 1963In 1963, Fred Vine, Drummond Matthews, andothers found that the crust surrounding themidocean ridges showed alternating bands -- eachband magnetized with a polarity opposite thesurrounding bands. They suggested that as new sea-floor crust was formed around the rift in themidocean ridge, it magnetized differently,depending upon the polarity of the planet at thattime. This supported the theory that Harry Hesshad put forth, that the ocean progressively widensas new sea floor is created along a crack that

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follows the crest of midocean ridges.In 1966, earth scientists first identified theJaramillo Event, the wholesale reversal of Earth'smagnetic fields some 900,000 years ago. Thisconfirmed the theory that Earth's magnetic field hadflip-flopped through the planet's life, and it madeMatthews and Vine's 1963 finding quite clear. Theyrealized that the pattern of reversals matchedperfectly the magnetic profile they had compiledof the sea floor. This discovery, together with datafrom a 1964 research vessel, transformed the fieldof geology. It confirmed sea-floor spreading ashypothesized by Hess, and thus "continental drift,"originally proposed by Alfred Wegener back in1912. It convinced many that plate tectonics wasthe best theory to unify nearly all the previouslyaccumulated, but disjoint geological data.ForcesThe land masses continued to move apart, ridingon separate plates, until they reached the positionsthey currently occupy. These continents are still onthe move today.Exactly what drives plate tectonics is not known.One theory is that convection within the Earth'smantle pushes the plates, in much the same waythat air heated by your body rises upward and isdeflected sideways when it reaches the ceiling.

Another theory is that gravity is pulling the older,colder, and thus heavier ocean floor with more forcethan the newer, lighter seafloor.

Whatever drives the movement, plate tectonicactivity takes place at four types of boundaries:divergent boundaries, where new crust is formed;convergent boundaries, where crust is consumed;collisional boundaries, where two land massescollide; and transform boundaries, where two platesslide against each other.

DivergentBoundaryAlso known as spreading boundary, a divergentboundary occurs where two plates move apart,allowing magma, or molten rock, to rise from the

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Earth's interior to fill in the gap. The two platesmove away from each other like two conveyor beltsmoving in opposite directions.Convergent BoundaryConvergent plate boundaries are locations wherelithospheric plates are moving towards one another.The plate collisions that occur in these areas canproduce earthquakes, volcanic activity and crustaldeformation.Continent and oceanWhen continental and oceanic plates collide thethinner and more dense oceanic plate is overriddenby the thicker and less dense continental plate. Theoceanic plate is forced down into the mantle in aprocess known as "subduction". As the oceanicplate descends it is forced into higher temperatureenvironments. At a depth of about 100 miles (160km) materials in the subducting plate begin toapproach their melting temperatures and a processof partial melting begins. This partial meltingproduces magma chambers above the subductingoceanic plate. These magma chambers are lessdense than the surrounding mantle materials andare buoyant. The buoyant magma chambers begina slow asscent through the overlying materials,melting and fracturing their way upwards. The sizeand depth of these magma chambers can bedetermined by mapping the earthquake activityarround them. If a magma chamber rises to thesurface without solidifying the magma will breakthrough in the form of a volcanic eruption. TheWashington-Oregon coastline of the United Statesis an example of this type of convergent plateboundary. Here the Juan de Fuca oceanic plate issubducting beneath the westward moving NorthAmerican continental plate. The Cascade MountainRange is a line of volcanoes above themelting oceanic plate. The Andes Mountain Rangeof western South America is another example of aconvergent boundary between an oceanic andcontinental plate. Here the Nazca Plate issubducting beneath the South American plate.Effects of a convergent boundary between anoceanic and continental plate include: a zone ofearthquake activity that is shallow along thecontinent margin but deepens beneath the continent,

sometimes an ocean trench immediately off shoreof the continent, a line of volcanic eruptions a fewhundred miles inland from the shoreline, destructionof oceanic lithosphere.

Ocean to oceanWhen a convergent boundary occurs between twooceanic plates one of those plates will subductbeneath the other. Normally the older plate willsubduct because of its higher density. Thesubducting plate is heated as it is forced deeperinto the mantle and at a depth of about 100 miles(150 km) the plate begins to melt. Magma chambersare produced as a result of this melting and themagma is lower in density than the surroundingrock material. It begins ascending by melting andfracturing its way throught the overlying rockmaterial. Magma chambers that reach the surfacebreak through to form a volcanic eruption cone. Inthe early stages of this type of boundary the coneswill be deep beneath the ocean surface but latergrow to be higher than sea level. This produces anisland chain. With continued development theislands grow larger, merge and an elongatelandmass is created.Japan, the Aleutian islands and the EasternCaribbean islands of Martinique, St. Lucia and St.Vincent and the Grenadines are examples of islandsformed through this type of plate boundary. Effectsthat are found at this type of plate boundary include:a zone of progressively deeper earthquakes, anoceanic trench, a chain of volcanic islands, and thedestruction of oceanic lithosphere.Continent to continent

This is a difficult boundary to draw. First it iscomplex and second, it is poorly understood whencompared to the other types of plate boundaries. Inthis type of convergent boundary a powerful

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collision occurs. The two thick continental platescollide and both of them have a density that is muchlower than the mantle, which prevents subduction(there may be a small amount of subduction or theheavier lithosphere below the continental crustmight break free from the crust andsubduct).Fragments of crust or continent marginsediments might be caught in the collision zonebetween the continents forming a highly deformedmélange of rock. The intense compression can alsocause extensive folding and faulting of rocks withinthe two colliding plates. This deformation canextend hundreds of miles into the plate interior. TheHimalaya Mountain Range is the best activeexample of this type of plate boundary; where theIndian and Eurasian plates are currently in collision.The Appalachian Mountain Range is an ancientexample of this collision type and is also markedon the map. Effects found at a convergent boundarybetween continental plates include: intense foldingand faulting, a broad folded mountain range,shallow earthquake activity, shortening andthickening of the plates within the collisionzone.Transform Boundary`A transform boundary occurs where two platesslide against each other. But rather than slidingsmoothly, the plates build up tension, then releasethe tension with a spurt of movement. Thismovement is felt as an earthquake

Transform boundaries neither create nor consumecrust. Rather, two plates move against each other,building up tension, and then releasing the tensionin a sudden and often violent jerk. This sudden jerkcreates an earthquake. The San Andreas Fault isundoubtedly the most famous transform boundaryin the world. To the west of the fault is the Pacificplate, which is moving northwest. To the east isthe North American Plate, which is movingsoutheast. Los Angeles, located on the Pacific plate,is now 340 miles south of San Francisco, locatedon the North American plate. In 16 million years,the plates will have moved so much that LosAngeles will be north of San Francisco!Mechanism:On the basis of above details and a variety of otherevidences, plate tectonics got acceptance amongthe scientific community.It provides a frameworkwith which we can understand and relate a widerange of internal processes and topographic patternsaround the world.The lithosphere is mosaic of rigid

lithospheric plates floating over the underlyingasthenosphere.The main features of plate tectonics are:• The Earth's surface is covered by a series of

crustal plates.• The ocean floors are continually moving,

spreading from the center, sinking at the edges,and being regenerated.

• Convection currents beneath the plates movethe crustal plates in different directions.

• The source of heat driving the convectioncurrents is radioactivity deep in the Earthsmantle.

Advances in sonic depth recording during WorldWar II and the subsequent development of thenuclear resonance type magnometer (proton-precession magnometer) led to detailed mappingof the ocean floor and with it came manyobservation that led scientists like Howard Hessand R. Deitz to revive Holmes' convection theory.Hess and Deitz modified the theory considerablyand called the new theory "Sea-floor Spreading".Among the seafloor features that supported the sea-floor spreading hypothesis were: mid-oceanicridges, deep sea trenches, island arcs, geomagneticpatterns, and fault patterns.Mid-OceanicRidgesThe mid-oceanic ridges rise 3000 meters from theocean floor and are more than 2000 kilometers widesurpassing the Himalayas in size. The mapping ofthe seafloor also revealed that these hugeunderwater mountain ranges have a deep trenchwhich bisects the length of the ridges and in placesis more than 2000 meters deep. Research into theheat flow from the ocean floor during the early1960s revealed that the greatest heat flow wascentered at the crests of these mid-oceanic ridges.Seismic studies show that the mid-oceanic ridgesexperience an elevated number of earthquakes. Allthese observations indicate intense geologicalactivity at the mid-oceanic ridges.

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Geomagnetic AnomaliesOccasionally, at random intervals, the Earth'smagnetic field reverses. New rock formed frommagma records the orientation of Earth's magneticfield at the time the magma cools. Study of the seafloor with magnometers revealed "stripes" ofalternating magnetization parallel to the mid-oceanic ridges. This is evidence for continuousformation of new rock at the ridges. As more rockforms, older rock is pushed farther away from theridge, producing symmetrical stripes to either sideof the ridge. In the diagram to the right, the darkstripes represent ocean floor generated during"reversed" polar orientation and the lighter stripesrepresent the polar orientation we have today.Notice that the patterns on either side of the linerepresenting the mid-oceanic ridge are mirrorimages of one another. The shaded stripes alsorepresent older and older rock as they move awayfrom the mid-oceanic ridge. Geologists havedetermined that rocks found in different parts ofthe planet with similar ages have the same magneticcharacteristics.

Deep Sea TrenchesThe deepest waters are found in oceanic trenches,which plunge as deep as 35,000 feet below theocean surface. These trenches are usually long andnarrow, and run parallel to and near the oceansmargins. They are often associated with and parallelto large continental mountain ranges. There is alsoan observed parallel association of trenches andisland arcs. Like the mid-oceanic ridges, thetrenches are seismically active, but unlike the ridgesthey have low levels of heat flow. Scientists alsobegan to realize that the youngest regions of theocean floor were along the mid-oceanic ridges, andthat the age of the ocean floor increased as thedistance from the ridges increased. In addition, ithas been determined that the oldest seafloor oftenends in the deep-sea trenches.Island ArcsChains of islands are found throughout the oceansand especially in the western Pacific margins; theAleutians, Kuriles, Japan, Ryukus, Philippines,Marianas, Indonesia, Solomons, New Hebrides,

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and the Tongas, are some examples.. These "Islandarcs" are usually situated along deep sea trenchesand are situated on the continental side of the trench.These observations, along with many other studiesof our planet, support the theory that underneaththe Earth's crust (the lithosphere: a solid array ofplates) is a malleable layer of heated rock knownas the asthenosphere which is heated by radioactivedecay of elements such as Uranium, Thorium, andPotassium. Because the radioactive source of heatis deep within the mantle, the fluid asthenospherecirculates as convection currents underneath thesolid

lithosphere. This heated layer is the source of lavawe see in volcanos, the source of heat that driveshot springs and geysers, and the source of rawmaterial which pushes up the mid-oceanic ridgesand forms new ocean floor. Magma continuouslywells upwards at the mid-oceanic ridges (arrows)producing currents of magma flowing in oppositedirections and thus generating the forces that pullthe sea floor apart at the mid-oceanic ridges. Asthe ocean floor is spread apart cracks appear in themiddle of the ridges allowing molten magma tosurface through the cracks to form the newest oceanfloor. As the ocean floor moves away from the mid-oceanic ridge it will eventually come into contactwith a continental plate and will be subductedunderneath the continent. Finally, the lithospherewill be driven back into the asthenosphere whereit returns to a heated state.Hot SpotsVolcanism also occurs in areas that are notassociated with plate boundaries, in the interior ofplates. These are most commonly associated withwhat is called a hot spot. Hot spots appear to resultfrom plumes of hot mantle material upwellingtoward the surface, independent of the convectioncells thought to cause plate motion. Hot spots tendto be fixed in position, with the plates moving overthe top. As the rising plume of hot mantle movesupward it begins to melt to produce magmas. Thesemagmas then rise to the surface producing avolcano. But, as the plate carrying the volcano

moves away from the position over the hot spot,volcanism ceases and new volcano forms in theposition now over the hot spot. This tends toproduce chains of volcanoes or seamounts (formervolcanic islands that have eroded below sea level).Volcanism resulting from hotspots occurs in boththe Atlantic and Pacific ocean, but are more evidenton the sea floor of the Pacific Ocean, because theplates here move at higher velocity than those underthe Atlantic Ocean. A hot spot trace shows up as alinear chain of islands and seamounts, many ofwhich can be seen in the Pacific Ocean. TheHawaiian Ridge is one such hot spot trace. Herethe Big Island of Hawaii is currently over the hotspot, the other Hawaiian islands still stand abovesea level, but volcanism has ceased. Northwest ofthe Hawaiian Islands, the volcanoes have erodedand are now seamounts. The ages of volcanic rocksincrease along the Hawaiian Ridge to the northwest

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of Hawaii. The prominent bend observed wherethe Hawaiian Ridge intersects the EmperorSeamount chain has resulted from a change in thedirection of plate motion over the hot spot. Note

Seafloor spreding. Convection currents bring magma from the asthenosphere up through fissures in theoceanic lithosphere at the midocean ridge. The cooled and therefore solidified magma becomes a newportion of ridge along the ocean floor and the two sides of the ridge spread away from each other asindicated by the arrows. Where denser ocean lithosphere converges with less-dense continental lithosphere,the oceanic plate slides under the continental plate in a process called subduction, Magma produced bythis subduction rises to form volcances and igneous intrusions.

that when the Emperor Seamount chain wasproduced, the plate must have been moving in amore northerly direction. The age of the volcanicrocks at the bend is about 50 million years.

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4. CHANNEL MORPHOLOGYGeomorphology is the study of landforms includingthe origin, evolution and the processes that formthem. These processes are each influenceddifferently by climate, ecology, and human activity.Present day structures are studied not only to helpus understand the geological features we see todayand the forces that created them, but also to helpus understand the features from previous ages thatwe see recorded in geological history. Tectonics,volcanism, and glaciation have shaped the regionallandscape and resulted in different geologic terrainsthat influence topography, drainage development,sediment production, and hydrology.Fluvial geomorphology deals with the landformsdeveloped by flowing water. This seemingly simplesubject encompasses a rather complex set ofinterrelated processes that produce a diverse arrayof fluvial forms. We restrict the content of thiscompendium includes many types of streams andrivers. For simplicity, we use "streams" as an all-encompassing term that includes all channels withflowing water (creeks, brooks, tributaries, rivers,etc.), regardless of the absolute size or the timingand frequency of flows carried by the channels.This provides an overview of the key factorsdetermining the morphology of watershed streams.The morphology of the river channel, including itscross-sectional shape, size, longitudinal profile andplanform pattern, is the result of sediment erosion,transport and deposition processes taking placewithin the controls imposed by the geology andterrain of the drainage basin. Rivers are constantlyevolving and adjusting as a response to thesequence of normal flow, flood peaks and droughts,which are controlled by the regional climate, localweather and catchment hydrology. In this respectthe channel geomorphology can be best explainedif distinctions are made between those factors whichdrive the fluvial system in producing the channel,those which characterize the physical boundariesof the channel and those which respond to thedriving and boundary conditions and define thechannel form. In brief, we can say that channelmorphology is part of fluvial morphology. As itdeals exclusively with the channel /river in differentphysical background e.g. in arid areas or humidareas.

There are four principal erosion processes:1. Corrasion takes place when the river picks up

sediment, which acts like sandpaper and wearsaway the rock by abrasion. It is effective duringflood stages and is the major process by whichrivers erode horizontally and vertically. It canproduce bedrock hollows and potholes, where

gravels can be trapped in pre-existing holes, andbecause the current is turbulent, the gravel canbe swirled around in these holes which enlargevertically.

2. Corrosion or solution is where the rock isdissolved in the water. It occurs ubiquitouslyand continuously and takes place independentof river discharge or velocity, although it canbe affected by the chemical composition of thewater, particularly from the soil system. Forexample, where there is limestone bedrock thesoil carbon dioxide and humic acids can producehigh calcium concentrations in the water.

3. Attrition occurs where the sediment particlesbeing transported, particularly as bed load,collide with one another, which may causebreakage into smaller fragments. This accountsfor originally angular material becomingprogressively more rounded with time anddistance downriver.

4. Hydraulic action takes place where the sheerforce of the turbulent water hitting river banks,as on the outside of meander bends, is forcedinto cracks. Here the air in the cracks may becompressed, increasing pressure and causingexpansion of the cracks and eventually bankcollapse. Cavitation is part of this hydraulicaction, where air bubbles collapse, producingshock waves which weaken the channelperimeter as they hit.

Streams of all sizes have the same basic function;they move water, sediment, and other matter(organic, chemical, biological materials, etc.) overthe land surface and, ultimately, empty all of thesematerials into the ocean or a lake. The detailsregarding time and space scales are numerous; toenable us to consider these factors. A watershedincludes the entire stream network upstream of apoint on the main stem, as well as the hillslopescontributing water, sediment, and woody debris tothe network. The fluvial dynamics at a given pointalong a stream are influenced by the variousprocesses occurring in the watershed upstream,which is why it is necessary to consider processesthat function at a watershed scale. The ridgeseparating the two watersheds is called drainagedivide. Principal causative factors affecting thechannel morphology are as follows:Climate describes the fluctuations in averageweather. Although the weather is always changing,longer-term characteristics such as seasonal andinter-annual variations can be defined .Climatechange occurs when this envelope shifts and a newrange of climatic conditions arises.Tectonics refers to the internal forces that deform

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the Earth's crust. These forces can lead to largescale uplift, localised subsidence, warping, tilting,fracturing and faulting. Where uplift has occurred,inputs of water have to be lifted to a greaterelevation, increasing energy availability; some ofthe highest rates of sediment production in the worldare associated with areas of tectonic uplift. Valleygradients are altered by faulting and localizeduplift, which may in turn affect channel pattern.Lateral (sideways) tilting can cause channelmigration and affect patterns of valleysedimentation.Base level is the level below which a channelcannot erode. In most cases this is sea level. If thereis a fall in sea level relative to the land surface,more energy is available to drive flow and sedimentmovement. Conversely a relative rise in base levelmeans that less energy is available, resulting in netdeposition in the lower reaches of the channel. Overtime these effects may be propagated upstreamthrough a complex sequence of internal adjustmentsand feedbacks, base level first formulated byPowell in 1875.He wrote, "We may consider thelevel of sea to be a grand base level, below whichthe dry lands cannot be eroded; but we may alsohave, for local and temporary purposes, other baselevels of erosion, which are the levels of the bedsof the principal streams which carry away theproducts of erosion. The base level would in fact,be an imaginary surface inclining slightly in allparts towards the lower end of principal stream".Powell's definition of base level, thus includes threebasic ideas; namely(i) The ultimate limit of sub-aerial erosion of the

continent is the base level of the sea.(ii) Locally resistant rocks in the path of the

stream, lakes in the stream path, or otherobstacles can produce temporary base level.

(iii) Tributaries may not erode below that of themain stream, and since mainstream will always

have some slope, the base level need not alwaysbe a flat surface.Thus, sea level may be considered as a general,permanent base level which fluctuates from timeto time but which remains normally within a rangeof a few metres. Local base levels such as rockoutcrops and lakes are temporary; changes in baselevel cause changes in the mainstream, tributariesand subtributaries.

Besides, human activities play a very imoportantrole in channel morphology through constructingdam and other activities.Concept of Graded streamGeo-morphologists have used the concept ofequilibrium in streams. A stream in equilibrium iscalled a graded stream or a poised stream. Mackin(1948) has given the following definition of agraded stream:"A graded stream is one in which, over a period ofyears, slope is delicately adjusted to provide,with available discharge and with prevailingchannel characteristics, just the velocity required

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for the transportation of load supplied from thedrainage basin. The graded stream is a system inequilibrium; its diagnostic characteristic is that anychange in any of the controlling factors will causea displacement of the equilibrium in a direction thatwill tend to absorb the effect of the change".Thus, the four variables related to the concept of agraded stream are slope, discharge, channelcharacteristics and sediment load. In a naturalstream, the discharge is continuously changing dueto precipitation, infiltration, evaporation andwithdrawals. Although stream tends to pick upsediment or deposit it until load equals capacity,because of rapid variations in flow it cannot do so.Hence, in very short times the stream cannot be inequilibrium. Similarly, since the tendency of thestreams is to lower the land surface to the sea level,over very long periods the stream cannot be inequilibrium. Thus, neither in very short not verylong periods can a natural stream be considered tobe in true equilibrium.Yet, for all practical purposes, most of the alluvialstreams are in equilibrium over periods of the orderof a few decades. In such streams, the bed may godown during high flows and fill back during lowflows; yet the net amount of change is notsufficiently large to be detected by quantitativemeasurements. Most of the alluvial streams whichare not affected by human interferences can be saidto be graded or in equilibrium. Construction ofdams, withdrawal or addition of clear water,addition of sediment load, contraction of streamand cutting off the bends are some the ways inwhich the equilibrium of the stream is disturbedby human activities.Characteristics of Graded StreamsTo get better appreciation of the stream morphologythe characteristics of graded streams are brieflyenumerated here. Firstly the slope of a gradedstream, in general, decreases in the downstreamdirection yielding a concave profile. Secondly,partly as a consequence of decreasing slope in thedownstream direction, the stream drops the coarsermaterial that it cannot transport, a phenomenonknown as sorting; and partly due to abrasion, thebed material of an alluvial stream becomes finerin the downstream direction. Thirdly in humidregions as more and more tributaries join the mainstream; the discharge increases in the downstreamdirection. However, if the stream passes througharid region, the discharge can actually decrease indownstream direction as in the case of theEuphrates in Iraq. This is primarily due to seepageand evaporation. In addition, the upper part of thedrainage basin is the main source of sediment eventhough the runoff from this part of the catchments

may be small. The runoff from the rest of the basinis large but it carries relatively less sediment. Thisleads to decrease in the average concentration ofsediment in the downstream direction necessitatinga smaller slope. Lastly, because of finer material,streams usually have relatively narrow channelsi.e. larger width to depth ratio, in the downstreamdirection. As a result the stream has greaterhydraulic efficiency and flows with a smaller slope.A graded stream may show aggradational tendency,albeit temporary, under the following conditions.1. If dissection of upland region is in progress and

a vast number of smaller new valleys andravines come into existence in the stage of youth.To carry relatively higher load stream mayincrease slope by aggradation.

2. If the river after it is graded flows in a widerchannel than it has hitherto had in youth, lossof depth in the stream may rebuilt in a reductionof velocity and transporting power that it needssteeper slope to carry the load.

3. As a river develops increasingly large curvesby lateral corrasion, its length increases andslope decreases and hence carrying powerdecreases resulting in aggradation.

4. Decrease in water volume due to infiltration,evaporation or withdrawal can cause increasein slope due to aggradation.

As the controlling variables usually change morefrequently than the time taken for the channelproperties to respond, a graded stream displays aquasi-equilibrium rather than a true steady state.How Rivers Shape Their ChannelsRivers and streams continuously shape and reformtheir channels through erosion of the channelboundary (bed and banks) and the reworking anddeposition of sediments. For example, erosion andundermining of the banks can lead to channelwidening. Scouring of the channel bed deepens thechannel, while sediment deposition reduces thedepth and can lead to the formation of channel bars.These are just some of the ways in which channeladjustment takes place.There is an important balance between the erosivepower of the flow and the strength, or resistance,of the bed and bank material to erosion. Duringmajor flood events, when the erosive power of theflow is greatly increased, there can be dramaticchanges in channel form. Just how dramatic thesechanges are depends on how much resistance isprovided by the bed and banks. Channels formedin unconsolidated alluvium offer much lessresistance to erosion than those cut in bedrock. Infact, most flows are able to shape channels formedin sandy alluvium because relatively little energy

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is required to set the individual sand grains inmotion. Because silts and clays are smaller thansand grains, you might expect these particles to beeasier to erode. However, they tend to stick togetheras a result of cohesive (attractive) electrochemicalforces between the particles. This means thatchannel boundaries with a high proportion of siltand clay are actually more resistant to erosion thanthose formed in sand and fine gravel. The amountand type of vegetation growing along the banks isalso significant, since this can provide additionalresistance to erosion.In many cases, it is only extreme floods that arecapable of significantly modifying bedrockchannels. The comparative rarity of these very largefloods means that channel adjustments tend to occursporadically, being interspersed by long periods oflittle change. Alluvial channels dominated bycobbles and boulders may also be relativelyunaffected by most flows, which are not powerfulenough to move such coarse material.The energy needed to carry out geomorphologicalwork is provided by the flow of water through thechannel. For any length of channel, energyavailability is dependent on two things: the flowdischarge and the steepness of the channel slope.Increases in either of these will increase the streampower and therefore the potential to carry outgeomorphological work. However, before sedimenttransport and erosion can take place, a surprisingamount of energy has to be used simply to movewater through the channel. This is because ofvarious types of flow resistance, including frictionbetween the flowing water and the channelboundary. This can be particularly high in rough,boulder bed channels but is also significant forchannels formed in finer substrates. Energy is alsoexpended when the channel impinges against thevalley walls when the flow moves around bendsand as it cascades over steps and waterfalls.Friction is even generated within the flow itself asa result of eddies and turbulence. It is estimatedthat 95 per cent of a river's energy is used inovercoming flow resistance, leaving just 5 per centto carry out geomorphological work.Flow and sediment supplyThe flow in natural channels constantly fluctuatesthrough a continuous series of normal flows, floodsand droughts. Sediment supply also varies throughtime. Rivers continuously adjust their form inresponse to these fluctuations, which in turninfluences the flow of water and sediment transportthrough the channel. Because the flow of water ina river provides the energy required to shape thechannel, the characteristics of that flow are veryimportant in determining channel form. As

previously mentioned, the mean discharge usuallyincreases with the size of the upstream drainagearea. However, the mean discharge does not reflectthe way in which flow varies through time. Thesevariations are described by the flow regime, whichcan be thought of as the 'climate' of a river.Characteristics of the flow regime include seasonalvariations in flow and the size and frequency offloods.Processes of erosion, transport and depositionwithin a channel reach are influenced by the supplyof sediment at the upstream end as well as sedimentthat is locally eroded from the bed and banks. It isnot only the volume of sediment that is important,but also its size distribution. Processes of sedimenttransport are very different for coarse and finesediment, so sediment supply has an importantinfluence on channel form and behaviour. The finermaterials - clay particles, silts and sands - arecarried in the flow as suspended load. This can betransported over considerable distances. Coarsesediment, because it weighs more, is transportedclose to the channel bed as bedload. Compared withthe suspended load, bedload movement is morelocalised, involving much shorter travel distances.Deposits of coarse material form the channel barsthat characterize many alluvial and bedrockchannels, although finer grained sand and silt barsare also common.In short, the form of a given reach (length) ofchannel is controlled by the supply of flow andsediment to its upstream end. Also significant arethe channel substrate, valley width, valley slopeand bankside vegetation. All these controls vary,both between rivers and along the same river. Thiscreates a huge range of fluvial environments andresultant channel forms. The three dimensionalshape of a river is described in terms of its planform,slope and cross-sectional shape. Riverscontinuously adjust their channels in response tofluctuations in flow and sediment supply. Animportant balance exists between the erosive forceof the flow and the resistance of the channelboundary to erosion. Four main types of alluvialchannel form can be identified: straight,meandering, braided and anabranching. Bedrockchannels also exhibit a wide variety of differentforms.The Planform of A Channel/Channel shape andpatternWith so many environmental variables influencingchannel form, an enormous range of differentchannel forms and behaviour is possible. It shouldbe pointed out that not all rivers fit neatly into oneof these categories - there are many examples oftransitional rivers that have characteristics

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associated with more than one channel type.(1) Alluvial channel formFour main types of alluvial channels are generallyrecognised: straight, meandering, braided andanabranching.

Straight channelsAlthough there are many examples of streams andrivers that have been artificially straightened forengineering purposes, naturally straight channelsare rare. Even where they do exist, variations areusually seen in flow patterns and bed elevation.Straight channels are relatively static, with ratesof channel migration limited by a combination oflow energy availability and high bank strength. Thisis especially true where the channel banks areformed from more resistant material, such ascohesive silts and clays.Meandering channelsMeanders form in a variety of bedrock and alluvialsubstrates. Associated with moderate streampowers, alluvial meanders may develop in gravels,sands, or fine-grained silts and clays. An interestingcharacteristic of meanders is that they are scaledto the size of the channel, being more widely spacedfor larger channels. The degree of meanderingvaries greatly, from channels that only deviate

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slightly from a straight line to sequences of highlyconvoluted meander bends. Variations are also seenin the regularity of meander bends, many of whichare rather more irregular. Meandering channelsevolve over time as individual bends migrate acrossthe floodplain. Erosion is usually focused at theoutside of meander bends, which gradually eat intothe floodplain as the channel migrates laterally. Atthe same time, deposition on the inside of the bendallows the channel to maintain its width.Braided channelsBraided rivers are characterised by wide, relativelyshallow, channels in which the flow divides andrejoins around bars and islands. The appearanceof a braided channel varies with changing flowconditions. During high flows, many of the barsbecome partly or wholly submerged, giving theappearance of a single wide channel. At low flows,extensive areas of bar surface may be exposed .Inorder for bars to form, an abundant supply ofbedload is required. Much of this is supplied fromthe upstream catchment area, with additionalcontributions from bank erosion. The barsthemselves can be formed from sand, gravel orboulders. Braided rivers, are associated with highrates of energy expenditure, which is involved inthe transport of large volumes of sediment. Theyoften have steep channel slopes, although there areseveral examples of large braided rivers that flowover low gradients, such as the lower reaches ofthe vast Brahmaputra River in India andBangladesh. Erodible banks are also required forthe channel to become wide enough to allow forthe growth and development of channel bars.Braided channels are highly dynamic, with frequentshifts in channel position. Modifications, such asthe dissection and reworking of bars and theformation and growth of new bars, occur overrelatively short periods of time (days to years). Thepresence of bars leads to complex patterns of flowwithin the channel, and there can be sudden shiftsin the location of sub-channels. Individual channelscan be abandoned or reoccupied in the space of afew days.The Brahamputra a few km aboveGauhati in Assam, the Kosi in Bihar, the Gangabelow its confluence with the Gandak etc. are goodexamples of Braided river.Anastomosing channelsAnastomosing channels have a set of distributariesthat branch and rejoin. They are suggestive ofbraided channels, but braided channels are single-channel forms in which flow is diverted aroundobstacles in the channel, while anastomosingchannels are a set of interconnected channelsseparated by bedrock or by stable alluvium. Theformation of anastomosing channels is favoured by

an aggradational regime involving a high suspendedsediment load in sites where lateral expansion isconstrained. Anastomosing channels are rare: theRiver Feshie, Scotland, is the only example in theUK.Anabranching channelsAnabranching channels, where the flow is dividedinto two or more separate channels, are relativelyrare in comparison to braided and meanderingchannels. The separate channels, calledanabranches, are typically cut into the floodplain,dividing it up into a number of large islands.Individual anabranches can themselves be straight,meandering or braided. Unlike braided channels,rates of lateral channel migration are typically verylow. The islands are stable features and, dependingon climatic conditions, are often well vegetated.However, new channels can be cut whenfloodwaters breach the channel boundary and spillout on to the floodplain. Other channels areabandoned as the flow is diverted elsewhere, orwhen they become infilled with sediment.(2) Bedrock channelsBedrock channels also show a wide diversity ofform. In comparison with alluvial channels,bedrock and mixed bedrock-alluvial rivers havereceived relatively little attention until recently.These channels often behave in a different way toalluvial channels, being strongly influenced by theresistant nature of their substrate. Structuralcontrols, such as joints, bedding planes and theunderlying geological strata can all have asignificant effect on flow processes and rivermorphology.Cross and Longitudinal profileAnalysing the bank to bank profile across the widthof a river, resulting from horizontal and verticalcutting of valley is called as cross profile of theriver. On the other hand, source to mouthobservation of gradient and morphological featureson valley bed and along the bank is longitudinalprofile.

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River Cross ProfilesRiver cross profiles show you a cross-section of ariver's channel and valley at certain points in theriver's course. The cross profile of a river changesas it moves from the upper to lower course as aresult of changes in the river's energy and theprocesses that the river carries out.In the upper course, the valley and channel arenarrow and deep as a result of the large amount ofvertical erosion and little lateral erosion. The sidesof a river's valley in the upper course are very steepearning these valleys the nickname "V-ShapedValley" since they look like a letter V. The river'svalley can be anything from a few meters to a few

hundred metres in width depending on the lithologybut the channel rarely more than 5m or 6m wide.In the middle course, the valley has increased inwidth due to the increase in lateral erosion but itsdepth hasn't changed significantly because verticalerosion has slowed down. Similarly, the channel'swidth has increased but it's still roughly the samedepth. The land to either side of the channel in thevalley is now the river's floodplain and the valley'ssides are much more gentle.In the lower course the valley is now very wide(often several kilometres) and the floodplain hasincreased greatly in size. The channel is a littlewider but not much deeper.

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The Long ProfileThe long profile shows how a river's gradientchanges as it flows from its source to its mouth.You can draw a diagram of a river's long profileeither by drawing a quick sketch based on someprevious knowledge or by plotting the height of theriver above sea level at various points in its course.A sketch of a long profile would look somethinglike this:

The long profile shows how, in the upper stage of ariver's course, the river's gradient is steep but itgradually flattens out as the river erodes towardsits base level. One thing to note is the presence ofknickpoints in the long profile. These are points

where the gradient of the river changes suddenlyand can be caused by landforms like waterfalls orlakes, where the lithology of the river changes anddifferential erosion takes place. Knickpoints canalso be the result of rejuvenation, where the baselevel of the river falls giving it some extragravitational potential energy to erode vertically.Throughout the long profile of a river, depositionand erosion are balanced meaning that, givenenough time, the river's long profile would becomea smooth, concave, graded profile and all theknickpoints would be eliminated as they are eithereroded or filled in by deposition. It would take along time for a river's long profile to become agraded profile though so the idea of a graded profileis, essentially, theoretical as it doesn't really occurin nature. It's not too hard to imagine what a gradedprofile would look like but here's a sketch of oneanyway:

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Channel's Geomorphic unit.The longitudinal profile or long profile of a river isthe gradient of its water-surface line from sourceto mouth. Streams with discharge increasingdownstream have concave long profiles. This isbecause the drag force of flowing water dependson the product of channel gradient and water depth.Depth increases with increasing discharge and so,in moving downstream, a progressively lowergradient is sufficient to transport the bed load.Many river long profiles are not smoothly concavebut contain flatter and steeper sections. The steepersections, which start at knickpoints, may result fromoutcrops of hard rock, the action of local tectonicmovements, sudden changes in discharge, or criticalstages in valley development such as activeheadward erosion.ValleysValleys are so common that are seldom defined and,strangely, tend to be overlooked as landforms. Truevalleys are simply linear depressions on the landsurface that are almost invariably longer than theyare wide with floors that slope downwards. Underspecial circumstances, as in some over deepenedglaciated valleys , sections of a valley floor maybe flat or slope upwards. Valleys occur in a rangeof sizes and go by a welter of names, some of whichrefer to the specific types of valley - gully, draw,defile, ravine, gulch, hollow, run, arroyo, gorge,canyon, dell, glen, dale, and vale. As a general rule,valleys are created by fluvial erosion, but often inconjunction with tectonic processes.Like the rivers that fashion them, valleys formnetworks of main valleys and tributaries. Valleysgrow by becoming deeper, wider, and longerthrough the action of running water. Valleys deepenby hydraulic action, corrasion, abrasion, potholing,corrosion, and weathering of the valley floor. Theywiden by lateral stream erosion and by weathering,mass movements, and fluvial processes on thevalley sides.

Rapids and cascadesLike step-pool sequences, these are associated withsteep channel gradients. Rapids are characterisedby transverse, rib-like arrangements of coarseparticles that stretch across the channel, whilecascades have a more disorganised, 'random'structure. Rapids and cascades are stable duringmost flows because only the highest flows arecompetent to move the coarser cobbles and bouldersthat form the main structure.Interlocking Spurs - As the river cuts its deep V-shaped valley in its upper course, it follows thepath of the easies trock to erode. Thus it tends towind its way along, leaving the more resistant areasof rock as interlocking spurs.Alluvial fansAn alluvial fan is a cone-shaped body that formswhere a stream flowing out of mountains debouchesonto a plain. The alluvial deposits radiate from thefan apex, which is the point at which the streamemerges from the mountains. Radiating channelscut into the fan. These are at their deepest near theapex and shallow with increasing distance from theapex, eventually converging with the fan surface.The steepness of the fan slope depends on the sizeof the stream and the coarseness of the load, withthe steepest alluvial fans being associated withsmall streams and coarse loads. Fans are commonin arid and semi-arid areas but are found in allclimatic zones.

River terracesA terrace is a roughly flat area that is limited bysloping surfaces on the upslope and downslopesides.River terraces are the remains of old valleyfloors that are left sitting on valley sides after riverdowncutting. Flat areas on valley sides - structuralbenches - may be produced by resistant beds inhorizontally lying strata, so the recognition of

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terraces requires that structural controls have beenruled out. River terraces slope downstream but notnecessarily at the same grade as the activefloodplain. Paired terraces form where the verticaldowncutting by the river is faster than the lateralmigration of the river channel. Unpaired terracesform where the channel shifts laterally faster thanit cuts down, so terraces are formed by being cut inturn on each side of the valley. The floor of a rivervalley is a precondition for river terrace formation.Two main types of river terrace exist thatcorrespond to two types of valley floor: bedrockterraces and alluvial terraces.

Alluvial bedforms

River beds develop a variety of landforms generatedby turbulence associated with irregularcrosschannel or vertical velocity distributions thaterode and deposit alluvium. The forms are riffle-pool sequences and ripple-antidune sequences .FloodplainsMost rivers, save those in mountains, are flankedby an area of fairly flat land called a floodplain,which is formed from debris deposited when theriver is in flood. Small floods that occur frequentlycover a part of the floodplain, while rare majorfloods submerge the entire area. The width offloodplains is roughly proportional to riverdischarge. The active floodplain of the lowerMississippi River is some 15 km across. Adjacentfloodplains in regions of subdued topography maycoalesce to form alluvial plains.MeanderingThe word meandering comes from the name of thestream in south eastern Turkey, which was at onetime known as Buyuk Meanderes .A stream havinga winding course and having either regular sinuouspattern or irregular pattern is known as ameandering stream. There are some streams whichfollow sinuous or irregular path, but which havecut into solid rock or hard strata in deep gorges.These are called incised or entrenched meanders.Entrenched meanders can also form in the floodplain when winding pattern is formed in a matureor old stream and rejuvenation takes place whereit starts cutting down again. The ratio of streamlength to valley length is known as the sinuosity.Because of changing conditions of flow, streamslope, sediment size, sediment load and lithologythe meandering pattern along the length can beregular or can change along its length; the latter

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are then called irregular meanders. The irregularityresults

from variation in discharge along the length due totributaries, withdrawal of water, presence of lakes,rock outcrops, weirs and barrages, and non-homogeneity of strata through which the streamflows. Leopold and Wolman (1957) have set anupper limit of sinuosity of 1.5 for differentiatingstraight streams from and meandering streams. Forsome Indian rivers sinuosity values up to 2.5 havebeen reported whereas a value of about 5.5 isconsidered to be the upper limit. The shape ofmeanders is rarely truly sinuous; it is many timesarc of a circle, parabola or some other curve. Onemay sometimes come across a case where thestream has a primary meandering pattern on whichis superposed a meander pattern of smaller meanderlength and belt. This happens if the stream has morethan one dominant discharge.Natural Levees:Natural levees are long embankments formed bythe deposition of alluvial material by the riverswhen they overflow their banks. When streamsoverflow their banks the velocity is appreciablyreduced and hence the carrying capacity of the flowis decreased. This causes deposition of some of thecoarser sediment load resulting in the formationlow ridge along the banks of the stream; these are

called natural levees. Natural levees are highestnear the riverbank and slope gradually away fromit. Natural levees may be one or two kilometre inwidth. They cause the present meander belt of theriver to stand up above the flood plain as a lowalluvial ridge. These levees may be built up untilthe river channel is several meters above the generallevel of the flood plain. This has occurred in thecase of the Yellow river in China and theMississippi river in U.S.A. In many cases tributarystreams have difficulty in breaching the naturallevees and many flow in the same flood plain formany kilometres before breaking through the leveeto join the main stream.BenchesBenches are flat-topped, elongated, depositionalfeatures that form along one or both banks ofchannels.They are typically found on the inside ofbends and along straight reaches, and areintermediate in height between the level of thechannel bed and floodplains.BarsBars are in-channel accumulations of sedimentwhich may be formed from boulders, gravel, sandor silt. Bars can be divided into two broad groups:unit bars and compound bar. Unit bars are relativelysimple bar forms whose morphology is mainlydetermined by processes of deposition .PlayasPlayas are the flattest and the smoothest landformson the Earth . A prime example is the Bonnevillesalt flats in Utah, USA, which is ideal for high-speed car racing, although some playas containlarge desiccation cracks so caution is advised.Playas are known as salinas in South America andsabkhas or sebkhas in Africa. They occur in closedbasins of continental interiors, which are calledbolsons in North America. The bolsons aresurrounded by mountains out of which flood watersladen with sediment debouch into the basin. Thecoarser sediment is deposited to form alluvial fans,which may coalesce to form complex sloping plainsknown as bajadas. The remaining material - mainlyfine sand, silt, and clay - washes out over the playaand settles as the water evaporates. The floor ofthe playa accumulates sediment at the rate of a fewcentimetres to a metre in a millennium. As waterfills the lowest part of the playa, deposited sedimenttends to level the terrain. Playas typically occupyabout 2 to 6 per cent of the depositional area in abolson. Many bolsons contained perennial lakesduring the Pleistocene.Bedrock terracesBedrock or Strath terraces start in valleys where ariver cuts down through bedrock to produce a

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Vshaped valley, the floor of which then widens bylateral erosion . The flat, laterally eroded surfaceis often covered by a thin layer of gravel. Reneweddowncutting into this valley floor then leavesremnants of the former valley floor on the slopesof the deepened valley as rock-floored terraces.Rock-floored terraces are pointers to prolongeddowncutting, often resulting from tectonic uplift.The rock floors are cut by lateral erosion duringintermissions in uplift.

Alluvial terracesAlluvial or accumulation terraces are relicts ofalluvial valley floors. Once a valley is formed byvertical erosion, it may fill with alluvium to createa floodplain. Recommenced vertical erosion thencuts through the alluvium, sometimes leavingaccumulation terraces stranded on the valley sides.The suites of alluvial terraces in particular valleyshave often had complicated histories, with severalphases of accumulation and downcutting that areinterrupted by phases of lateral erosion. They oftenform a staircase, with each tread (a terrace) beingseparated by risers.Geomorphic cycleIt is subdivided into parts of unequal duration, eachpart being characterised by the degree and thevariety of relief and by the rate of change, as wellas by the amount of change that has beenaccomplished since the initiation of the cycle. Thevarious stages in the geomorphic cycle aredescribed in terms of age beginning with youth,which passes into maturity and then into old age.The topographic features of the first stage arespoken of as young or youthful, later ones as matureand those of the last stage as old, with furthersubdivisions when desirable such as, for example,early and late maturity. It may also be mentionedthat each of these stages need not be of the same

duration. It should also be noted that the blendingof types of topographies is the rule rather than anexception. Thus in a region of general youthfulcharacteristics, some streams and valleys may bemature. Similarly in mature plateaus there will besome youthful streams actively engaged indeepening their valleys. The span of time involvedin a complete transformation of landscape may runinto millions of years. Consequently during a periodof scientific observation the changes in topographymay be unnoticeable.

Youthful TopographyYouthful topography is characterised bycomparatively few streams but usually they havehigh gradients. Drainage may be poor with lakesand swamps on the divides between the streams.Streams flow in deep walled canyons or V-shapedvalleys; these will be shallow or deep dependingon the height of the region above sea level. Usuallystreams are actively engaged in cutting their valleysdeeper. Youthful topography also possesses rapidsand falls. There will be general lack of developmentof flood plain except along trunk streams.Mature TopographyWhen the region advances from youth to maturityin the cycle of erosion, the drainage is betterdeveloped with the number of streams increasing.The streams cut their valleys to lowermost levels,their tributaries are well established and lakes,swamps and rapids disappear. Meanders may exist.Since streams start eroding laterally, valleys areflat but the widths of the valley floors do not greatlyexceed the width of the meander belt. If streamsflow through homogenous rocks, tree-like drainagepattern known as dendritic pattern is developedduring maturity of topography. In the regions offolded beds the drainage pattern is rectangular i.e.tributaries meet their main streams at right angles.

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` Old TopographyIn old topography, all main streams have very flatslope and are meandering back and forth over theirflood plains. Valleys are extremely broad and slopegently both laterally and longitudinally; valleywidths are considerably greater than the widths ofmeander belts. Their velocities are low andtransporting power for sediment very limited. Thewhole landscape is gently rolling. Occasionallyerosional remnants stand above the general landsurface. Lakes, swamps and marshes may bepresent but they are on flood plains and not in inter-stream tracts as in youth. The topography tendstowards the ultimate form namely peneplain whichis a large land area of low relief that has beenreduced to nearly base level by the combined actionof weathering and streams. As a rule, surfaces ofpeneplains are not flat but gently rolling with lowhills standing island-like erosion remnants in thegeneral surface of lands(Worcester 1948).

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5. EROSION-PLANATION SURFACE"At the very outset, it is recommended thatplanation surface needs to look at, along with cycleof erosion and slope models of three big names ofgeomorphological history, Davis, Penck and King.Davis remained most influential in coming yearsdue its noticing presence on different academicbodies both in America and Europe, Penck wasmost misrepresented geomorphologist because oflate publication of his book and partly due toconfusions created by Davisian school .King gotits foundation both from Davis and Penck, at someplaces like upliftment of peneplain, he is in supportwith Davis of a rapid rise of area, at other placelike slope analysis, he is more near to the conceptof Penck i.e. retreating slope or slope replacement.Both Davis and King devoted a long period indeveloping theories on the basis of observation indifferent regions, but Penck lived a very short lifeof 35 years was unable to reshape and review thetheories as others did.Also,they worked in differentphysical environment with varied endogenic andexogenic forces carved the differences in theirmodels. Davis worked in stable New Englandregion of U.S.A, while Penck in the unstableAndese in South America and King in arid andsemi-arid regions of Africa. So the conflictingmodels are not the dichotomies but to look at thelandscape from different point of view dependingon the physical milieu of differentgeomorphologists. Planation surfaces can easily be

understand as changing slope angle to the gentlerside due to erosional effect on uplifted landform ineither Davis or Penck's way. The transformationof summit to planation surface can easily beimagined as the slope decline or retreat of generalslope of landscape and through the declining valleyslopes of fluvial channel from lateral cutting ofwater divides or ridges between the channels, andprogressively coalescence of remnant featureswhether monadnocks, pediments or inselbergs. Itis also worth noting that even in arid cycle oferosion, Davis and king combined the role of riverchannels in shaping the geomorphic features. Davisclearly stated that we are not talking about plaindeserts but mountainous desert where role of fluvialchannels are very important.Old landscapes, like old soldiers, never die.Geomorphic processes, as effective as they are atreducing mountains to mere monadnocks, fail toeliminate all vestiges of past landforms in all partsof the globe. Old plains (palaeoplains) survive thatare tens and hundreds of millions of years old. Theseold plains may be various kinds of erosion surface,peneplains formed by fluvial action, pediplains andpanplains formed by scarp retreat and lateralplanation by rivers respectively, etchplains, orexhumed surfaces. Exhumed sur faces andlandforms are old landforms that were buriedbeneath a cover of sediments and then later re-exposed as the cover rocks were eroded. Severalexhumed palaeoplains and such other landforms

Figure: planation surface and formation of monadnocks through lateral cutting

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as reef knolls have been discovered. Stagnantlandscapes are geomorphic backwaters where littleerosion has occurred and the land surface has beenlittle altered for millions of years or far longer. Theyappear to be more common than was once supposed.

Fig: Formation of pediplain and inselberg Planation Surface:

Denudation chronologists once eagerly soughterosion surfaces. However, the search for erosionsurfaces became unfashionable, particularly inBritish geomorphological circles, during the secondhalf of the twentieth century, with manygeomorphologists questioning their existence. Thecurrent consensus is that they do exist, and a revivalof interest in them is apparent. As Ollier (1981)not so tactfully put it, 'Most people who are notblind or stupid can tell when they are in an area ofrelatively flat country: they can recognize a plainwhen they see one'. Of course, a plain may bedepositional, constructed from successive layers ofalluvial, lacustrine, marine, or other sediments.Erosional plains that cut across diverse bedrocktypes and geological structures are all planationsurfaces of some kind. They occur in low-lyingareas and at elevation. Elevated plains sometimesbear signs of an erosional origin followed bysubsequent dissection. A good example is a bevelledcuesta. Planation or erosional surfaces(paleoplain)are the end product or near end product of theprocesses of different agents of erosion, like wind,running water, glacier etc., in different climaticregions (humid, arid).It has been defined by Gilbertas the 'process of carrying away the rock so as toproduce an even surface, and at the same timecovering it with an alluvial deposit'. Thesegeographically plain surface or faint relief isproduced in the last phase of cycle of erosion. Somegeomorphologists, mainly the 'big names' in thefield; have turned their attention to the long-termchange of landscapes. Starting with William Morris

Davis's 'geographical cycle', several theories toexplain the prolonged decay of regional landscapeshave been promulgated. Walther Penck offered avariation on Davis's scheme. According to theDavisian model, uplift and planation take place

alternately. But, in many landscapes, uplift anddenudation occur at the same time. The continuousand gradual interaction of tectonic processes anddenudation leads to a different model of landscapeevolution, in which the evolution of individualslopes is thought to determine the evolution of theentire landscape (Penck 1924, 1953).PeniplainThe 'geographical cycle', expounded by WilliamMorris Davis, was the first modern theory oflandscape evolution (e.g. Davis 1889, 1899, 1909).It assumed that uplift takes place quickly.Geomorphic processes, without furthercomplications from tectonic movements, thengradually wear down the raw topography.Furthermore, slopes within landscapes declinethrough time - maximum slope angles slowly lessen. So topography is reduced, little by little, to anextensive flat region close to baselevel - a peneplain- with occasional hills, called monadnocks afterMount Monadnock in New Hampshire, USA,which are local erosional remnants, standingconspicuously above the general level. Thereduction process creates a time sequence oflandforms that progresses through the stages ofyouth, maturity, and old age. The 'geographicalcycle' was designed to account for the developmentof humid temperate landforms produced byprolonged wearing down of uplifted rocks offeringuniform resistance to erosion. It was extended toother landforms, including arid landscapes, glaciallandscapes, periglacial landscapes, to landformsproduced by shore processes, and to karstlandscapes.

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In the above three dimensional diagram, fig iii, isthe planation surface or peneplain, formed over avery long geological time from an uplifted land dueto the combined effect of vertical and lateral cuttingof fluvial channels resulting slope decline. FromDavisian view point, this almost featureless plainwith some presence of monadnocks got shaped aftera long crustal stability.

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EndrumphA variation on Davis's scheme was offered byWalther Penck. For the end product of cycle oferosion, he used the term Endrumph with isolatedfeatures of inselbergs. According to the Davisian

model, uplift and planation take place alternately.But, in many landscapes, uplift and denudationoccur at the same time. The continuous and gradualinteraction of tectonic processes and denudationleads to a different model of landscape evolution,in which the evolution of individual slopes isthought to determine the evolution of the entirelandscape (Penck 1924, 1953).Three main slopeforms evolve with different combinations of upliftand denudation rates. First, convex slope profiles,resulting from waxing development (aufsteigendeEntwicklung), form when the uplift rate exceedsthe denudation rate. Second, straight slopes,

resulting from stationary (or steady-state)development (gleichförmige Entwicklung), formwhen uplift and denudation rates match one another.And, third, concave slopes, resulting from waningdevelopment (absteigende Entwicklung), formwhen the uplift rate is less than the denudation rate.

Later work has shown that valley-side shapedepends not on the simple interplay of erosion ratesand uplift rates, but on slope materials and thenature of slope-eroding processes.According to Penck's arguments, slopes may eitherrecede at the original gradient or else flatten,according to circumstances. Penck (1953) arguedthat a steep rock face would move upslope,maintaining its original gradient, but would soonbe eliminated by a growing basal slope. If the cliffface was the scarp of a tableland, however, it wouldtake a long time to disappear. He reasoned that alower-angle slope, which starts growing from the

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bottom of the basal slope, replaces the basal slope.Continued slope replacement then leads to aflattening of slopes, with steeper sections formedduring earlier stages of development sometimessurviving in summit areas (Penck 1953). In short,Penck's complicated analysis predicted both sloperecession and slope decline, a result that extendsDavis's simple idea of slope decline. Field studieshave confirmed that slope retreat is common in awide range of situations. However, a slope that isactively eroded at its base (by a river or by thesea) may decline if the basal erosion should stop.Moreover, a tableland scarp retains its anglethrough parallel retreat until the erosion removesthe protective cap rock, when slope decline sets in(Ollier and Tuddenham 1962).As a result ofdifferent processes of slope change due to erosionaleffect of fluvial channels, the uplifted land geteroded to its base level and change the wholelandscape to a featureless Endrumph with isolated

inselbergs. In the above diagram, pediment has beenused with penck, one should not get confuse,features are almost same only name varies, Penckhimself used Endrumph, later geographers used theterm pediplains or pediment like for Penck too.Pediplains and panplainsPenck's model of slope retreat was adopted byLester Charles King, who, in another model oflandscape evolution, proposed that slope retreatproduces pediments and that, where enoughpediments form, a pediplain results (King 1953,1967, 1983). King envisaged 'cycles ofpedimentation'. Each cycle starts with a suddenburst of cymatogenic diastrophism and passes intoa period of diastrophic quiescence, during whichsubaerial processes reduce the relief to a pediplain.However, cymatogeny and pediplanation areinterconnected. As a continent is denuded, so theeroded sediment is deposited offshore. With some

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sediment removed, the continental margins rise. Atthe same time, the weight of sediment in offshoreregions causes depression. The concurrent upliftand depression institutes the development of amajor scarp near the coast that cuts back inland.As the scarp retreats, leaving a pediplain in itswake, it further unloads the continent and placesan extra load of sediment offshore. Eventually, afresh bout of uplift and depression will produce anew scarp. Thus, because of the cyclicalrelationship between continental unloading and theoffshore loading, continental landscapes come toconsist of a huge staircase of erosion surfaces(pediplains), the oldest steps of which occur wellinland. Another variation on slope retreat concernsthe notion of unequal activity espoused by ColinHayter Crickmay (1933, 1975).Davis's, Penck's,and King's models of landscape evolution assumethat slope processes act evenly on individual slopes.However, geomorphic agents act unequally. For thisreason, a slope may recede only where a stream(or the sea) erodes its base. If this should be so,then slope denudation is largely achieved by thelateral corrasion of rivers (or marine erosion at acliff foot). This will mean that some parts of thelandscape will stay virtually untouched by sloperecession. Some evidence supports this contention.Crickmay opined that lateral planation by riverscreates panplains. panplain (panplane, planplain)Area of very subdued relief that consists ofcoalesced floodplains. It is therefore due to lateralstream migration and is a component of a peneplain.Good examples are found in the Carpentaria regionof Australia.

EtchplainsTraditional models of landscape evolution assumedthat mechanical erosion predominates. It wasrealized that chemical weathering reduces the massof weathered material, but only on rocks especiallyvulnerable to solution (such as limestones) werechemical processes thought to have an overridinginfluence on landscape evolution. However, it nowseems that forms of chemical weathering areimportant in the evolution of landscapes.Groundwater sapping, for instance, shapes thefeatures ofsome drainage basins.Some geomorphologistssuspect that chemical weathering plays a starringrole in the evolution of nearly all landscapes. Intropical and subtropical environments, chemicalweathering produces a thick regolith that erosionthen strips . This process is called etchplanation. Itcreates an etched plain or etchplain. The etchplainis largely a production of chemical weathering. Inplaces where the regolith is deeper, weakly acidwater lowers the weathering front, in the same waythat an acid-soaked sponge would etch a metalsurface. Some researchers contend that surfaceerosion lowers the land surface at the same ratethat chemical etching lowers the weathering front .This is the theory of double planation. It envisagesland surfaces of low relief being maintained duringprolonged, slow uplift by the continuous loweringof double planation surfaces - the wash surface andthe basal weathering surface.Whatever the detailsof the etching process, it is very effective in creatinglandforms, even in regions lying beyond the presenttropics. The Scottish Highlands experienced amajor uplift in the Early Tertiary. After 50 millionyears, the terrain evolved by dynamic etching withdeep weathering of varied geology under a warmto temperate humid climate (Hall 1991). Thisetching led to a progressive differentiation of relieffeatures, with the evolution of basins, valleys,scarps, and inselbergs. In like manner,etchplanation may have played a basic role in theTertiary evolutionary geomorphology of thesouthern England Chalklands, a topic that hasalways generated much heat. There is a growingrecognition that the fundamental erosional surfaceis a summit surface formed by etchplanation duringthe Palaeogene period, and is not a peneplainformed during the Miocene and Pliocene periods.

Exhumed landformsExhumed landscapes and landforms are common,preserved for long periods beneath sediments andthen uncovered by erosion. They are common onall continents. Exhumed erosion surfaces are quitecommon. The geological column is packed withunconformities, which are marked by surfaces

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dividing older, often folded rocks from overlying,often flat-lying rocks. Some unconformities seemto be old plains, either peneplains formed by coastalerosion during a marine transgression or by fluvialerosion, or else etchplains formed by the processesof etchplanation. The overlying rocks can bemarine, commonly a conglomerate laid downduring a transgression, or terrestrial. Theunconformity is revealed as an exhumed erosionsurface when the overlying softer rocks areremoved by erosion. It is debatable how theexhumed erosion surface relates to landscapeevolution. If a thin cover has been stripped, thenthe old erosion surface plays a large part in themodern topography, but where hundreds orthousands of metres of overlying strata have beenremoved the exhumed erosion surface is all but achance component part of the modern landscape,much like any other structural surface (Ollier 1991,97).The Kimberley Plateau ofWestern Australiabears an erosion surface carrying striationsproduced by the Sturtian glaciation some 700

million years ago and then covered by a glacialtill. The thin till was later stripped to reveal theKimberley surface, the modern topography ofwhich closely matches the Precambrian topographyand displays the exhumed striations. The reliefdifferentiation on the Baltic Shield, once thoughtto result primarily from glacial erosion, isconsidered now to depend on basement-surfaceexposure time during the Phanerozoic aeon. Innorthern England, a variety of active, exhumed, andburied limestone landforms are present (Douglas1987).They were originally created bysedimentation early in the Carboniferous period(late Tournaisian and early Viséan ages).Subsequent tectonic changes associated with a tilt-block basement structure have effected a complexsequence of landform changes .

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6. CYCLE OF EROSIONThe Cycle of Erosion or 'The Geographical Cycle'was formulated in the latter years of the nineteenthcentury by W.M. Davis (e.g. Davis 1899). It wasthe first widely accepted modern theory oflandscape evolution. Davis regarded landscapes asevolving through a progressive sequence of stages,each of which exhibited similar landforms. In theDavisian model, it was assumed that uplift takesplace quickly. The land is then gradually worn downby the operation of geomorphological processes,without further complications being produced bytectonic movements. It was believed that slopesdeclined in steepness through time until anextensive flat region was produced close to BASELEVEL, though locally hills called monadnocksmight rise above it. This erosion surface was termeda peneplain. The reduction in the landscape createsa time sequence of landforms progressing throughthree stages: youth, maturity and old age. Initiallythe Davisian model was postulated in the contextof development under humid temperate ('normal')conditions, but it was then extended to otherlandscapes including arid (Davis 1905), glacial(Davis 1900), coastal (Johnson 1919), karst(Cvijic´ 1918) and periglacial landscapes (Peltier1950). Davis's model was immensely influentialand dominated much of thinking in Anglo-Saxongeomorphology in the first half of the twentiethcentury, contributing to the development ofDENUDATION CHRONOLOGY. Davis was averitable 'Everest' among geomorphologists .Themodel was largely deductive and theoretical andsuffered from a rather vague understanding ofsurface processes, from a paucity of data on ratesof operation of processes, from a neglect of climatechange, and from assumptions he made about therates and occurrence of uplift. However, it waselegant, simple and tied in with broad, evolutionaryconcerns in science at the time. Nonetheless, bythe mid-1960s the concept was under attack(Chorley 1965). The Davisian model was neveruniversally accepted in Europe, where the viewsof W. Penck were more widely adopted. Penck'smodel involves more complex tectonic changes thanthat of Davis, and regards slopes as evolving in adifferent manner (slope replacement rather thanslope decline) through time . An alternative modelof slope development by parallel retreat leading topediplanation was put forward by L.C. King (e.g.King 1957). Another evolutionary model oflandscape evolution was produced by Budel (1982),who developed the concept of etching, etch plainand etch planation.W.D. THORNBURY defined Geographic Cycle as"The various changes in surface configuration

which a landmass undergoes as the processes landsculpture act upon it"An initial surface underlain by a certain type ofgeologic structure, upon which geomorphicprocesses operate ?=sequential development oflandforms.Davisian modelThe model of landscape evolution usually knownas cycle of erosion was developed by WilliamMorris Davis between 1884 and 1899, publishedin 1899 in the geographical journal royalgeographical society. Accordingly, Landscapes canbe arranged in evolutionary sequences illustrativeof cyclical changes. Since fluvial action iswidespread over the earth's surface in all areasexcepting that of cold and hot deserts. Major partsof the world (except the cold and hot deserts)experience fluvial actions. This remained afoundational concept in geomorphology for manyyears, formed basis for i n t e r p r e t i n glandforms. The idealized sequence of landscape/landform evolution begins with uplifted, virginlandscape culminates with featureless plane erodedto base level, in between passes through stages,each with a set of r e c o g n i z a b l elandforms. Principle of uniformitarianism "novestige of a beginning and no prospect of an end"was the inspiration for his cyclic concept, besidesDarwin and Lamarck.Basic Concepts:1. In a similar way to life forms (influenced from

the Darwin's and Lamarckian ideas) land formscan be effectively analyzed in terms of theirevolution. He regarded landscapes as evolvingthrough a progressive sequence of stages eachexhibiting characteristics landforms.

2. Second key concept was thermodynamicsalthough not explicitly referred by Davis, as theterm developed latter. The landforms initialuplift is the chief source of energy in the formof potential energy and that; thereafter there isan irreversible equalization of energy levels.Throughout the landform assemblage, leadingultimately to a spatially uniform terrain whichDavis called Peneplain.

3. The upliftment of the land is rapid and whilethe landmass is being uplifted, there is very littleor no erosion.

4. Uniform lithology( physical and chemicalstructural characteristics of rocks)

5. Davis divided stages into youth, mature and old.These were however, relative time scale andcannot be related to any particular timeframework.

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6. The streams will erode up to base level i.e., alevel below which streams cannot erode theirvalley floors, further he, supplemented the ideaof base level with the concept of 'grade'- thebalance between erosion and deposition;gradation= aggradation - degradation.

7. In the trio of Davis" structure, process andstage" structure denotes lithology, composition,texture of earth materials; process denotesagents of weathering & erosion whichearth's surface undergoes modification andstage denotes the successive phases of landformevolution, i.e. youth, maturity and old stage.

Figure:1

Fig:2- recapitulation of landscape from peneplainto the uplifted land mass

Figure: 3-

Figure: 4-Figure 1 exhibits the cycle explaining themorphological changes, any landscape passesthrough, as Hutton pointed out that there is novestige of a beginning and no prospect of an endand hence cycle goes on. Figure 2 shows therecapitulation of a landform, which passes throughdifferent stages.Figure 3 and figure 4 also helps inunderstanding the sequential change of landscape.Figure 5 and figure 6 are the diagrammaticrepresentation of landform evolution as shown byDavis to explain the undergoing processes fromyouth to old stage, which a landform passes throughfrom sequential change resulting from endogenicand exogenic forces and carving out themorphological features turns the uplifted landscapeto a peneplain.

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figure-5

figure-6Erosion starts after the completion of upliftment,as Davis said there is very little or no erosion duringrapid upliftment of uniform structure.The abovegraph represents the model of geographical cyclewherein UC (Upper curve) and LC (Lower curve)denote the hill tops of crests of water divided(absolute relief from mean sea level) and valleyfloors lowest relief (from mean sea-level)respectively. The horizontal line denotes timewhereas vertical axis depicts altitude from sea-level. AC represents maximum absolute reliefwhereas BC denotes initial average relief. Initialrelief is defined as difference between upper curve(summits of water divides) and lower curve (valleyfloors) of a landmass. In other words, relief isdefined as the difference between the highest andthe lowest points of a landmass ADG line in denotesbase level of erosion which represents sea-level.No river can erode its valley beyond base levels(below sea-level). The upliftment of the landmassstops after point C as the phase of upliftment iscomplete. Now, erosion starts and the whole cyclepass through the following three stages.1. Youthful stageThe upliftment is complete and has stopped anderosion starts after the completion of the upliftment

of the landmass. The top-surfaces or the summitsof the water divides are not affected by erosionbecause the rivers are small and widely spaced.Small rivers and short tributaries (consequentstreams) are engaged in head ward erosion due towhich they extend their length. Because of steepslope and steep channel gradient rivers activelydeepen their valleys through vertical erosion andthus there is gradual increase in the depth of rivervalleys. The valley becomes deep and narrowcharacterized by steep valley side slopes of convexplan. The lower curve falls rapidly because of valleydeepening but the upper curve remains almostparallel to the horizontal axis because the summitsor upper parts of the landmass are not affected byerosion. Increased relief heralds the beginning ofmature age, indicated by widening of the gapbetween lines 'A' and 'B'.2. Mature stageThe early mature stage is herald by marked lateralerosion and well integrated drainage network.Vertical erosion or valley deepening is remarkablyreduced or vertical erosion slows down andhorizontal actions increases. The summits of waterdivides are also eroded and hence there is markedfall in upper curve (UC) i.e. there is markedlowering of absolute relief, both decreases. The

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lateral erosion leads to valley widening whichtransforms the V-shaped valleys with uniform orrectilinear valley side. The coming closer of lines'A and B' indicates emergence of gentle slope. Thesubsequent streams gain importance now. Themarked reduction in valley deepening (verticalerosion or valley incision) is because of substantialdecrease in channel gradients, flow velocity andtransporting capacity of the rivers. How soon youthwill turn into maturity depends to a large extent onthe texture of the drainage. The number of streamsand their relative distances will determine the speedwith the inter steam areas are being denuded.3. Old stage A gentle gradient, accentuated by horizontal actionand deposition, reduces the erosion intensity. Oldage in characterized by almost total absence ofvalley incision but lateral erosion and valleywidening is still active process. Water divides are

more rapidly eroded. In fact, water divides arereduced in dimension by both, down wasting andbackwasting. Thus, upper curve falls more rapidlymeaning thereby, there is rapid rate of decrease inabsolute height. Relative or available relief alsodecreases sharply because of active lateral erosionbut no vertical erosion. Near absence of valleydeepening in due to extremely low channel gradientand remarkably reduced kinetic energy. The valleybecomes almost flat or get mellowed lines "A andB" run parallel to each other with concave valleyside slopes. The entire landscape is dominated bya graded valley sides and divide crests, broad, openand gently sloping valleys having extensive floodplains, well developed meanders, residual

Monadonocks (residual hills, Davis called suchresidual hills by this name after mountainMonadonock in New Hamshire. Thus, the entirelandscape is transformed into Peneplane sometimesduration of old stage in many times as long as youmaturity combines together.Evaluation of the Davisian Model of landformsDevelopmentThe Davisian model of cycle of erosion despite thecriticism and reinterpretation of the importance ofspatial and temporal scales in geomorphologicalstudies and the subsequent orientation ofgeomorphology towards process studies, theDavisian cycle of erosion still holds a special placein the education of most university geographystudents.Every geomorphological studies giveatleast a passing reference to the Davisian cycle,which dominated the world for 60 years after itsinception in 1899.

Positive AspectsThe Davisian cycle affords a genetic classificationand nomenclature of landscape, as compared to amorphological one and provides the means ofexpressing texture and the build of a landscape.Davis Model of 'geographical cycle' is simple andapplicable.Davis presented his Model in a very lucid,compelling and disarming style using very simplebut expressive language and his model is based ondetailed and careful field observation.The consideration of Davis of change in base levelas indication of initiation of a new cycle has certainadvantages. One, the base level change can be

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considered a unit of time compared to the geologists'stratigraphical time unit. Two, the base levelchanges during glaciations are accommodated.This model is capable of both predictions andhistorical interpretation of landform evolution. Thismodel also synthesized the current geologicalthoughts.Davis ideas have been overthrown in the secondhalf of this century. Though the model faces manychallenges, and being criticized from the verybeginning of its postulation. One of the main criticsof Davis was Walther Penck, followed by S. Judsonand C.G. Higgins.Criticism1. For the sake of simplicity and according to

Davis, erosion is insignificant during the phaseof upliftment, Davis has assumed in his theorythat erosion begins only after the uplift has takenplace and that upliftment occurs very rapidlyand then ceases for the rest of the period of time.But it is natural facts (process) that as land rises,erosion begins and In fact erosion and upliftmentgoes hand in hand. As soon as a landmass beginsto be formed by uplift erosion immediately startsworking on it.

2. There is no logical ground for the assumptionthat flat slopes are old and steep slopes areyoung. Other variables controlling the slope are

nature of soil material and the bedrock, climate,vegetation and downslope factors acting at thefoot slope.

3. The ideal normal cycle of erosion is onlypossible, if there is long crustal stabilityallowing to pass through every stages, but earthis very much unstable. The plate tectonics alsorevealed that plates are always mobile and hencelong period of crustal stability is a remotepossibility.

4. Too much of generalization in the Davisiancycle presents an inadequate framework forlandform interpretation.

5. There is little evidence to prove that landformsactually evolve to an end product calledpeneplane.

6. Davis has over emphasized time. Hisinterpretation of geomorphic processes wasentirely based on empirical observation ratherthan a field instrumentation and measurement.

Penck's model of geographical erosion:Penck made certain deviations from the views ofDavis. One, the erosion does not remain suspendedtill the uplift is complete. In fact, he said, thegeomorphic forms are an expression of the phaseand rate of uplift in relation to the rate ofdegradation, and that interaction between the twofactors, uplift and degradation is continuous, also

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the rate of uplift keeps changing.W.Penck, perhaps the most misunderstoodgeomorphologist's work got published in 1924posthumously by his father A.Penck.He was greatcritic of Davisian Geographical cycle especiallyabout the crustal stability and absence of erosivework during rapid upliftment. He observed thegeomorphological features in tectonically unstableregion of Andese in South America.Basic Concepts:1. The morphological characteristics of any region

of the earth's surface is the result of competitionbetween crustal movement and denudationprocesses.

2. On the basis of morphological characteristicstectonic movements can be explained and theircausal factors ascertained.

3. Development of landscape is not time-dependent.

4. The shape of the hillslope depends on therelative rates of valley incision by rivers andremoval of debris from the hill-slope.

5. Three crustal states are witnessed: (a) state ofcrustal stability with no active displacement; (b)state of initial domed uplift in a limited areafollowed by widespread uplift; and (c) state ofextensive crustal upliftment.

6. Upliftment and erosion are always coexistent.7. Three states of adjustment between crustal

movement and valley deepening are observed:(a) if for a longer time crustal upliftment remainsconstant, the vertical erosion by the river is suchthat there is balance between the rate ofupliftment and erosion; (b) if the rate of upliftis more than the rate of valley deepening, thenthe channel gradient continues to increase tillthe rate of valley deepening matches with therate of upliftment and the state of equilibriumis attained when both become equal; and (c) ifthe rate of valley deepening is more than therate of crustal uplifment, then the channelgradient is lowered to such an extent that therates of upliftment and erosion become equaland the state of equilibrium is attained.

In the place of stage he used the term stage(entwickelung); in the place of youth, mature andold stages, he used the term; waxing or acceleratedrate of development; uniform rate of development;and wanning or deccelarating rate ofdevelopment.He used the word primarumf to theland before upliftment and endrumpf for theresidual plain after completion of stages, for whichdavis used peneplain.stage 1:

With uplift, the interfluves, as well as the lowerparts, rise. There is a lack of brisk undercutting.Penck used the term 'Primarumpf' to represent thecharacteristic landscape before upliftment.Primarumpf is, in fact, initial surface or primarypeneplane representing either newly emergedsurface from below sea level or a 'fastenbene' or'peneplane' type of land surface converted intofeatureless landmass by uplift.Stage 2:Here, the rate, of downcutting is less than the rateof uplift. There is not much change in relief.Stage 3:Rate of downcutting becomes equal to the rate ofuplift. Again, there is not much change in relief.Stage 4:Uplift comes to an end and the downcutting furtherintensifies. The height of interfluves decreases.Deepening of valleys accelerates. A convex sloperesults: this is the stage of waxing erosion orGleichformige Entwickelung.Stage 5:The downcutting and the deepening of valleys slowsdown. The interfluves are rounded and furtherlowered. A concave slope results: this is the stageof waning erosion or Absteigende Entwickelung.Stage 6:Uniform erosion or Gleichformige Entwickelungcharacterises the end product- endrumpf or residualplain like peneplain.Among the above mentioned 6 stages, first stage isthe phase of waxing(accelerating) rate of landformdevelopment. Here, land surface rises slowlyinitially, but after some time rate of upliftment isaccelerated. Continuous active down cutting andvalley deepening result deep narrow v shape valley.As the erosion fails to cope with rate of upliftment,absolute height continues to increase, also relativerelief between highest point and lowest pointcontinues to grow.2, 3 and 4th stage means uniform development oflandforms or Gleichformige Entwickelung. Stage5 and 6 means waning development of landscape,during which landscape is progressively dominatedby the process of lateral erosion and markeddecrease in valley deepening.Absolute reliefdecreases remarkably because of total absence ofupliftment but continued downwsating of dividesummits.Relative relief continues to decreasebecause of divide summits is being eroded.Parallelretreat of valley side slope continues, the upper partof this slope is called as gravity slope orboschungen, and lower segment is called as wash

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slope or Haldenhang.The process continues andinitial landscape transforms in to inselberg and thenfinally to endrumpf.Positive Points:1. Penck followed a deductive approach and did

not restrict himself to any particular condition.2. Compared to the Davisian cycle, Penck's

approach was forward looking.3. Penck, quite appropriately, emphasised the

mutual relation between uplift and the deepeningof valleys. This indicates Penck's respect forgeological evidence. Penck's third stage isevident in the Middle Alps.

Negative Points1. Penck gave too much importance to the role of

endogenetic forces.2. The orderliness in landform changes, as

assumed by Penck, may be difficult to achieve.3. Inadequate knowledge about the initial pristine

landscape does not allow much verification.

4. The concept of geographical cycle of erosionitself has been criticised by many, since manyof the cyclic generalisations are based onuntested assumptions. An overemphasis onhistorical and evolutionary studies in landformsresults in the reconstruction of stages ofevolution becoming the focus of study.

King's cycle of erosionL.C. King's theory of landform development isbased on his studies of landforms in arid, semi-arid and savanna regions of South Africa. Heformulated a set of cyclic models (such as landscapecycle, epigene cycle, pediplanation cycle, hillslopecycle, etc.) and asserted that these are practicablein other parts of globe as well. The reference systemof King's model says "there is uniform development

of landforms in varying environmental conditionsand there is insignificant influence of climaticchanges in the development of fluvially originatedlandforms.Major landscapes in all the continents have beenevolved by rhythmic global tectonic events. Thereis continuous migration (retreat) of hillslope andsuch retreat is always in the form of parallelretreat." For King, the profile of an ideal hillslopeconsists of all four elements of slope, viz., summit,scarp, debris slope and pediments and suchhillslopes develop in all regions and in all climateswhere there is sufficient relief and fluvial processis the dominant agent of denudation.Pointing out that the Davisian model of arid cycleof erosion was inadequate to explain all types oflandscapes, King, in the 1940s, propounded a newcyclic model of pediplanation (or pediplanationcycle) to explain the unique landscapes that heobserved in the arid, semi-arid and savanna partsof Africa. According to King, the African landscapeconsisted of three basic elements: (a) rockpediments flanking river valleys and having

concave slope varying in angle from 1.5° to 7° cutinto solid rocks; and (b) scarps having steep slopesbounding the uplands and varying in angle from15° to 30° and experiencing parallel retreat due tobackwasting by weathering and rainwash; (c) steepsided residual hills known as inselbergs(bornhardts) which vary in size and shape. The sizeof the inselbergs is dependent on the magnitude oferosion and their shape on the nature of underlyingstructure.It is worth noting that King's concept of upliftmentand crustal stability is similar to the concept ofDavis. The cycle of pediplanation is performed bytwin processes of scarp retreat and pedimentation.Each cycle begins with rapid rate of upliftmentfollowed by long period of crustal (tectonic)

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stability. The cycle of pediplanation begins withthe uplift of previously formed pediplains and notof any structural unit. The pediplanation cyclepasses through the stages of youth, mature and oldas in the Davisian cycle of erosion.However, there are certain differences between themodels of King and Davis. Davis's peneplain isformed due to down wasting while King's pediplainis formed due to coalescence and integration ofseveral pediments which are formed due to parallelscarp retreat. Once formed, Davis's peneplain doesnot experience further growth until it is re uplifted.When uplifted, new erosional cycle is initiated andthe rivers are rejuvenated.On the other hand, King's pediplain once formedfurther grows headward. New scarp is initiated atthe far end of the previously formed pediplain whichis progressively consumed by the retreat of newscarp and thus second pediplain is formed whilethe former pediplain experiences decrease in itsextent. The process continues and a series ofintersecting pediplains are formed which extendheadward. Hence, King's pediplains, so formed, areanalogous to Penck's piedmont treppen.

King's model was subjected to many criticisms:(a) King's model was limited to the African

experience.(b) It is doubtful to assert that there is uniform

development of landscapes in differentenvironmental conditions.

(c) King's concept of antique pediplanationremains questionable.

Important terms need attention1. Palimpsest topography.2. Base level of Erosion3. Profile of Equilibrium4. Graded Profile5. Waxing, waning and uniform slope.6. Gravity Slope or Boschungen7. Wash Slope or Haldenhang8. Piedmont treppen or Treppen concept9. Epigene Cycle of king10.Bornhardt11.Dynamic Equilibrium theory12.Cymatogeny

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7. SLOPE EVOLUTIONOf all landforms, slopes are clearly the mostcommon and often the most overlooked.Understanding slope processes is of particularinterest to landuse planners,and because slopesoften reflect changes in lithology, they are ofparticular interest to bedrock mappers. Slope is anupward Or downward inclination of surfacebetween hills and valleys and form most significanaspect of landscape assemblages. In mountainousareas, it is more perceptible. In fact, morphologicalcharacterestics of a given region are determinedby slopes of that region, because physicallandscapes are the result of combinations of slopes;it enhances the complexity of landscapes of aregion. The slope study involves classification,development and evolution of hill slopes. Historyof slope studies, in its initial phase was dominatedby interpretation of different aspects of hill slopesor valley slopes through field visits e.g. slopemodels of Davis, Penck, King, Woods etc, in laterphase, slope models are based on quantitativeanalysis of slope data derived from topographicalmaps, like Young, Mosley etc. In the longitudinalprofile of hill slopes, it is found punctuated withconvexity, concavity, rectilinearity (in straight linewith constant degree of slope) and free face (walllike).

Summital convexity is convexed upper segment ofcrest, also called as waxing slope. It is the mainsource of debris flows downward in the initial stageor even later. Free face is wall like precipitous slopedevoid of any deposition. Below this rectilinearelement is that part of slope, which is gentle innature and covered by debris due to down slopetransport of materials, at the bottom concave slopeexists forms due to active denudation, mainlyrainwash, rill and gully erosion.it is also known aswaning slope.Slopes can be divided laterally into sections basedon surface flow. Imagine or image formulation,drawing flow lines from the highest contour to thebase of the slope. Diverging flow lines define

diverging sections, parallel flow lines parallelsections, and converging flow lines convergingsections.

Plan view of a slopeSlopes can be genetically categorizes into primaryslopes, formed by processes that tend to promoterelief, and secondary slopes, formed by processestending to decrease relief. Secondary slopes evolvefrom the erosion and modification of primaryslopes. The distinction is not always clear becauseprimary and secondary processes do not operateindependently. However, it's important tounderstand to what degree a slope is the result ofprimary and secondary processes. Many slopes arepaleo slopes formed under a different climaticregime. This is especially true in New Englandwhere slopes occupy the flanks of relict glacialfeatures, such as drumlins, moraines, glacialtroughs, and meltwater valleys. A slope's shape isgoverned by its internal structure and externalprocesses, such as slope wash, creep and othermechanisms of sediment transport. Materialdeposited while in transit down the slope is termedcolluvium--an unsorted mixture of rock andsediment derived from the slope face.Origins of primary slopes• Tectonic : fault scarps• Depositional: volcanoes, glacial moraines,

drumlins, dunes, alluvial fans, delta foreset(foreset is inclined part of delta), etc.)

• Erosional (glacial and riverine valleys, etc.)• Human activity (blasted rock slopes, hydraulic

mining, tailings piles, etc)Processes acting on slopes* Mass Wasting• creep, flow, fall, etc* Action of water• raindrop impact (aids in the suspension of

sediment)

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• slope wash (Horton overland flow*, sheet flow)• Horton overland flow describes the tendency

of water to flow horizontally across landsurfaces when rainfall has exceeded infiltrationcapacity and depression storage capacity

• channelized flow (rills)• Subsurface flow (eluviation and solute

transport, sapping etc.) Mass movement and morphology of slope.

• creep leads to the development of convexupward slope segments

• solifluction, slumps, and flows commonly resultin concave upward profiles at their heads andconvex toes of colluvium

• rock fall forms a talus (scree slope)beneath afree face (cliff)

* slope of talus is governed by:• angularity of sediment• Rate of rock fall vs. rate of weathering and

erosion of talus* Pediment surfaces that lack significant debris

beneath the free face develops because talus isweathered and removed faster than it isproduced

Effects of water* surface flow (Horton overland flow, or slope

wash, and channel flow):• aids the development of concave upward

profiles in valleys and• convex upward profiles along divides* subsurface flow (downward percolation,

through flow and groundwater flow)• aids in eluviation (minor mechanism of slope

decline)• aids in the formation of earthflows and

solifluction• may lead to surface channel formation by piping

(sapping).Other factors influencing slope morphology1. Geology: Slope composition and structure

controls the detachability of slope material bya particular process

• Rock slopes: Slope is controlled by rockstrength and structure.

• rock strength: high strength promotes thedevelopment of a free face low strengthpromotes flatter slopes

• structure: orientation, type and abundance ofplanes of weakness (e.g. bedding planes &

joints)• fall faces typically occur where,• there is an active geologic agent oversteepening

the slope• previously oversteepened slope has not yet been

deeply weathered or consumed by colluvium• Change in base level exhumes buried

topography• Soil slopes: Shape controlled more by processes• Erosion by water is influenced by permeability

and erodibility of slope materials and vegetativecover

• sharp divides typically develop on poorlyvegetated, impermeable and easily erodedslopes

• Mass wasting is influenced by sedimentcharacteristics (cohesiveness, grain size, sortingand angularity), degree of consolidation, andstructure.

2. Climate• controls intensity of chemical vs. mechanical

weathering• controls vegetation and water content• In arid landscapes lacking vegetation, such as

those shown in figures 1 and 2, fluvial erosionis quite effective.

Figure 1. Slope developed on horizontalsedimentary rock, Grand Canyon, AZ. Variationsin lithology strongly influence the rock slopes thatflank the canyon. Cliffs of limestone and sandstonealternate with gentle slopes composed of shale.

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Figure 2. Slopes developed in playa sediments(Furnace Creek Formation near Zabrinskie Point,Death Valley, CA)..• Continues…..Generalizations regarding the

effects of climate:• Humid• Slope form is controlled by processes acting on

regolith: slopes tend to be transport limited• Arid/semiarid• Lack of vegetation increases the efficiency of

water and wind• slope form is controlled by bedrock strength and

characteristics: slopes tend to be weatheringlimited

3. Local activity: Rates of mass-wasting arepromoted by:

• Proximity to stream, shoreline, etc.• activity of man• rate of uplift and incision; reliefTerminology used to describe slopes• Slope angle. See Table• Transport limited: Rate of transport is lower

than regolith formation: Weathering and soilformation rates are faster than rates of removal.Slope form is greatly controlled by creep,solifluction and similar mass movementprocesses, and slope wash.

• Weathering limited: Rates of regolithformation is slower than transport: Erosionalprocesses, such as mass-wasting, slope wash,fluvial activity, etc., are faster than weathering(soil-forming) processes. Slopes are steep andhave little to no soil. Structure and lithologycontrol the shape of the slope.

Angle Description0°-0° plain0°-30' slightly sloping

2°-5° gently inclined5°-15° strongly inclined15°-25° steep25°-35° very steep35°-55° precipitous55° and greater verticalProfile ShapesSlope profiles can have several manifestationsdepending on the factors which have been discussedby many scholars with qualitative and quantitativeapproach.Some models have been discussed asbelow.Early geomorphologist, such as W.M. Davis, L.King and W. Penck each devised differentconceptual models for slope retreat for which theystrongly advocated. However, their models are notnecessarily contradictory, but reflect the differentclimates, tectonic settings, lithologies and processesaffecting the slopes where they studied. WilliamMorris Davis studied slopes in New England wherethe climate is humid and temperate and the tectonicsetting is stable. Lester King worked in the-semiarid climate of South Africa and Walther Penck inthe tectonically active Andes.1. Slope Decline (By W.M. Davis, 1899)Upper slope weathers and erodes at at faster rateso there is progressive decline of slope angle occurs.The slope becomes progressively decrease in theangle of slope in each phase of their development.It becomes less steep and a concavity develops atthe base, the convexity extends in length andbecomes more gently curved, i.e. smoothly convex-concave forms and in particular long and gently-curved convexities associated with decline. Cause:it is equilibrium between the rates of weatheringand transport- In stage 1, the elimination of the free faces isdone by the processes of fall and slump of thebedrock until the slope is gentle enough to developa cover of regolith.- Stage 2 shows this phase which is called thegraded slope. The regolith maintains a constantthickness over the slope and all the weatheredmaterials is transported by mass movements andwash. The form of the slope is concave-convex.- Stage 3 & 4, the length of the straight segmentincreased. The curvature of the elements decreasesas the slope continues to decline, and the length ofstraight segment diminishes. The upper convexityexperiences more and more output than input,whereas the lower concavity receives more inputthan what it can output.

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- Lower part of the slope - with the accumulationof the transported regolith, lowering is less.- Davis, the original proponent of this theory, basedhis arguments on visual assessment of slopes inhumid temperate areas.

A:Slope decline of Davis; B: Slope replacement ofPenck; C:Parallel retreat of King with scarpOr free face; D:Parallel retreat without freeface(after young,1972)2.Slope Replacement and retreating slope ByWalter PenckSlope replacement means original steep slopesbeing replaced by lower angle slopes which extendupwards from the base at a constant angle. A freeface slope is slowly buried by scree whichaccumulates at the base of cliff. (It means thereplacement of a cliff by scree). All parts of thecliffs face are exposed to weathering. The screeaccumulating at the base increase in height and ifit is not removed, it will eventually replace theentire cliff by a gentle slope, the angle of rest.Continued weathering and removal leads to anupward extension of this gentler slope. Thiscontinues until the whole of the slope has beenreplaced from below by the gentler slope.Penck also discussed about the parallel retreat ofslope. Penck's model of retreating slope profile,where evolution of the profile is controlled by rateof output (river erosion) at the base and rate ofuplift of the land. He was able to deduce variousslope profiles for different combinations of rivererosion, uplift and rock resistance, by assuming thatstronger rock requires steeper slopes for the samerate of denudation. He also modelled streamlongitudinal profile as controlled by uplift, rock typeand stream discharge.Slope angle and lengths remain uniform as the sloperetreats parallel to itself. Hillslopes are esentiallyfree of sediment. Parallel retreat occurs whereunderlying strata are protected by a resistant cap

rock, such as a layer of sandstone, limestone, orlava. Failure of the caprock occurs only whenerosion has removed the weaker rock supportingit. Parallel retreat is responsible for the classicstepped topography of the Colorado Plateau, andthe formation of flat-topped buttes, mesas, and

pinnacles.(Evaluation of Penck's model is hindered by itshurried writing, posthumous publication andconfused representation in English, includingmisrepresentation by Davis who was defending hisown ideas. Although there were important flawsand contradictions in Penck's work, and it waspoorly translated or misrepresented especially byadherents to the Davisian school, it was the onecomprehensive alternative to the cycle of erosionand thus was a focus for contrary ideas, such asemphasis on process rate (both endogenic andexogenic) and greater attention to slope retreat. Soone should not get confuse because. King also usedparallel retreat of slope, the works of L.C. king isquite in consonance with Penck, he differed fromDavis at so many points)

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3. Parallel Retreat (By King)Concave-convex slopes are, by no meansubiquitous. In many semi-arid areas, more complexprofile are common, comprising an upper convexity,a free-face, a rectilinear debris slope, and a gentlyconcave pediments. Each of the upper parts of theslope retreats by the same amount and maintainthe same angle. Therefore, the convexity, free faceand debris slope all retain the same length.

The concavity extends in length and becomesslightly gentler in angle. The pediment isMonument Valley, Utah exhibits some of the bestexamples of slope retreat. Slopes are weatheringlimited and reflect the angle of repose of the variouslithologies. Bryce Canyon Utah exhibits sloperetreat where weak rocks are protected by resistantcaprock. Once removed the slope undergoe rapiddecline.

the name given to the gently concave area whichextends from the foot of the debris slope andbecomes wider and wider as the slopes retreat. Thepediment is generally slightly concave but theslopes are very gentle, only 3-5 degree, and oftenthey appear to be completely flat. The pediment isonly covered by a thin sheet of regolith. In the aridarea, where chemical weathering is greatlyimpeded. The supply of regolith is limited; the inputof transported materials at the bottom of debrisslope is little, with respect to the rate of output. Asa result, the debris slope is able to maintain aconstant angle, with an continual extension ofpediments primarily by the process of sheetwash.Late in the erosion cycle, the hills are left asisolated, steep-sided relicts, called inselbergs in

After RJ Small 1970

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Africa, or buttes and mesas in North America.Parallel retreat in brief(a) Of a slope containing a free face and debrisslope,~ the angle of the free face is determined by thestrength of the rock. A strong will often form avertical free face.(b) Of a slope without a free face~ In less resistant rocks, the free face may not bepresent. Weathering reduces the slope to acontinuous debris-covered slope.Four unit slope model (Wood, 1942)The four unit slope is best developed on a highinitial (primary) slope composed of strong rock andthe absence of local undercutting. As the steep fallface retreats the base is covered by a straight talusslope.

Figure: Four Unit slope model modified from Wood(1942)9-Unit Slope Model of Dalrymple et al, 19681.interfluve: divide area characterized by largelyvertical subsurface water and soil movement2. seepage slope: gently dipping portion dominatedby downward percolation3. convex creep slope: upper convex zonecharacterized by creep and terracette formation4. fall face: Cliff face characterized by rapiddetachment of material or bedrock (weatheringlimited) exposure.5. transportational mid-slope: Active regioncharacterized by mass movement, terracetteformation, slope wash and subsurface water action.Figure:9-unit slope model.5. transportational mid-slope: Active regioncharacterized by mass movement, terracetteformation, slope wash and subsurface water action

Figure:9-unit slope model.6. colluvial foot slope: Depositional region.Material is further transported down slope by creep,slopewash and subsurface flow7. Alluvial toe-slope: region of alluvial deposition(e.g. levee deposits)8. Channel wall: removal by corrasion, slumping,fall etc.9. Channel bed: Down stream transport of materialBesides, many analysis have been done by moderngeomorphologist through quantitative methods onthe basis of topographical data. There are actuallyno universal pattern of slope evolution, slopes arerelated to rock type, vegetation and variousweathering and sediment transport processes.