EARTH SCIENCES CENTREGÖTEBORG UNIVERSITYB193 1999
THE GEOMORPHOLOGICAL EVOLUTIONOF TILTED BLOCK MOUNTAINS
- a case study from Sierra Nevada, California
Kerstin Ericson
Department of Physical GeographyGÖTEBORG 1999
GÖTEBORGS UNIVERSITETInstitutionen för geovetenskaperNaturgeografiGeovetarcentrum
THE GEOMORPHOLOGICAL EVOLUTIONOF TILTED BLOCK MOUNTAINS
- a case study from Sierra Nevada, California
Kerstin Ericson
ISSN 1400-3821 B193 Projketarabete
Göteborg 1999
Postadress Besöksadress Telefo Telfax Earth SciencesCentre Geovetarcentrum Geovetarcentrum 031-773 19 51 031-773 19 86 Göteborg UniversityS-405 30 Göteborg Guldhedsgatan 5A S-405 30 Göteborg
SWEDEN
“The Ninth Symphonyis the Yosemite of music!
Great is GraniteAnd Yosemite is its prophet!”
Rev. Thomas Starr King
2
3
Abstract
Landscape evolution is complex and extends over a long geological time period where temporalvariations in crustal movement, eustatic levels and climate are established factors. Hence, theseparameters will determine whether deep weathering, pedimentation, glacial, fluvial or eolian erosionetc. will be the dominating exogenic process. The aim of this study is to describe crystalline bedrockforms in order to genetically classify landforms and evaluate the relative importance of differentprocesses for the present morphology according to this form-process concept.
The westward tilted Sierra Nevada batholith was studied owing to its well known and remarkablegeological history. The batholith started to form during the early Mesozoic, 210 Ma, and theintrusions took about 130 Ma to complete. At the end of the Cretaceous period Sierra Nevadabegan its uplift from the sea. Hence periods of orogenic activity such as faulting, uplift and westwardtilting succeeded each other during late Cretaceous-early Tertiary. In connection with the orogenicactivity deep weathering and subsequent stripping and fluvial incision have remodeled the landscape.During Pleistocene major parts of Sierra Nevada was exposed to heavy glaciation.
The gross morphology in Sierra Nevada suggest a structurally controlled landscape exposed to deepweathering. Remnants of saprolites cover parts of the western side of Sierra Nevada and the slopestowards the Central Valley of California. The relief on exposed plateaus suggest that the majorlandforms in Sierra Nevada are inherited and preserved predating the Pleistocene glaciation. Theglacial erosion has been confined to the more elevated High Sierra and has only remodeled thevalleys. The study points at important steps in the geomorphic evolution of tilted block mountainswhich may be applicable to the evolution of formerly glaciated continental margins such as theScandinavian peninsula and the North American Cordillera.
4
Sammanfattning
Landskapsutveckling är komplex och verkar över en lång tidsperiod där temporala variationer ijordskorpans rörelser, eustatiska nivåer samt klimatförändringar är etablerade faktorer. Följaktligenbestämmer dessa parametrar om djupvittring, pedimentation, glacial, fluvial eller eolisk erosion etc.kommer att bli den dominerande exogena processen. Ändamålet med denna studie är att beskriva dekristallina berggrundsformerna för att klassificera landformernas genes och värdera den relativabetydelsen de olika processerna har för den nuvarande morfologin enligt detta form-processbegrepp.
Den västligt lutande Sierra Nevada batoliten studerades på grund av dess välkända ochanmärkningsvärda geologiska bakgrund. Batoliten började formas under tidig Mesozoikum, 210Ma, och intrusionerna tog cirka 130 Ma att fullfölja. Vid slutet av Krita påbörjade Sierra Nevadaupplyftningen från havet. Under sen Krita-tidig Tertiär följde orogenisk aktivitet såsom förkastningar,uplift och västlig lutning på varandra. I samband med den orogena aktiviteten omformade bland annatdjupvittring med påföljande stripping och fluvial nedskärning landet. Under Pleistocen var stora delarav Sierra Nevada utsatt för glaciation.
Stormorfologin i Sierra Nevada tyder på ett djupvittrat strukturkontrollerat landskap. Saprolitrestertäcker delar av den västra sidan av Sierra Nevada samt sluttningarna ner mot Great Valley ofCalifornia. Reliefen på exponerade platåer påvisar att de huvudsakliga landformerna i Sierra Nevadaär nedärvda samt antedaterar den Pleistocena glaciationen. Den glaciala erosionen har varitbegränsad till det högre liggande High Sierra och har endast omformat dalarna. Studien påvisar fleraviktiga steg i den geomorfologiska utvecklingen av lutande bergsmassiv vilket kan appliceras påutvecklingen av tidigare nedisade kontinentala gränser såsom den Skandinaviska halvön och denNordamerikanska Cordilleran.
5
CONTENTS
1 INTRODUCTION............................................................................................................................................................. 7
2 THE STUDY AREA - SIERRA NEVADA, CALIFORNIA.......................................................................................... 8
2:1 TOPOGRAPHY ..............................................................................................................................................................82:2 GEOLOGIC SETTING ....................................................................................................................................................82:3 PHANEROZOIC EVOLUTION.......................................................................................................................................9
2:3.1 The origin and history of the present geomorphology ............................................................................. 92:3.2 Faulting, uplift and westward tilting.........................................................................................................10
2:4 QUATERNARY EVOLUTION AND THE GREAT ICE AGE......................................................................................112:5.1 Glacial reshaping of the landscape ...........................................................................................................12
2:6 FORM-PROCESS RELATIONS.....................................................................................................................................142:6.2 Bedrock forms related to deep weathering processes and stripping...................................................142:6.3 Conditions favorable for glacial erosion..................................................................................................152:6.4 Bedrock forms related to glacial erosion..................................................................................................15
3 METHODS.......................................................................................................................................................................16
3:1 DEMARCATION OF STUDY AREAS..........................................................................................................................163:2 MAPS AND TOPOGRAPHIC PROFILES.....................................................................................................................163:3 FIELD MAPPING.........................................................................................................................................................173:4 MANUAL FOR FIELD MAPPING...............................................................................................................................18
4 RESULTS.........................................................................................................................................................................19
4:1 TOPOGRAPHICAL PROFILES OVER TIOGA PASS QUADRANGLE........................................................................194:2 TOPOGRAPHICAL PROFILES OF YOSEMITE VALLEY..........................................................................................214:3 YOSEMITE NATIONAL PARK – FIELD MAPPING.................................................................................................25
4:3.1 Yosemite Valley ..............................................................................................................................................264:3.2 Yosemite Valley - weathering forms............................................................................................................264:3.3 Yosemite Valley - glacial forms ...................................................................................................................274:3.4 Tamarack.........................................................................................................................................................274:3.5 Tamarack - weathering forms......................................................................................................................274:3.6 Foresta.............................................................................................................................................................284:3.7 Foresta - weathering forms ..........................................................................................................................284:3.8 Foresta - glacial forms..................................................................................................................................304:3.9 Road sections in Yosemite National Park.................................................................................................30
4:4 HETCH HETCHY ........................................................................................................................................................314:4.1 Weathering forms ...........................................................................................................................................324:4.2 Glacial forms...................................................................................................................................................334:4:3 Roadside geomorphology between Hetch Hetchy reservoir and Poopenaut ....................................33
4:5 KINGS CANYON & SEQUOIA NATIONAL PARKS..................................................................................................344:5.1 Kings Canyon.................................................................................................................................................354:5.2 Generals Highway .........................................................................................................................................354:5.3 Generals Highway - Silliman Creek...........................................................................................................364:5.4 Moro Rock.......................................................................................................................................................37
4:6 KERN RIVER VALLEY ...............................................................................................................................................384:7 BIG PINE LAVA FIELDS .............................................................................................................................................394:8 STRUCTURE OF DIFFERENT FORMS .......................................................................................................................41
5 DISCUSSION..................................................................................................................................................................43
5:1 YOSEMITE VALLEY...................................................................................................................................................435:2 STRUCTURE OF DIFFERENT FORMS .......................................................................................................................44
5:2.1 Valleys with vertical joint controlled walls: Yosemite Valley, Hetch Hetchy Valley,Kings Canyon and Kern River................................................................................................................................445:2.2 Forms on plateaus outside Yosemite Valley, Hetch Hetchy Valley, and Kings Canyon ..................455:2.3 Forms in and about Foresta: landscape with no recent glaciation ....................................................455:2.4 Pediments east of the Sierra Nevada fault - Big Pine lava fields..........................................................46
6
5:2.5 Relative importance of different processes for the present forms..........................................................46
6 CONCLUSIONS .............................................................................................................................................................49
ACKNOWLEDGMENTS....................................................................................................................................................50
REFERENCES.......................................................................................................................................................................51
FURTHER READING..........................................................................................................................................................53
APPENDIX............................................................................................................................................................................54
7
1 Introduction
The evolution of denudational landforms/landscapes developed in hard crystalline rocks is oftencomplex and extends over a long geological time period. In the long term perspective landscapeevolution depends on temporal variations in crustal movement, eustatic levels and climate. Thoseparameters will in turn determine whether sediment burial, river incision, pedimentation, deepweathering, glacial erosion etc. will be the dominant process.
In unglaciated shield areas, such as Australia, the long term development of landforms can be quitewell explained and dated by the existence of saprolites and cover rocks of different ages (Young1983; Bird and Chivas 1993; Ollier 1995; Twidale & Campbell 1995). In glaciated shield areas, onthe other hand, the preglacial regolith as well as landforms have been more or less completelyeroded as a consequence of repeated glaciations during Pleistocene. However, the significance ofglacial erosion has been re-evaluated during the last decade in formerly glaciated areas such asScotland (Hall 1986; Hall & Sugden 1987), Canada (Bouchard et al. 1995) and France (Battiau-Queney 1997).
The evolution of passive margins formed by Mesozoic and Cenozoic uplift leading to rejuvenationand fluvial incision in the newly created continental scarp has been in focus during the last decades(Ollier 1982, 1991; Summerfield & Thomas 1987; Thomas 1995). Landforms that have beenpreserved in the scarp hinterland form elevated paleoplains where the sub continent of southernAfrica as well as eastern Australia are excellent examples.
In contrary the evolution of similar features such as tilted block mountains has been less studied,however, their geomorphological uplift geometry share features with passive margins. Ollier (1991)points at the characteristically geomorphological evolutionary trend of tilted block mountains leadingto major landscape rejuvenation in the uplifted parts and preservation in the less uplifted parts of theblock. Hence, both passive margins and tilted block mountains may contribute to the understandingof the dynamic geomorphic system and denudation rates and patterns through time.
This work focus on the advantage of studying bedrock forms in order to genetically classifylandforms and evaluate the relative importance of different processes for the present morphology incrystalline rock. This kind of work has been appreciated during the last decade in Sweden (Lidmar-Bergström 1989, 1997; Lidmar-Bergström et al. 1997; Olvmo et al. in press) as a useful tool inunderstanding the long term evolution of landforms as well as to understand the magnitude of glacialerosion.
In this study the geomorphological evolution of the Sierra Nevada batholith, which constitute of oneof the most well known granite landscapes on earth, is studied by mapping characteristic landformassemblages. The study has taken place along a transect crossing the tilted block from west to eastperpendicular to the uplift axis. The purpose is to define different landform zones, ranging frompreviously glaciated areas to areas with no connotations of ice ever operating on them. The studytakes place on different elevations along the transect which in turn can be used to outline the mainsteps of landform development in this particular area. Furthermore an attempt is made to evaluate therole different processes has played for the present landscape.
8
2 The study area - Sierra Nevada, California
2:1 Topography
Running half the length of California, Sierra Nevada is the largest single mountain range in thecontiguous U.S. It is 80-130 km wide and runs almost 650 km in the north-south direction, coveringnearly as much area as the French, Swiss and Italian Alps combined (Matthes 1930; Shelton 1966).The mountain range is strongly asymmetric with a steep eastern escarpment, with a dip of 25° atparts, and a gentle westward slope with a minor dip of 3° towards the Central Valley of California(fig. 1).
Regarding altitude the Sierra Nevada out-rivals all the other mountain ranges in the U.S. Not only isMount Whitney, 4,417 m, the highest summit in the Lower 48 but the range as a whole stands higherabove its immediate base than any other range. The peaks at the east base stands about 3,300 mabove Owens Valley and at the west base they stand about 4,200 m above the Central Valley ofCalifornia (fig. 1). This can be compared with the Rocky Mountains which stand only 2,700 mabove the Great Plains (Shelton 1966).
2:2 Geologic setting
Sierra Nevada is one of Earth’s grandest examples of granitic terrain. The batholith is formed byhundreds of granite intrusions, 70-200 Ma, ranging from less than 2 km2 to 1,300 km2. Furthermore,the granite plutons have sharp crosscutting relationships which indicate multiple injections (Batemanet al. 1963). The plutonic rocks of Sierra Nevada is built of five minerals: quartz, potassiumfeldspar, plagioclase feldspar, biotite, and hornblende (Huber 1989). This leads to a rockcomposition ranging from diorite and gabbro, quartz-monzodiorite, quartz-diorite, tonalite,granodiorite to granite.
Throughout most of the area the granitic rocks are jointed, commonly spaced from 0.6 to more than3 m apart. At outcrop scale, three sets of joints are generally present, two nearly vertical, almostperpendicular to each other, and a third nearly horizontal creating approximately rectangular blocks.The joints are zones of weaknesses, a gateway for water and humic acids which may weather thesehard and usually erosion-resistant rocks.
Remnants of metamorphic rocks can be seen along the eastern margin in the summit area and whileapproaching the western edge of the foothills (fig. 2), moreover, they are considered to be the oldestrocks within the Sierra, 440 million years old (Huber 1989). The rocks consist of schist, slate,quartzite, marble, calc-silicate hornfels, amphibolite and serpentine. Bedded and foliated bodies of
Fig. 1. Diagram of the tilted Sierra Nevada,arrows show the direction of movement.The height and slant of the range areexaggerated and streams are shown flowingin the general direction that Sierran streamsflow. West of the range is the Central Valleyof California filled with sediments derivedfrom the mountains. Owens Valley is markedon the east side. (Modification afterMatthes 1930).
OwensValley
9
these rocks, 1.5-225 km wide and 7.5-400 km long, generally strike north-west and dip steeply tobeing almost vertical (Wahrhaftig 1965). The metamorphic rocks underlie approximately 15-20 percent of the western slope of the southern Sierra Nevada below 2,700 meters. Furthermore, they aremore resistant than the granitoids and generally rise above the immediately adjacent granitic terrain toform rugged, sharp-crested mountains with long even side slopes (Wahrhaftig 1965).
Fig. 2. Schematic section over the present geologic setting in California from the Pacific Plate defined by the SanAndreas fault in the west to the Basin and Range area in the east. The Coast Range and the western edge of thefoothills is built by metamorphic rocks. Remnants of metamorphic rocks cover some Sierran summits. Sedimentsderived from the mountains cover the Central Valley of California. Sierra Nevadas uplift and westward tilt isclearly defined via the steep east escarpment demonstrated by the Sierra-Mono fault on the eastern side of themountain range. To be followed by the Basin and Range most westerly outpost, Owens Valley, where volcanoesstill are active. Not to scale. (Modification after Hill 1975; Uppsala excursion guide 1980).
2:3 Phanerozoic evolution
2:3.1 The origin and history of the present geomorphology
The geologic time-scale used in this study is presented in the appendix, page 53. In the Paleozoic thecontinents were joined together as one landmass and throughout most of the Paleozoic sea coveredthe surface that was going to become the Sierra Nevada. Consequently, thousands of meters ofsediments such as clay, mud, sand as well as volcanic ash from submarine volcanoes settled on thebottom of the sea. In the early Mesozoic, about 210 Ma, the continents began to drift apart and thebatholith that was going to become Sierra Nevada started to form. Hot, molten magma pushed thesediments aside transforming them into metamorphic rocks. However, it took about 130 millionyears to complete the granite intrusions and the subsequent cooling and the land did not begin itsuplift from the sea until the end of the Cretaceous period, 65 Ma, (Bateman et al. 1963; Shelton1966; Hill 1975).
Owing to its latitudinal position the Sierran landscape was subjected to subtropical climate with meantemperatures of 20° C during the latter part of Cretaceous until late Oligocene, 80-25 Ma, (Hill1975). Geomorphic processes such as weathering and erosion could operate more swiftly andgradually the upper parts of the Sierran batholith was exposed. Due to the uplift parts of the granitecore were probably still hot while the top parts were denuded. The result of the Paleogenedenudation is a flat denudation surface with low topographical relief, today appearing as a summitlevel surface.
10
2:3.2 Faulting, uplift and westward tilting
During the Miocene, 22-5 Ma, the movement between the North American plate and the Pacificplate turned to dextral strike-slip faulting, the San Andreas fault. At the same time began the westernmargin of the American landmass uplift (Hill 1975; Huber 1989). Parts of the Pacific plate slidbeneath the North American plate in a subduction zone commencing partial melting of the Pacificplate which in turn caused melting of the lower parts of the North American plate. Extensivevolcanism began and large areas in northern Sierra Nevada were covered by lava, ash and lahars.Due to subduction of the Pacific plate volcanoes have continued to erupt in Sierra Nevada to thepresent day.
During the Neogene (latter part of Tertiary) up to the present day Sierra Nevada has gone throughrepeated uplift events and westward tilting which has formed the present mountain range with thegently sloping west side and steep east escarpment (figs.1 and 2). Owing to the uplift and subsequenttilting, weathering and stripping accelerated during Neogene and Quaternary which in turn initiatedrenewed deep weathering and fluvial incision. Furthermore, most of the major streams in the SierraNevada follow their ancient river channels, incisions started prior to the uplift. The rivers flowwestward into the San Joaquin or Sacramento rivers and hence to the sea while just a few riversflow eastwards into Nevada.
Matthes (1930) argued that Sierra Nevada had gone through three major Tertiary uplifts intervenedby pauses. Each uplift initiated a new cycle of erosion and produced a more pronounced landscapeincision with a greater topographical relief (fig. 3).
Fig. 3. Bird’s-eye view over the development in Yosemite Valley. A. the first uplift during the Paleocene causedheadworth growth of the Merced River and by the Miocene the broad-valley stage with meandering streams anda rolling surface of rounded hills was developed; B. the second uplift throughout Pliocene, the mountain-valleystage, steepened the stream gradients and deepened the valleys; C. the third and greatest uplift during latePliocene-early Pleistocene developed the canyon stage and Merced River deepened the valley further. (AfterMatthes 1930).
Selected landforms identified:E Echo Peak ND North Dome LC Liberty Cap CR Cathedral RocksC Clouds Rest TC Tenaya Creek SD Sentinel Dome LT Leaning TowerSM Sunrise Mountain IC Indian Creek G Glacier Point DP Dewey PointM Mount Maclure HD Half Dome SR Sentinel Rock RC Ribbon CreekL Mount Lyell BP Bunell Points SC Sentinel Creek BV Bridalveil CreekF Mount Florens LY Little Yosemite Valley EP Eagle Peak MR Merced RiverMW Mount Watkins B Mount Broderick YC Yosemite Creek R Royal Arches
CB CBA
11
BD Basket Dome CC Cascade Cliffs EC El Capitan W Washington Column
By choosing Yosemite Valley as reference Matthes explanations of the stages are; The first upliftduring the Paleocene epoch was slow moving, raising the Sierran crest about 1,200 m. MercedRiver drainage system evolved and by the beginning of the Miocene the broad-valley stage wasdeveloped (fig. 3A); The second uplift took place in late Miocene, adding 900 m to the crest. TheMerced River deepened the valley with 210 m and the mountain-valley stage was developed by latePliocene (fig. 3B); The third and greatest uplift during late Pliocene-early Quaternary added 1,800 mto the crest and the current heights were achieved. The Merced River deepened the valley further byabout 390 m and the canyon stage was fulfilled (fig. 3C). The canyon-stage may also give aprobable view over Yosemite Valley prior to the great Ice Age.
Matthes explanation over Sierra Nevada’s development has been challenged over the years,nevertheless, his thoughts remain and are still used to point out a possible course of event.Wahrhaftig (1965) demonstrated that the three surfaces are better explained by stepped topography,which could have developed simultaneously during late Cenozoic time primarily in the last 10 millionyears. Schaffer (1997) came to the same conclusion. However, he argued that the topography wasinitiated during the Late Cretaceous epoch and was pronounced by early Cenozoic time. Otherstudies (Christensen 1966; Huber 1981) have pointed out that the uplift began slowly andaccelerated over time. Indeed the major uplift was followed by a continuous slower moving upliftthat still proceeds with an approximate rate of 4 cm (1½ inch) per 100 years (Huber 1989).Anyhow, most present studies point out that Sierra Nevada’s long-term evolution includes two orthree phases of uplift and a succession of late Cenozoic volcanism.
2:4 Quaternary evolution and the Great Ice Age
During the last 2 million years throughout the Pleistocene until early Holocene, 10,000 years ago,parts of Sierra Nevada were repeatedly glaciated. It should be pointed out that these mountainglaciers had no connections with the great continental ice sheets to the north and east even thoughthey acted at the same time. The conception of the Quaternary evolution in Sierra Nevada is builtupon stratigraphy of glaciogene sediments as well as glacial geomorphology.
Table 1. Approximate timing of the mentioned major Quaternary glaciations of the western SierraNevada. Sherwin is one of pre-Tahoe glaciation’s advances. Modifications after Matthes 1930;Huber 1981; Ericsson & Gembert 1991 and Schaffer 1997.
Sierran glaciation Approximate age in years European correlationTioga 35,000-13,000 BP
glacial maximum about 15,000-20,000 BPLate Weichselian
Tahoe glacial maximum about 60,000-75,000 BP Mid Weichselian (?)Sherwin, pre-Tahoe older than 700,000 years Menapia (?)Pre-Tahoe initiation 2.5 Ma BP Pretiglia
It is still unknown how many glaciations the Sierras went through owing to fragmentary remnants ofolder glacial deposits. Nevertheless, extensive field studies (Whitney 1869; Matthes 1930; Huber1981; Schaffer 1997) have demonstrated that Sierra Nevada has undergone at least three majorperiods of glaciation, i. e. pre-Tahoe (oldest), Tahoe and Tioga (youngest) (Table 1 and fig. 4).
12
Fig. 4A shows how Yosemite Valley might have appeared during Sherwin, the largest pre-Tahoeglaciation, when the valley was completely filled with ice some 700,000- 800,000 years ago. Fig.4C gives a probable limelight over the Tioga glaciation which had its maximum between 15,000-20,000 years ago. Fig. 4B gives a glimpse of the valleys’ possible appearance after Sherwin,700,000 years ago. When Sherwin withdrew it probably left behind the largest, deepest, postglacialLake Yosemite ever, about 11 km long and up to 600 m deep (Matthes 1930; Schaffer 1997).
Fig. 4. Bird’s eye view of Yosemite Valley as it might have appeared during different ice ages; A. during Sherwin,the largest pre-Tahoe glaciation, 700,000-800,000 BP; B. after the Sherwin glaciation, 700,000 years ago; C. duringthe Tioga glaciation, 15,000-20,000 BP. See fig. 3 for identification of landforms. (After Schaffer 1997).
2:5.1 Glacial reshaping of the landscape
There have been several discussions over the development in Sierra Nevada and Yosemite Valley inparticular. Muir’s research from the middle of the 19th century and onward proposed that theglaciers had done most of the excavating in and about Yosemite Valley (1912, placing togetherearlier research from the19th century). Moreover, Muir suggested that glaciers once had completelycovered the Sierra Nevada to the Central Valley and beyond. Whitney (1869) claimed that theoverriding process creating Yosemite Valley was down-faulting followed by long-term streamerosion and that a glacier merely had occupied the valley, not widening it. Thus, glaciers may havetransported debris out of the valley. Matthes (1930) gave both right to a certain extent claiming thatthe area had been glaciated, however, neither as heavy or vast as Muir stated nor as little as Whitneyproposed. The glacial transformation of valleys in the central part, like Yosemite Valley andTuolumne Valley, is highly variable and make a strong contrast to the undulating higher sierras.Glacial striae and other glacial forms indicate glaciation on some parts of the higher elevations,however the transformation has not been to the same extent as the valleys (Huber 1989; Schaffer1997).
In fig. 5 present-day Yosemite Valley is illustrated (Huber 1989). Bear in mind Matthes earlierdrawings of the valley (fig. 3), especially the canyon-stage, and compare them with Hubers’. It isclear that the succeeding glaciers have widened the pre-glacial valley by removing all the weatheredmaterial as well as mass wasted rocks. The glaciers have, as Whitney proposed, been acting as acleansing agent sweeping the valley clear of all debris as well as having an erosive effect on themountain sides, leaving a U-formed valley behind. However, the glaciers were relatively powerlesswhen dealing with the massive granite monoliths in Sierra Nevada. Grand examples are El Capitan,Half Dome and Cathedral Rocks in Yosemite Valley. Matthes (1930) suggested that a large pre-
B CA
13
Tioga advance carried out most of the Quaternary glacial erosion in the lower and upper YosemiteValley, 150 m and 460 m respectively. Huber (1990) concluded that the longitudinal profile of theTuolumne Valley east of the Grand Canyon of the Tuolumne was primarily due to glacial erosion.
The U-formed valleys make a sharp contrast to the ”High Sierra”, a term coined by Whitneyregarding the higher region of the Sierra Nevada most of it laying above the timberline. It is possiblethat most of the High Sierra was glaciated during pre-Tahoe stages and partly glaciated during theTioga and Tahoe stages, however the relief features show hardly any signs of glacial erosion. Insteadthe High Sierran landscape is characterized by bare mountain areas where large scale sheeting playsa great part for the geomorphology, giving the multiple granite domes a character of undulatingrounded hills, see the back parts on fig. 5. This Paleogene denudation surface share similarities withthe “Paleic surface” in Norway thought to reflect weathering in a tropical climate (Gjessing 1967). Inthis thesis the High Sierras may at some occasions be referred to as the Paleic surface.
Fig. 5. Bird’s-eye view of present-day Yosemite Valley, selected landforms identified (Huber, 1989).
14
2:6 Form-process relations
2:6.1 Form-process relationsThis study is based upon the concept of form-process relations in landscape evolution. In order tounderstand the involved weathering and stripping processes a brief explanation over deep weatheringand glacial erosion is taken in.
2:6.2 Bedrock forms related to deep weathering processes and stripping
The climate plays a significant role in rock weathering because decomposition of rocks are favoredin warm and humid climates. High temperature as well as good supply of water, e. g. highprecipitation, favors biologic activity producing organic acids. The water transports humic acidsthrough the cover soil towards fresh bedrock where the acids leach metal bases from the silicatelattice. Hence the weathering front, the advance of weathering into fresh bedrock (Mabbutt 1961),may proceed more rapidly in the humid tropics due to the high precipitation and high temperatures inthat environment. It is suggested that the chemical weathering rate is four times higher in humidtropical conditions compared to higher latitudes or higher altitudes (Thomas 1994).
Deep weathering products such as clay rich saprolites (weathering mantles), core-stones, sheetingwith straight joint controlled bedrock surfaces, and smoothly rounded sweeping forms, are used asconnotations of chemical weathering in a former warm and tropical climate (Twidale 1993; Lidmar-Bergström et al. 1997). This can be compared with saprolites with a high gruss contents, andcoarser rock surfaces which are believed to belong to colder but yet humid climates (Twidale 1993;Lidmar-Bergström et al. 1997).
Fig. 7. Weathering of joint blocks and stages in theformation of core stones (Huber, 1989).
Solid rock
Weatheredrock
Joints
Core stones
Three surfacesweatheringcorners rounded
Two surfacesweatheringedges rounded
One surfaceweathering
The different stages of weathering in heavilyjointed granite bedrock is illustrated in fig. 7.Solid granite rock being exposed to warmthand chemical solution is susceptible to moistureattack and decomposes between joints. Thisopens up pathways for more moisture andweathering can accelerate. Weathering doesnot advance universally, but may attackdivided rock masses from all sidestransforming sharp corners to rounded edges.When the granite has transformed into saproliteit will remain with the same structure as thefresh bedrock had before weathering as longas the vegetation above is intact and protectthe rock from erosion. The transition to freshrock may also be gradual and no identifiablesurface will then separate the unaltered rockfrom the weathered mantle (Thomas, 1994).The duration of weathering in connection withthe fracture system in the bedrock are the maincauses to which core stones owe
Fresh rock, no visiblesign of rock materialweathering
15
their shape and size. The time deep weathering can act on the bedrock is also crucial for thesaprolite formed. Thus, the top parts of the saprolite is more transformed into clay than the partscloser to the bedrock.
If the vegetation cover vanish the possibilities of marine, fluvial and/or glacial erosion of the saproliteincrease. The weathering front can under these circumstances be exposed, i. e. stripped, leavingtors and core stones behind. Dismantling will inhibit further deep weathering since precipitation willdrain off the exposed rock surface, besides, the water does not contain the humic acids necessaryfor continued deep weathering. Furthermore, dry granite is very stable and upon stripping of an areathe landforms created will remain relatively unaltered because sub-aerial weathering is now thedominant factor until a vegetation cover, no matter how thin, has been reestablished and deepweathering can be revived (cf. Ollier 1991; Thomas 1994).
Saprolites are seldom found in former glaciated landscapes mainly depending on the powerfulstripping effect of glacial erosion. There have been no deep weathering since the beginning of theGreat Ice Age nor since the Tioga ice left Sierra Nevada (Hill 1975). Hence, the expansion or lackof deep weathering profiles or clay-rich saprolites in Sierra Nevada may give some answersregarding the ice’s effectiveness.
2:6.3 Conditions favorable for glacial erosion
The variation in the intensity of glacial erosion is highly depending on the thickness of the ice, timeand rate of glacial movement and the ground surface. Whether the ice has reached pressure meltingpoint leading to a more erodable ice, or if the ice is frozen to the base with a less erodable ice as aresult. The nature of the ground surface; topography; rock composition; jointing; chemicaltransformation of the rock; permeability as well as the composition of the regolith are all main factorsof the surface beneath the glacier. Other erodable factors are the shape, abundance and hardness ofthe rock fragments contained in the ice at the base of the glacier. Thus, glacial erosion is moreeffective in the beginning of glaciation when the ground is softer and covered with saprolite. (Sugden& John 1976).
2:6.4 Bedrock forms related to glacial erosion
The most prominent and eye-catching feature of glacial erosion in hard rock is abrasion and pluckingof fractured and loosened rock fragments leading to the creation of roche moutonnées. However,glacial striae is of more vital importance since the loosened debris from a highly fractured rock havebeen detached for glacial transport and used as abrasive tools.
Sub-glacial melt water is a great manufacturer of softly rounded concave or trough shaped crosssections such as p-forms, longitudinal furrows and flutes (Dahl 1965; Benn & Evans 1998).However, the erosive power from sub-glacial melt water can above all be seen in the outstandingpotholes.
16
3 Methods
3:1 Demarcation of study areas
Field areas within Sierra Nevada were chosen from legal accessibility such as; National Parks:Yosemite, Sequoia and Kings Canyon (fig. 6); National Forests: Sierra, Sequoia and Inyo; as wellas several road cuttings near state highways; 120, 140, 41, 49, 180, 198, J22, 155, 178, 190. InYosemite National Park the areas Yosemite Valley; Tamarack; Foresta; and Hetch Hetchy areaswere chosen for a more detailed survey (fig. 6). In Sequoia and Kings Canyon National Parks thesurvey was defined to Kings Canyon, Generals Highway and Moro Rock. In Sierra NationalForest the study was restricted to road cuttings while the Kern River Valley was studied moreclosely in Sequoia National Forest. Big Pine lava fields, an area between the two towns Big Pineand Independence, was studied in Inyo National Forest.
Fig. 6. Location of the principal study areas in California, USA.
3:2 Maps and topographic profiles
All data regarding maps are based upon United States Geological Survey, U.S.G.S., 15 inchquadrangle topographic maps at a scale of 1:24,000 and 1:62,500 with contour intervals of 40feet and 80 feet, respectively. The maps used in this study are: Wawona, Mariposa Grove,Kinsley, El Portal, El Capitan, Half Dome, Tamarack Flat, Yosemite Falls, Tenaya Lake, LakeEleanor, Hetch Hetchy Reservoir, Muir Grove, Giant Forest and Lodgepole.
Large scale maps were created to give a more lucid picture over the study areas. While convertingelevation data from feet to meter together with the subsequent rounding accurate elevation mighthave been lost. However, the redrawn maps in this study will by no means claim to be appropriatefor further work, they are just a pointer over the research areas locality. Thus, it is not advisable toreconvert the figures for use with the topographic maps.
In order to achieve an appropriate picture over Yosemite Valley’s change from U-valley shape informer glaciated areas to V-valley shape in areas with minor or none glacial influence fourteencross sections over Merced River canyon were drawn at different locations. The distance on theprofiles was set to five kilometers and notes in feet were taken down regarding the elevation above
17
sea level on every 100 meter. Hence conversions from feet to meter were calculated upon whichthe profiles were drawn.
To elucidate an area in the High Sierras that had been heavily glaciated during the Pleistoceneglaciation topographical profiles, 10 km long, as well as a map over Tioga Pass Quadrangle weredrawn. The procedure followed the above description.
While constructing the profiles the surface elevation of lakes and rivers has been used. Larger lakes(fig. 8-9 p. 19-20), as well as areas with meandering rivers (fig. 10-12 p. 21-23), will then appearas flat surfaces. This may be in bad congruity with reality regarding the lakes, however it was asimple way to illustrate the valley glaciers sculpturing effect.
3:3 Field mapping
The intention was to study different morphological regions within the Sierra Nevada i. e. to evaluatethe role of different processes for the development of major valleys, top surfaces and batholithmargins. With respect to the Yosemite Valley, it was chosen for a more detailed study of formervalley glaciers work on the granite domes. Upon it, Hetch Hetchy and Kings Canyon were used ascomparative sites.
The original intention was also to make a comparison of landforms in former heavy glaciated areason higher elevations. Unfortunately this could not be carried out owing to the extreme bad weatherwhere three meters of snow covered the ground on elevations above 2,000 m. Instead Big Pine inMono Valley, east of Sierra Nevada, was studied. Hence, a comparison of deep weatheredlandforms in an area exposed to volcanism could be accomplished.
The eight main areas with surroundings were mapped during 15 field days. Yosemite Valley, HetchHetchy and Foresta pretty thoroughly while the other locations were mapped more perspicuously.See paragraph 3:4, Manual for field mapping, regarding mapped landforms. Small, 0.1-10 m, tomedium scale, 10–100 m, bedrock forms were classified either as glacial or non-glacial. The non-glacial forms were subdivided according to process of formation, e. g. surface weathering, etchforms or water induced forms. The glacial surfaces were used as a time reference and the otherforms were classified as older or younger than the glacial surface.
In pursuit of weathered material, e. g. decomposed granite, road cuttings and primary rock witheasy accessibility were examined. Road cuttings, core stones and other geomorphologicphenomena were documented by several photographs and drawings.
The found saprolites have just been visually inspected during field study. In order to facilitate datingof the weathered material a more thorough grain analysis is recommended. Upon it a comparisonregarding the composition of other saprolites produced at different time periods and/or environmentsmay be made.
A following-up field study regarding heavy glaciated areas on higher elevations is also recommendedin order to get a full picture over glacial vs. deep weathered influence in the area.
18
3:4 Manual for field mapping
During field work a modification of a manual for mapping landforms worked out by Lidmar-Bergström 1986, revised by Lidmar-Bergström and Olvmo 1995, was used (unpublishedmanuscript).
Following forms were mapped during field survey:Substratum:
• Primary rock• Glacial till
Bedrock-forms:• Flat rock face• Rock hill
Primary forms - caused by rock type or cleavage:• Sheeting - pressure release fractures• Dome formations: cliff domes, dome formed rock hills• Block formations: cliff mounds• Columnar formations: (pseudo)rauk, tor(mounting)
Secondary forms - caused by weathering, glacial erosion or other erosion:• Sharp edges due to frost action or other cracking• Weathering induced forms:
• edge rounded convex forms• rounded, gently sweeping forms• bulging, bending outwards, flared slope• mushroom forms
• Glacial forms:• roche moutonnées
• Wind polish, direction• Abrasion by running water/waves, potholes, polished surfaces, p-forms
Deep weathering:• Gruss weathering, with core-stones• Clay weathering, with core-stones
Surface weathering - above all exfoliation with rough surfaces:• Exfoliation, small-scale => large-scale• Spherical weathering, block weathering• Enlarged joints• Differentiated weathering:
• weathered path-ways, depressions or dikes• rough pitted surfaces, stand-up quartz and feldspar crystals
• Alveoli weathering:• tafoni, weathering hollows, tubular hollows• handle and knob forms
• Frost weathering and transport of sharp edged blocks:• short transfer or clitter, widespread blocks• talus• sheeting-pressure release fractures
19
4 Results
The presentation of the results follow the evaluated different landform zones. The zones range fromelevated areas with heavy glaciation, Tioga Pass, to areas with local glaciation, Yosemite, KingsCanyon and Sequoia National Parks, and finally leading to areas lacking glacial influence, Kern RiverValley and Big Pine.
4:1 Topographical profiles over Tioga Pass Quadrangle
The locations of the profiles over Tioga Pass Quadrangle are presented in fig. 9. The interpretationof the topographical profiles was made from larger profiles in juxtaposition with accuratetopographical maps. Hence, differences in height and distance between the profiles appeared moreclearly than presented in this study.
Mount Conness
2800
3100
3400
3700
4000
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000
distance, in meter
ele
va
tio
n,
in m
ete
r
asl
NW SE
She
ep P
eake
Mt C
onne
ss
Roosevelt Lake
trough v
alle
y
cirques
cirque
Roosevelt
2800
3100
3400
3700
4000
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000
distance, in meter
ele
va
tio
n,
in m
ete
r
asl
SSWNNE
Roosevelt Lake
Rag
ged
Pea
k
I
cirques
I
cirque
cirqueConnessCreek
Fig. 8. Profiles covering an area north of Tuolumne Meadows, Sierra Nevada, showing how glaciers havetransformed the landscape, see fig. 9 for location. ”Mount Conness” demonstrates how glaciers on differentelevations have had a disparate influence on the landscape. ”Roosevelt” emphasize one valley glaciers lengthwithin an area, namely Roosevelt Lake, which is a former ice lake. The riverbed of Conness Creek, a recipient ofglacial meltwater, can be noticed by the dip in the profile’s 6,500 m distance.
20
Fig. 9. Location of drawn profiles on Tioga Pass Quadrangle map. The straight line shows the ”Roosevelt” profileand the dotted line shows the ”Mount Conness” profile. Major glacial incisions are marked.
Selected landforms identified: SP Sheep Peake MC Mount Conness RP Ragged Peak LD Lembert Dome
The area has been heavily glaciated where glaciers have worked their way little by little into themountain sides, the last ice leaving the area 10,000 years ago. What is striking is the disparateinfluence but yet congruent of the glaciers incisions. By following the contour intervals on the mapone can tell how ancient valley glaciers have transformed the area. The general trend seem to bebasins all along a northwestern-southeastern line which indicates an ice flowing from northeasttowards south-west. Several cirque glaciers on the northeastern hillsides occupy former glacial
RPRP
LD
21
basins. There are about 70 tiny glaciers in the sierra today, almost all being cirques’, living whollywithin basins carved by the giant valley glaciers of the Pleistocene glaciation. However, these tinyglaciers are not, as one might believe, remnants of glaciers from the Great Ice Age but remnants fromglaciers that started to grow during the Little Ice Age, 1500-1850 (Hill 1975).
The profile Mount Conness (fig. 8) visualize how several glaciers on different elevations havetransformed the landscape. The incision in the landscape has varied which may be depending on thebedrock’s mineral contents, the glacier’s size, and the elevation it was working on. The profile hascontact with several ancient glacier basins of which the valley glacier by Roosevelt Lake has carvedits way into the mountainside while more southeastern glaciers have made minor incisions on higherelevations. One possible cause could be that the western side of the line received the main portion ofprecipitation fallen while the eastern side lay in rain shadow. Hence, northeastern laying glaciers werejust as cold but had less snow and therefore might have been frozen fast to the ground leading tominor incisions.
The profile Roosevelt (fig. 8) shows the extension of a former valley glacier which basin is now filledwith the water of Roosevelt Lake. The glacial sculpturing of the landscape has been disparate andoccurred during different glaciations. One glacier has deepened the canyon immediately south ofRoosevelt Lake. Conness Creek’s riverbed, a recipient of glacial melt water, can be noticed by thedip in the profile’s 6,500 m distance. Following the profile from south to north reveals how an icehas carved its way around Ragged Peak. North of Ragged Peak the ice has filled the lowland as highas the base of the vertical cliffs. Conness Creek is the main recipient of melt water from higherelevations, the creek’s deep incision may indicate a pre-glacial riverbed. Some of the dip in thelandscape may also be due to powerful tapping of melt water from former cirques, now ice-dammedlakes Young Lakes, northeast of Ragged Peak south of Roosevelt Lake.
4:2 Topographical profiles of Yosemite Valley
Fig. 10. Location of drawn cross sections/profiles in Yosemite Valley, the name of each profile is placed by thecorresponding line.
22
The locations of the following profiles are presented in fig 10. The profiles in fig. 11 and 12 revealshow glacier tongues via two valley-beds, Tenaya Creek and Merced River, worked their way downand into Yosemite Valley (Tenaya Canyon, Half Dome and Sierra Point). Royal Arches showswhere the two tounges joined and how they step by step transformed the valley to U-valley shape(Glacier Point, Eagle, Sentinel, El Capitan and Bridalveil Meadow). The ice seems to haveceased and become less erosive by Rainbow View, and Merced River’s incision could once againdetermine the shape of the valley floor, V-shape (Turtleback Dome, Elephant Rock, Big Meadowand Foresta). The westward tilting and Merced Rivers incision may be illustrated by following theelevations from east to west.
Tenaya Canyon
750
1250
1750
2250
2750
0 1000 2000 3000 4000 5000
distance, in meters
ele
va
tio
n,
in m
ete
rs SSE NNW
Tena
ya
Cre
ek
l
asl
Half Dome
750
1250
1750
2250
2750
0 1000 2000 3000 4000 5000
distance, in meters
ele
va
tio
n,
in m
ete
rs SE NW
Hal
f Dom
e
Mer
ced
Riv
er
asl
l
Sierra Point
750
1250
1750
2250
2750
0 1000 2000 3000 4000 5000
distance, in meters
ele
va
tio
n,
in m
ete
rs SW NE
Mer
ced
Riv
er Sie
rra
Poi
nt
asl
l
Royal Arches
750
1250
1750
2250
2750
0 1000 2000 3000 4000 5000
distance, in meters
ele
va
tio
n,
in m
ete
rs
S N
Merced River
asl
l l
Glacier Point
750
1250
1750
2250
2750
0 1000 2000 3000 4000 5000
distance, in meters
ele
va
tio
n,
in m
ete
rs
Gla
cier
P
oint
SSW NNE
Mer
ced
Riv
er
Nor
th
Dom
e
l
asl
Eagle
750
1250
1750
2250
2750
0 1000 2000 3000 4000 5000
distance, in meters
ele
va
tio
n,
in m
ete
rs SSE NNW
Mer
ced
Riv
er
asl
Fig. 11. Cross sections of Yosemite Valley, Sierra Nevada, see fig. 10 for location. The cross sections are drawnfrom east, Tenaya Canyon, where a glacier tongue came down into the valley, to Eagle, where the valley isperfectly U-shaped. Crossing l in the profile indicates Merced River’s location. Furthermore, by looking at theelevation the mountains gentle slope westward can also be notified.
23
Sentinel
750
1250
1750
2250
2750
0 1000 2000 3000 4000 5000
distance, in meters
ele
va
tio
n,
in m
ete
rs SE NW
Mer
ced
Riv
er
asl
l
El Capitan
750
1250
1750
2250
2750
0 1000 2000 3000 4000 5000
distance, in meters
ele
va
tio
n,
in m
ete
rs
S N
Mer
ced
Riv
er
Cat
hedr
al R
ocks
El C
apita
n
El Capitan Meadowl
asl
Bridalveil Meadow
750
1250
1750
2250
2750
0 1000 2000 3000 4000 5000
distance, in meters
ele
va
tio
n,
in m
ete
rs S N
Mer
ced
Riv
er
Bridalveil Meadow
Cro
cker
P
oint
asl
l
Rainbow View
750
1250
1750
2250
2750
0 1000 2000 3000 4000 5000
distance, in meters
ele
va
tio
n,
in m
ete
rs SW NE
Mer
ced
Riv
er
Rai
nbow
V
iew
l
asl
Turtleback Dome
750
1250
1750
2250
2750
0 1000 2000 3000 4000 5000
distance, in meters
ele
va
tio
n,
in m
ete
rs SSE NNW
Mer
ced
Riv
er
Turtl
ebac
k D
ome
The
Cas
cade
s
asl
l
Elephant Rock
750
1250
1750
2250
2750
0 1000 2000 3000 4000 5000
distance, in meters
ele
va
tio
n,
in m
ete
rs
Mer
ced
Riv
erNWSE
Ele
phan
t Roc
k
lasl
Big Meadow
750
1250
1750
2250
2750
0 1000 2000 3000 4000 5000
distance, in meters
ele
va
tio
n,
in m
ete
rs ESE WNW
Mer
ced
Riv
er
Big Meadow
lasl
Foresta
750
1250
1750
2250
2750
0 1000 2000 3000 4000 5000
distance, in meters
ele
va
tio
n,
in m
ete
rs
SSE NNW
Mer
ced
Riv
er
Fore
sta
lasl
Fig. 12. Cross sections of Yosemite Valley, see fig. 10 for location. The cross sections continue from east,visualizing the canyon’s change from U-valley shape in former glaciated areas, Sentinel, to a more V-valley shapein areas with minor or none recent glacial influence further west, Foresta. Furthermore, by looking at the elevationthe mountains gentle slope westward can also be notified. Crossing l in the profile indicates Merced River’slocation.
24
Several glaciations have worked their way through Yosemite Valley eroding the valley sides fillingthe floor with glaciofluvial debris. The maximum extent of the Pleistocene ice cover (fig. 13) filled thevalley to its brim reaching west of El Portal (Matthes 1930) further than fig. 10 goes. This is also thepoint where Merced River starts to meander more readily, indicating a river free to make its ownincision in the bedrock i. e. not depending on an earlier over-deepening by glaciers.
The elevation on the profiles (fig. 11 & 12) reveal some of the relative relief and the westward tilt. Acomparison between Tuolumne River and the inferred average trend of pre-glacial Merced River infig. 13 will better illustrate the latter event. The longitudinal profile of Merced River (fig.13) proposethat Yosemite Valley is almost entirely the result of earlier excavating glaciers. The valley floor standin direct connection with the sudden steps the hanging valleys Vernal and Nevada Falls make up aswell as Pywiack Cascade by Tenaya Creek. This should be compared with Tuolumne River which inspite of its recent heavy glaciation, Tioga, proceed with a more gradual climb to the higher Sierras.
The last glaciation, Tioga, was rather thin and did not reach further than Bridalveil Meadow during itsmaximum extent about 15,000 years ago (Huber 1989, 1990). While comparing the profiles with themaps and field studies the glaciers greatest erosive power has been between Royal Arches andBridalveil Meadow (fig. 10). The glacial erosion west of Bridalveil Meadow ceased gradually untilForesta (fig. 10) where an almost perfect V-shaped canyon appear (limit of Tioga glaciation fig. 13).The valley is once again widened west of Arch Rock to form a meadow at El Portal (fig.13).
Yosemite Valley is cut 750-1,050 m below the older upland surface, the contrast between the two isparticularly overwhelming by El Capitan which can be gathered by the steep cliff shown by thenamesake profile (figs. 10 and 12). While standing by the rim of the valley on the top surface theview of the undulating Paleic top surface is revealed.
Fig. 13. Longitudinal profile of Merced River and its tributary Tenaya Creek, Tuolumne River is outlined forcomparison. All profiles follow stream courses but ignore minor meanders. The Tuolumne plot is superimposed tointersect at the 1,200-m elevation which is the approximate elevation of the Merced-Tenaya rivers junction at thehead of Yosemite Valley. The dotted line indicates the bedrock basin in Yosemite Valley. The dashed lineindicates the inferred average trend of the pre-glacial Merced River without the Yosemite Valley basin, althoughsome excavation below the line probably resulted from stream erosion prior to glaciation. (Modification afterHuber, 1990).
25
4:3 Yosemite National Park – field mapping
Fig. 14. Geomorphologic map over Yosemite Valley and vicinity.
26
While reading the result of field mapping it is advisable to look at the field maps over YosemiteValley and vicinity (fig. 14) and over the Hetch Hetchy area (fig. 26 p. 32) which shows thewhereabouts of localized forms. A follow-up on U.S.G.S. 15 inch quadrangle topographical maps inscale 1:24,000 covering the study areas may also be recommended.
The field study over Yosemite National Park is divided into five parts namely Yosemite Valley,Tamarack, Foresta, Road sections and Hetch Hetchy (fig. 6 p. 16).
4:3.1 Yosemite Valley
Yosemite Valley is geomorphologically characterized by an over deepened trough valley with sheercliff walls. Truncated valley spurs bear evidence of a massive transformation and straightening byformer glaciers which have cleansed the valley from weathered and spalled material. The adjoiningtop surfaces are structurally controlled exfoliation domes and selective erosion has determined thepaths of glacier tongues forming hanging valleys from which waterfalls now cascade.
4:3.2 Yosemite Valley - weathering forms
The east side of the valley by Mirror Lake and Tenaya Creek show no or minor signs of deepweathered profiles, still some not to deep rounded joints are noticed. The area surrounding the trailfrom Mirror Lake to Happy Isles, west of Glacier Point, is framed by an extensive boulder field withblocks in various sizes ranging from 0.3-3 m in diameter. Some of the larger boulders look likeoverthrown tor-like pillars with rounded edges and corestone likecompartments. Others are erratics laying scatteredon the valley floor whereas the main part are rocksspalled off from the walls. Quite a few of the largerblocks show thin exfoliation as well as differentiatedpostglacial weathering and some have p-forms on thetop surface.
While following the trail from Happy Isles leading toVernal and Nevada Falls (fig. 14) the bases andsides of the cliff walls are covered with large boulderscreating a talus reaching Merced River. Higher upcloser to Vernal Falls the almost vertical wall showsigns of deep weathered joints. Between Vernal andNevada Falls several cliffs have edge roundedconvex forms. Below the waterfalls glacio-fluvial p-forms and potholes were found. The cliff walls in thevalley are sheer but where fractured the corners aresharp showing signs of extensive spalling due tofreeze-thaw actions. While trailing by the rock wallsboulders in various sizes make up magnificent smalland large talus where the Rockslides, west of ElCapitan, is the most powerful one.
Fig. 15. Bridalveil Falls, a hanging valley abovesheer cliff walls. Merced River with glaciofluvialdebris creating graded bedding in the foreground.
27
4:3.3 Yosemite Valley - glacial forms
Apart from the valleys U-shape glacial influence is evident when observing all the hanging valleys, ofwhich Bridalveil Falls is the most magnificent (fig. 15). Furthermore, Tioga-age terminal moraine isexposed in a road cut west of Bridalveil Meadow. Areas in and about Tenaya Creek and MercedRiver consist of glacio-fluvial debris and large erratics lay in both creeks. The river bank byBridalveil Meadow has a characteristic graded bedding (fig. 15) and cobblestones are seensoutheast of El Capitan. Owing to the extensive spalling glacial evidence on the bedrock are hard tofind but some glacial striae on the base of the wall northeast of Mirror Lake indicates glaciation.However, Tioga, was rather thin and did not reach high up on the walls. Personal communicationwith climbers gave more information about glacial polish on the apron of Glacier Point. Even soglacial marks are scarce on the upper parts of the valley’s walls which implies active slopeprocesses.
Glacial erosion has played a major role in remodeling Yosemite Valley, however the top parts of thevalley as well as the Paleic surface show minor signs of deep weathering. Hence, the deepweathered connotations are older than the glacial ones.
4:3.4 Tamarack
Northeast of Foresta and Big Meadow is a 0.5 km2 sitewith pronounced sheeting stooping east towards TamarackCreek. Geomorphologically the whole area showslandforms induced by deep weathering on a stripped etchsurface (fig. 16) on a slightly slanted plateau. No glacialforms are to be seen.
4:3.5 Tamarack - weathering forms
Rectangular blocks formed by intersecting joints cover asmall part of the site, joints are rinsed exposing edgerounded convex forms. Displacement along sheet fracturehas taken place on other parts of the site (fig. 17). The areacloser to the rim of Yosemite valley is more grussified, thegranite being decomposed and surfaces covered with a 1-5cm thick exfoliation. The latter area show minor pitting,saucer shaped depressions weathering hollows, severalmushroom weathering landforms (fig. 18) and tafoni. Thefound tafoni varies in size from 2-40 cm in diameter and 1-20 cm deep.
Fig. 16. Stripped etch surface showing deepweathered joint pattern system, TamarackCreek, elevation 1,500 m.
28
Fig. 17. Sheet fracture on edge convex rounded granite block formations. TamarackCreek, elevation 1,500 m.
Former weathering pathways have created shallow gutters inthe bedrock. The smallest ones being 0.5 m wide 0.1 mdeep, 10 m long respectively. A larger gutter, 2 m wide, 2-3dm deep, 13 m long, may be a rock levee initiation asexplained by Twidale (1993) in terms of the bedrock surfaceclose to the channel being exposed. The exposed channel isdry and sheds water, whereas weathering can continue onthe soil covered surroundings.
Tamarack was glaciated during Sherwin according toMatthes 1930 and Huber 1989. However, there are noglacial signs just deep weathered landforms.
4:3.6 Foresta
Foresta is a c. 4 km2 situated southwest of Tamarack. It is geomorphologically characterized by astripped etch surface dominated by a joint system running NNE-SSW. The granite is fine to mediumgrained but very dry and rotten, and it can be broken apart very easily while being held in hand.
4:3.7 Foresta - weathering forms
North of the study site Foresta (fig. 14) between Tamarack and Big Oak Flat Road an abrupt hillslope reveals a partly stripped c. 2.5 km2 large area with a multitude of edge rounded core stonescovered with a relatively thin vegetation cover, 0.5 m in places (fig. 19). Glacial till can be seen in thelower parts of the slope.
Fig. 19. Schematic profile over Foresta and surroundings suggesting a dome formed hill weathered within jointsfollowed by a stripped etch surface partially reshaped by glacial erosion and slope processes. Core stones andtors are seen resting on the slope and on the etch surface. Not to scale.
Partly stripped slopeetch surface
Fig. 18. Mushroom weathering, Tamarack Creek.
Dome formed hill,weathered within joints
TorsCorestones
Foresta
Weathering zone withincipient corestones
Glacial till in places
29
Foresta is dominated by numerous core stones and tors in various sizes, ranging from 0.5-5 m high,and tor-like pillars with a maximum height of 12 m covering the ground (fig. 20a). Quite a few of thetors are bedrock-rooted and have depressions on the surface, indicating pathways for water anddeep weathering (fig. 20b), others have weathering hollows or tafoni closer to the ground (fig.21a,b,c).
Fig 20, a, b. Stripped tors, exposed to deep weathering in Foresta. b) with a 0.6 m weathering depression.Elevation 1,300 m.
Some core stones make up inselberg-boulders or nubbins, about 20-50 m in diameter, 10-20 mhigh. The exposed boulders are often round and smooth but rough pitted surfaces are also verycommon. Indeed, pitting can be seen on almost every surface in all directions. When tapped onsome rocks sound fresh, others sound hollow indicating continued weathering behind a thin layer ofexfoliation. Stepwise and flake exfoliation is very common. Connotations of differentiated weatheringwere above all albeit veins protruding 1-3 cm, they are easily loosened but do not crumble up as thegranite does.
Fig. 21 a, b, c. Stripped tor-like pillar exposed to deep weathering 6 m high, 20 m wide facing west (a). East side ofthe tor only 3 m high with weathering hollow facing northwest (b). Close-up picture of the 0.6 meter deepweathering hollow (c). Foresta, elevation 1,300 m.
A mature well developed red clay-rich saprolite is found in a section east of Big Meadow. Thesection is moist and core stones in situ show signs of advanced stages of weathering having red clay
a b 1 m
b
a
c
30
in fractures and on the surface. Following the surface of the unraveled core stones a thin, 2-6 cmdeep, but fractured layer of flaky polygonal, mural, exfoliation weathering is conspicuous. Muralweathering suggests that the exposure of the boulder exposed an external slightly altered rock shellto sub-aerial environments which led to solutional loss and shrinkage (Thomas 1994).
4:3.8 Foresta - glacial forms
Foresta was last glaciated during Sherwin, 800,000 BP, (Matthes 1930; Huber 1989). When the icewithdrew Big Meadow was a lake which has choked-up over the succeeding years to its presentcondition of a marsh land. No connotation of glaciation or glacio-fluvial debris were to be foundapart from glacial till on the bottom of the meadow. The till is underlain by silty, sandy material at 3 mdepth, personal communication with a local person.
Both Tamarack and Foresta share landform similarities According to Matthes (1930) and Huber(1989) the areas were glaciated during Sherwin glaciation. Whether Matthes and Huber are right intheir assumptions is hard to judge from this study since exposed landforms have been subjected topost- glacial weathering. Thus, all eventual glacial marks, striaes etc. have weathered away duringlater Millennia. However, both stripped etch surfaces reveal deep weathering within joints.
4:3.9 Road sections in Yosemite National Park
Several road cuttings within the national park and its surroundings present deeply weathered profiles,5-15 m high, consisting of clay as well as coarser material with incipient core stones or core stonesin situ. The weathered material goes up to the surface but the depths of the profiles are hard todetermine since they go further down than the settings do. Some localities are overlain by glacial till.Within the west side of the park the most developed profiles can be seen on higher elevations, above1,500 m, while further west, beyond the parks limits, deep weathered profiles are found on lowerelevations depending on the westward tilt.
Road 41, Wawona Road, Big Oak Flat Road and Hetch Hetchy Road (fig. 14 p. 25) gives plenty ofbeautiful deep weathered examples (figs. 21- 24). The weathered granite is mainly medium grainedwith blond to light red color.
The core stones range from 0.3-2.5 m indiameter and on most sites they lay in situimbedded in gruss. Some road cuttingsshow settings with core stones in situ andtor-like pillars in the fore-ground. One ofthe sites is situated west of El Portal, rightby Merced River, about 500 m a.s.l. Herea narrow granite pluton has intruded in themetamorphic layer. The exposed granite isvery rotten, decomposed into gruss(Wagner, 1991) and core stones in situare seen like protruding rock eggs, 0.5-1.5 m in diameter (fig. 23). According toMatthes (1930) and Huber (1989) noglacier has ever reached this far west.
Fig. 21. Deep weathering along joints and incipient corestones. Core stones are also seen resting on the top surface.Road cut at Wawona Road, elevation 1,650 m. James Deanproviding scale.
31
Different types of deep weathered core stones are seen on several locations. A perfect example ofspheroidal weathering which via exfoliation show incipient unraveling of core stones can be seen byBig Meadow overlook (fig. 24). Mariposa Grove, the southern part of the park, exhibit several deepweathered convex edge rounded granite blocks (fig. 25).Owing to the snow cover glacial marks could not be seen apart from a 0.6 m pothole on a rock eastof Crane Flat, however this rock showed deep weathered features as well. A few road cuttingsexposed till and scattered erratics lay above or close to examined road cuttings.
Fig. 22. Deep weathered and protruding core stones in Fig. 23. Deep weathered and protruding core stones situ. Big Oak Flat Road, elevation 1,650 m. in situ. West of El Portal, elevation 600 m.
Fig. 24. Spheroidal weathering around core stone, 3 m Fig. 25. Deep weathered convex edge roundedwide 2 m high. Road cut at Big Meadow overlook, granite blocks, 3.5m wide 3m high. MariposaBig Oak Flat Road, elevation 1,650 m. Grove, elevation 1,900 m.
4:4 Hetch Hetchy
The geomorphological map over Hetch Hetchy and surroundings is presented in fig. 26. The HetchHetchy area resemble Yosemite Valley’s geomorphology in that it is an overdeepened canyon withsheer cliff walls. Some truncated valley spurs and hanging valleys with cascading waterfalls indicatethe valleys reshaping of a fluvial valley by former valley glaciers. The structural control of theadjoining top surface is also very obvious in this area. Since the valley is dammed, acting as reservoirfor San Francisco, it is impossible to judge the valleys complete appearance.
Immediately south of the dam’s head gate is a large roche moutonnée with its pluckside facing west.Despite being plucked the whole side show signs of deep weathered softly rounded edges where
32
most joints are deep and enlarged. The majority of the joints are between 0.1-0.9 m wide, 2-4 mdeep, and 4-10 m long. One incipient flared slope is facing southwest. The major part of the surfaceon the roche is thinly exfoliated and the rock sounds hollow where exfoliation is 0.5-3 cm. Pitting isseen on almost all surfaces as well as differential weathering and weathering hollows.On the top surface a large pothole, 1.5-2 m in diameter, and some minor pothole initiations can beseen. Glacial striae from northeast is mapped on several places. Several boulders, 0.5-1.5 m indiameter, lay scattered on the top surface. A lot of boulders of the local lithology have been movedby the ice southwest of the roche. Parts of the roche show signs of p-forms as if glacial melt waterhas formed it.
Fig. 26. Geomorphologic map covering studied areas in Hetch Hetchy and surroundings.
4:4.1 Weathering forms
Many deep weathered edge rounded convex formsoccur in between sheeting joints (fig. 27).
Differential weathering in the form of a diorite vein, 0.3m wide about 15 m long, protrudes 0.06 m on the
Fig. 27. Laminated weathering in joints, betweenTueeulala and Wanama Falls, elevation 1,200 m.
0.5 m
33
bedrock stooping towards the dam. Glacial striae facingsouthwest were found next to the vein. The rock surfaceis coherent and the vein protruding too much to beconsidered post-glacial weathering.
In the gap between Kolana Rock andHetch Hetchy Dome the evidence of thethree major uplifts with succeedingerosion and sheeting is outstanding (fig.28). The relative relief between HetchHetchy Reservoir and Hetchy Dome andKolana Rock is about 730 and 620 meterrespectively.
Vertical sheeting due to offloading areseen on both valley walls. On the south-west side of the canyon a deeply fracturedvertical joint controlled cliff is perceptible,some talus cover the base of the rock.
4:4.2 Glacial forms
By following the trail north of Hetch Hetchy Reservoir a wall with rounded edge surfaces is revealed,some plucking or spalling is noticeable. The granite is medium grained with a pink color indicating amore recent decomposition than in Yosemite Valley where the granite is having a more pale color.Moreover, decomposition has not been as heavy as in Yosemite Valley in that the granite is morecoherent and does not crumble as easily as in Yosemite Valley. At the base of the rock wall parts ofthe bedrock is overlain by glacial till and on some sites gruss overlay till. The latter case might bedownfall due to mass wasting.
West of Tueeulala Falls a couple of roche mouttonnéesfollowing sheeting planes are documented. One rochehas thin exfoliation on top, no glacial striae is seen.Small erratics pushed by the ice lay beside the roche.Talus cover the bedrock above the roches. Crescenticgouge marks in N-S direction are found south ofTueeulala Falls. Glacio-fluvial connotations such aspotholes in various sizes and pothole initiations wereseen below and east of Wanama Falls. By the sameplace large erratics were placed upon pedestal rocks.
4:4:3 Roadside geomorphology between HetchHetchy reservoir and Poopenaut
Upon leaving the Reservoir the road leading to theentrance is steep and narrow. The landscape form astepwise appearance, divided into levels with steepslopes followed by plateaus. The steps are at this
Fig. 28. Drawing over the gap between Kolana Rock andHetchy Dome. The gap gives evidence of the three major upliftsand the subsequent erosion and sheeting. The border betweeneach uplift is seen as darker lines on both domes.
Hetchy Dome
Hetch Hetchy Reservoir
Kolana Rock
N
Fig. 29. Corestone with incipient mushroomweathering, Hetch Hetchy, elevation 1,200 m.
34
location probably a visible evidence of the synergybetween uplift, sheeting, deep weathering and erosionindicating the evolution of different valley generations.At other places in the Sierras this isexplained as stepped topography (Wahrhaftig1965) considering the contrasted weathering ofwet and dry sites.
Exposed rock domes laying closer to themountainside show a weathered surfacedocumented by enlarged weathered joints. Closerto the rim of the valley glacial erosion is moreobvious in that a few roche moutonnées with asouthwest facing pluck side are exposed onsmooth polished granite plateaus. Quite often arethe joints on the plateaus slightly enlarged andhave edge rounded convex forms. The joints havemost likely been washed free of weatheredmaterial by glacio-fluvial meltwater.
The exposed road cuttings have no continuity, atsome sites large erratics lay on top of glacial till,next gruss weathering with core stones in situ willbe exposed, only to be followed by glacial till. Aswith the southern part of Yosemite the frequencyof gruss and core stones in situ increases withaltitude e. g. a couple of sites with spheroidal andmushroom weathering came on (fig. 29).Erratics invarious sizes lay scattered on both glacial till and the plateaus.
The Poopenaut area (fig. 26) is an undulating plateau which is abruptly cut off by the steep valleyside. The area is a salad bowl of all kinds of geomorphic forms. Displacement along sheetingfractures has taken place, small and large erratics lay on sheer polished granite, glaciofluvial meltwater has created p-forms and tiny pothole initiations. The rock surfaces expose small scaleweathering forms such as thin layered exfoliation, pitting, weathering pits of various sizes, and severalhandle and knob forms in a more fine grained composition. While standing on the rim of the valley avertically joint controlled wall (fig. 30), deep weathered within joints, was unveiled.
Despite the fact that the area has been recently glaciated by Tioga (Huber, 1989, 1990) the deepweathered surfaces are definitely in favor.
4:5 Kings Canyon & Sequoia National Parks
While approaching Kings Canyon driving through the foothills on road 180 (fig. 31) tors seem topop up everywhere. Closer to the park, on higher elevations, road cuttings present about the same
Fig. 30. Vertically joint controlled wall, deep weatheredwithin joints, Poopenaut, elevation 1,300m.
35
deep weathered profiles, 5-15 m, with incipient core stones or core stones in situ seen in Yosemite.Road cuttings by the road leading to Kings Canyon have a thin layer of exfoliated rock with a sandyappearance covering fresh rock. The core stones range from 0.3-2.5 m in diameter and the granite ismainly medium grained with a blond to pale red color. For locations of mapped locations see fig. 31.
4:5.1 Kings Canyon
At first sight the valley is V-shaped anddoes not look glaciated at all. However, itgoes through different faces and further eastit forms a soft U-shape, resembling TenayaCanyon, east of Yosemite Valley. Whilelooking on the mountain crest theexcavation of quite a few cirque glaciers areexposed, mainly on the north side of thewall. The mountain crest in the west side ofthe canyon show sharp corners at placesdue to extensive spalling, further east thecorners are more rounded. In road cuttingsby the foot of the rock wall glacial till wereexposed in one cutting, to be followed bygruss and core stones in situ in the next.Erratics of different sizes, 0.5-5 m indiameter are to be seen scattered at variouslocations, even on top of deep weatheredsites. The riverbed consist of glacio-fluvialdebris.
4:5.2 Generals Highway
Generals Highway runs between General Grant’s treein Kings Canyon Natl. Pk. to General Sherman´s treein Sequoia Natl. Pk. and beyond (fig. 31). At thejunction between Kings Canyon and Sequia Natl.Parks a dome formed rock hill was examined (A, fig.31). The soil contains a lot of gruss and cuttingsbelonging to the hill unveil incipient core stones andcore stones in situ. Quite a few core stones layscattered, however, some of them might be erraticsof the local lithology. Several large basal knobs arestanding on the hill, a couple by themselves but a fewin a group about 0.5-1 m apart. One of the largerknobs, 6 m high, 4 m wide, and 10 m long, has apronounced flared slope underlain by weatheringhollows 0.1-0.3 dm deep, 0.2-0.6 dm in diameter(fig. 32). On the top surface it had a 5 cm deeppothole initiation and glacio-fluvial marks, the latter
Fig. 31. Map over researched areas in Kings Canyonand Sequoia National Parks.
Fig. 32. Flared slope with weathering hollows.Glacio-fluvial p-forms on top surface. Generals
36
were also seen on other knobs and tors. A couple ofsmaller basal knobs and tors showed incipient andcompletely developed mushroom weathering. Small-scale surface weathering such as thin exfoliation,knob and handle forms, and the everywhereprevailing pitting as well asdifferential weathering were mapped on observed knobs and tors.
The same kind of small-scale forms were seen on gently rounded tors by Great Baldy viewpoint (Bfig. 31), approximate elevation 2,000 m a.s.l. Here a tor like pillar, more likely a protruding deepweathered basal knob, 6 m high 20 m long and 4 m across, has pothole initiation and p-forms on thetop surface. Some erratics lay below this pillar, probably descending from the same. Upon trailingbeneath the viewpoint glacial meltwater has stripped a 10 m long deep weathered joint patternsystem. The tors are 0.5-3 m high and 0.5-1 m wide with gently rounded surfaces. Fluvialdepressions follow the same pattern as seen in Foresta.
Quite a number of large sheeting planes that may have been fluvial polished with big and smallerratics on the surface were seen (fig. 33). One of them, a convex sheeting area facing north, 150 mwide and at least 300 m long, had several deep weathered core stones on top, of which at least twohad tafoni weathering (fig. 34). Again the bedrock exhibits thin exfoliation, enlarged joints, pitting,differential weathering, and even a protruding feldspar vein, 6 cm high.
Fig 33. Large scale sheeting plane. Generals Highway,elevation 2,100 m.
4:5.3 Generals Highway - Silliman Creek
Silliman Creek (fig. 36) is another extensivesheeting area having a slight concave formwhich may be a result of fluvial incision (C,fig. 31). Fluvial water has creatednumerous, at least 25, pot-holes in varioussizes, 0.4-2.5 m in diameter, 0.1-1 m deep,
Fig. 34. Large tor with incipient donughtformation on fluvial polished sheeting plane.Generals Highway, elevation 2,100 m.
0.5 m
37
within the creeks boundary. Boulders andgranite sheets, deriving from a structuralcontrolled convex sheeting area layingdirect south-east of the creek, have slid orbeen pushed by the ice into thewatercourse. The sheet joints follow theand are peeled off like an onion.On top of the sheeting plane is a common sight regarding deep weathered granite in the Sierras.Intersecting joints, 2 by 4 m, have created 0.5-1 m thick rectangular blocks with edge roundedconvex forms (fig. 35). On top of the rectangular blocks lay erratics as an evidence of glacial activity.Another glacial evidence was seen just a couple of miles down the road from Silliman Creek in thatglacial till was exposed in a road cutting.
Fig. 36. Schematic profile over Silliman Creek. The creek is incised in the bedrock. Intersecting rectangular blockslay on a structural controlled convex sheeting area. Some boulders derived from the local lithology lay on top ofthe blocks and in the creeks boundary. Not to scale
4:5.4 Moro Rock
Moro Rock (fig. 37) gives a perfect example of a vertical joint controlled rock where sheet jointsfollow topographic surfaces. The dome-shaped monolith formed by extensive spalling due tounloading is a very common, yet grand, sight in the Sierras. A smooth cliff wall is exposed onsheeting surfaces, however, the remains from spalled rocks have sharp corners. Being exposed tosub-aerial weathering the wind plays a major part in creating ventifacts and wind flutes (fig. 38).Knob handles in various sizes 5-30 cm in diameter are seen protruding 1-12 cm.
Silliman Creek
potholes
38
Fig. 37. Moro Rock, elevation 2,050 m.
4:6 Kern River Valley
Kern River Valley is cut into a deep weathered structural zone which has been successively incisedby Kern River. South of Lake Isabella flows Kern River through a successively incised V-shapedcanyon whose sides are fractured and deep weathered. At places the sides have a stepwiseappearance and core stones are left resting on the slopes (fig. 39), some tors have protruding knoband handle forms and pedestal rocks on top (fig. 40). On other sites tors and core stones make upcone shaped nubbin mounds.
Fig. 39. Schematic profile over Kern River Valley south of Lake Isabella. The canyon is deeply incised by KernRiver in a stepwise manner. Deep weathering takes place within joints and core stones are seen resting on theslopes. Not to scale.
Kern River
0.5 m
39
At the southern end of the valley the walls are controlled by vertical joints and bedrock slabs aresliced off tumbling into Kern River. Tor like pillars, tilted towards the river, come in all sizes whereone of the most magnificent is about 30 m high, 10 m across (fig. 41). Tors on higher elevations,closer to the crest, have a more rounded appearance than the core stones laying closer to KernRiver, i e lower in the original weathering profile.
The slopes on higher elevations are covered by a patchwork of core stones resting on the slopes.Owing to slope processes lower elevations are mainly covered by saprolites and detached corestones are often fractured on account of fall from the heights above.
Exposed road cuttings show structurally controlled joint systems with remarkable weathering alongjoints up to 50 m deep. Above the road cuts a thin weathering cover is partly stripped exposing aconvex etch surface with tors laying on top. Some road cuttings have an external shell of grussresembling cement which crumble up when hacked on. Beneath the outer shell the decomposed rockis moist and at 0.3 dm depth the rock is strong, i. e. fresh enough to give resistance. Grussweathering is also more evident on higher elevations. Weathering is more advanced on lowerelevations and quite a few mudslides reveal a mature clayey sandy soil.
Several convex sheeting planes are covered with edge rounded rectangular blocks. Once theboulders are exposed slope processes take over and the boulders start to slide off the sheetingplane. Some tors with tafoni were seen closer to the river.
The river must at some point have exerted a high pressure on its bed since potholes, 1 m in diameter,were seen in the riverbed. However, these must be normal fluvial potholes since the most southernpart of the system has not been subject to glacial transformation as the northern part of Kern Riverhas. South of the valley flows Kern River through softly rolling hills.
4:7 Big Pine lava fields
Big Pine is situated on Sierra Nevada’s eastern foothills bordering the structural basin and rangezone. The relative relief is about 3,000 m between Big Pine and the crest of the east escarpment.The escarpment is outstanding with an almost straight mountainside completely fractured andcracked open with rocks having very sharp edges. Frost and thawing processes in accordance withstrong slope processes acting on the cliffs have created massive taluses.
The foothills by Big Pine are characterized by tors, 2-4 m tall, and tor-like pillars, up to 10 m tall,surrounded by alluvium. Extrusive volcanic rocks of supposed Plio-Pleistocene origin rest
Fig. 40. Pedestal rock above Kern River.
Fig. 41. Torlike pillar c. 30 m high, 10 macross. Kern River Valley. Elevation 900 m.
40
unconformably on the granite. This implies that the tors and boulders are exhumed pre-volcaniclandforms, which suggest that also the pediment is of pre Plio-Pleistocene age (cf. Cooke et al,1993). Pedestal rocks lay on top of some tors and a multitude of core stones cover the ground.Most of the rocks have a thin, 1-5 cm deep, exfoliated layer on the surface, some exfoliation is akind of mural weathering, others are flaky or have a stepwise appearance (fig. 42). Knob handlesprotruding 4-10 cm, c. 4-15 cm wide and 5-15 cm long, were seen on several tors (fig. 43).
A couple of large boulder inselbergs, approximately 30 m high and 100 m in diameter weresurrounded by lava outcrop (fig. 44). However, no connotations of lava could be seen beneath thetors nor the core stones on the inselbergs.
Fig. 44. Boulder inselberg once surrounded by lava field, Big Pine. Elevation 1,400 m.
Fig. 43. Knob handle protruding 8 cm, 10 cm wide, Big Pine,elevation 1,400 m.
Fig. 42. Bedrock-rooted tor-like pillar with stepwise exfoliationfollowing the boulders surface. Big Pine, elevation 1,400 m.
0.5 m
41
4:8 Structure of different forms
An attempt to gather and structure the different forms found in the different landform zones wasmade. The result is presented in the following tables. In table 2 valleys with local glaciation as well asvalleys lacking glacial erosion is taken under consideration. It is striking that the same kind of etchforms, sub-aerial weathering forms and fluvial forms were found in all landforms zones. However, toa various degree.
Tab. 2. Structure of different forms found in valleys with vertical joint controlled walls: YosemiteValley, Hetch Hetchy Valley, Kings Canyon and Kern River Valley.
Etch forms Sub-aerial weatheringforms
Fluvial forms Glacial and glaciofluvialforms
core stone likecompartments
rough pitted surfaces polished surfaces striae on bedrock wallsand rock floor
tor-like pillars handle and knob forms potholes erraticsconvex edge roundedgranite blocks
thin/thick exfoliation1-5 cm / 10-20 cm
grooves potholes
weathered joints withlineation
enlarged joints hanging valleys
morainearête
Tab. 3. Structure of different forms found on plateaus outside Yosemite Valley, Hetch HetchyValley and Kings Canyon.
Etch forms Sub-aerial weatheringforms
Fluvial forms Glacial forms
core stone likecompartments
rough pitted surfaces grooves erratics
tor-like pillars handle and knob forms polished surfaces roche moutonnéeconvex edge roundedgranite blocks
thin/thick exfoliation1-5 cm / 10-20 cm
glacial polish
weathered joints withlineation
enlarged joints
Tab. 4. Structure of different forms found in Foresta. Landscape with no recent glaciation.
Etch forms Sub-aerial weatheringforms
Fluvial forms Glacial forms
core stone likecompartments
thin/thick exfoliation1-5 cm /10-20 cm.
polished surfaces erratics
tor-like pillars rough pitted surfacesconvex edge roundedgranite blocks
handle and knob forms
42
weathered joints withlineation
43
5 Discussion
5:1 Yosemite Valley
Owing to multiple glaciations, which has eroded the valley sides, overdeepened and filled the floorwith glaciofluvial debris, Yosemite Valley has gone through radical changes which today is presentedby a remarkable U-valley (figs.45, 46). The powerful U-shape has probably been accomplished bySherwin, the largest pre-Tahoe glaciation, which during its maximum extent filled the valley to itsbrim reaching west of El Portal (Matthes, 1930) (fig. 13 p. 24). This is also the point where Mercedstarts to meander more readily. The latest glaciation, Tioga, was rather thin and did not reach furtherthan Bridalveil Meadow during its maximum about 15,000 years ago according to Huber (1989).While comparing the profiles with the maps and field data the glaciers greatest erosive power hasbeen between Royal Arches and Bridalveil Meadow. The glacial erosion west of BridalveilMeadow cease gradually until Foresta where an almost perfect V-shaped canyon appear. The valleyis once again widened west of Arch Rock to form a meadow at El Portal. Postglacial fluvial erosionmust be considered insignificant seeing that no fluvial incision in the glacial valley is observed. Slightfluvial incision is restricted to the knick points between the main valley and the hanging valleys. In thethousands of years to follow glaciation freeze-thaw cycles have favored rock fall due to off-loadingcreating sheer and steep cliff walls with talus covered bases. Altogether this makes Yosemite Valleyan unsatisfactory place to come across glacial evidence such as striae or other geomorphiclandforms.
Fig. 45. View over Mirror Lake and Tenaya Creek. Fig. 46. View over Yosemite Valley looking east. ElNorth Dome to the left, facing south and lower parts Capitan to the left and Bridalveil Falls to the rightof Half Dome to the right. facing north. Half Dome is seen in the background.
While following the trail leading from the valley to Vernal Falls (fig. 14 p. 25) the change fromboulders with sharp corners and post glacial weathered erratics to bedrock walls with sheeting jointswith edge rounded surfaces is gradual. Close to the valley floor the steep wall facing west show nosign of glacial erosion nor deep weathering. On higher elevations and facing north the intersectingjoints are more enlarged and have the same edge convex rounded forms found in road cuttings. Thisindicates that the wall must have been formed by weathering along vertical joints and at the time ofglaciation the walls have probably been composed of grand tor-like pillars e. g. Like those seen inKern River Valley. The valley glacier has either had a gentle approach just pushing the pillars off thewalls and down into the valley. Or while the ice worked its way on the valley floor, cleansing therock base free of debris, it provided the pillars with an unstable base support and upon melting the
44
pillars were overthrown. During later millennia the wall has continued to spall. This possible cause ofevent could mean that glacial erosion was less effective on higher elevations.
Merced’s riverbed at Vernal Falls was not subject to glacial erosion during Tioga. One assumptionmay then be that this area reveals partly how the walls in Yosemite Valley looked like prior toSherwin glaciation. If so, there is no or little doubt that the valley has been deep weathered prior toglaciation. Since the weathered wall is facing north one possible thought regarding the deepenedjoints could be a dry cold based ice frozen fast to its bed preserving the landforms during glaciation(cf. Kleman 1994). Next glacial melt water cleansed the fractures setting them free of weatheredmaterial. The thousands of years which followed postglacial weathering has probably subsequentlyaccentuated the established deep weathered forms.
5:2 Structure of different forms
5:2.1 Valleys with vertical joint controlled walls: Yosemite Valley, Hetch Hetchy Valley,Kings Canyon and Kern River.
All rivers are entrenched in deep canyons on thewest side of the range, the deepest one KingsCanyon (fig. 47) having a relative relief of 2,400m which for instance is deeper than the GrandCanyon. Other canyons relative relief variesbetween 750 m, Hetchy Reservoir, 1,050 m,Yosemite Valley and up to 1,200 m, KernRiver. Nearly all the major canyons lay roughlyparallel to each other flowing west to the CentralValley of California. Faults and vertical fracturinghave played a major part in the valleys evolution.The canyons resemble one another and aremorphologically similar with joint aligned steepbedrock walls.
The deep weathered profiles documented specially in Kern River Valley suggests that deepweathering along orthogonal joint system plays a significant role for the development of valleys. Inthe Kern River Valley partial stripping during fluvial incision has resulted in exhumation of corestones, tor-like pillars and extensive weathering along sheeting joints.
In formerly glaciated terrain stripping of the saprolite is almost complete and has resulted in forinstance troughs, steep slopes, hanging valleys and local glacial over-deepening. Postglacial surfaceweathering is insignificant and has only influenced the landforms in establishing a layer of thinexfoliation and differential weathering. Postglacial slope processes, such as slab failures, are of localimportance.
Hetch Hetchy valley is intermediate between these two extremes. It has been covered to its brimwith ice, last by Tioga, and for that reason it exhibits several pieces of evidence of glacial erosion.On the other hand the ice has been less effective in some areas and large amount of deep weathered
Fig. 47. Kings Canyon facing west. The canyon may becompared with Yosemite Valley figs. 45 and 46.
45
landforms are detectable. Some weathered landforms found on the floor in Hetch Hetchy Valleyhave obviously survived repeated glaciations whereas some forms on higher elevations presentweathering patterns. This phenomenon has also been noted in Scotland where the variations on aregional scale seemed to be related to topography with pre-glacial surfaces preserved on uplandsand glacially-modified surfaces on lower grounds (Sugden 1989). In Hetchy the scale of variation atthe valley bottom could also be related to selected zones of the ice sheet having experienced locallyaccelerated flow due to bed deformation (Boulton & Jones 1979; Sharpe & Shaw 1989). Anotherexplanation could be an ice which in parts had a very low pressure melting point and as suchsuppressed glacial sliding and erosion or as Kleman (1994) argued, an ice frozen fast to its bed.
5:2.2 Forms on plateaus outside Yosemite Valley, Hetch Hetchy Valley, and KingsCanyon
Glacial erosion on plateaus has stripped the weathered material and the remaining surface beartraces of deep weathered joints. On some localities a pre-existing basal weathering front withtroughs, basins and gutters is exposed. According to Thomas (personal communication) this might beinherited tropical landforms where tropical pitting can initiate weathering and in turn create dambos,ill defined concavities in the landscape to which convergent water flow bringing sediment to thehollows (Thomas 1994). After erosion and stripping the dambos are cleansed and on someoccasions water fill the dambos deepening the same creating lakes. The most prominent glacialforms, roche moutonnées, are structurally controlled in that they follow the bedrock’s dome shapedstructure.
The bedrock on the slopes towards the central valley of California is normally covered by a more orless thick weathering cover. In the Mojave Desert, California, relations between modern pedimentsurfaces and potassium-argon-dated lava flows has recorded a four-million year history of pedimentevolution (Dohrenwend et al. 1987). This study gives general information of the minor changepediment domes have gone through since early Pliocene time. The result from the Mojave Desertmay also be compatible with the pediments applied to the Central Valley of California. In parts theweathering cover has been eroded and expose a weathering front showing characteristic landformshailing from pre-existing patterns which can be compared with forms found in the humid tropics. Thestripped surfaces follow convex sheeting planes on most locations and some etch surfaces arecovered by a mosaic of rectangular core stone blocks. Such block systems are also a common sighton elevated plateaus within Sierra Nevada. Twidale & Romani (1994) imply that fractionation duringcooling may give rise to core stone masses developing into boulders and Ollier (1988) stated that the”retention of weathering products will be favored by flat topography amongst other things”. Incombination and if applied to the Sierra Nevada this means that the joint pattern system is the oldestsheeting plane and it was formed during the granites cooling phase in the Mesozoic.
5:2.3 Forms in and about Foresta: landscape with no recent glaciation
During Pleistocene Foresta was completely glaciated during the Sherwin glaciation. Glacial erosionpartly stripped the area leaving an etch surface with saprolite remnants. Stripping has left nubbins,convex core stone like compartments, grand core stones and tor-like pillars to be exposed toexogenic processes. Whatever glacial evidence the rocks in this area have presented has been longgone owing to postglacial processes, irrespective of some erratics. Core stones have continued toweather post-glacially establishing a flaky form of layered thin exfoliation.
46
5:2.4 Pediments east of the Sierra Nevada fault - Big Pine lava fields
The landforms in Big Pine, eastern Sierra Nevada, show the same deep weathering pattern as foundin Kern River Valley and Mojave Desert (Oberlander 1972, 1974; Dohrenwend et al. 1986). Thelandforms emphasize further the establishment of deep weathered pre-existing landforms in graniticterrain.
Many studies in the semi-arid zone demonstrate that lowering of pediment surfaces takes place eithersimultaneously (Dohrenwend et al. 1987), or by alternate mantling and stripping (Mabbutt 1966;Oberlander 1974). By using the evidence of dated volcanic deposits on pediment surfaces and theweathered profiles beneath Oberlander (1974) concluded that the pediments in Mojave Desert wereinherited exhumed remnants of a sub-Pliocene landscape established through etch planation. Thisresult may also be applicable to landscapes laying further east and north of the Mojave Desert thus,Owens Valley.
5:2.5 Relative importance of different processes for the present forms
The purpose of this paper is to use Sierra Nevada as an example to draw attention to the land-forming effect of different exogenic processes on granitic landforms. The landforms characteristic ofthe Sierran granite are above all controlled by different joint systems, just as in areas far away fromPleistocene glaciation such as Australia, Spain and South Africa (Twidale & Campbell 1995;Twidale et al. 1996). However, in the absence of exogenic processes structures will never becomelandforms. In most areas deep weathering and subsequent stripping alone is responsible for the jointexploitation which is decisive for the landform development in this region. By structuring theprocesses involved (tab. 5) an attempt is made to grade the processes.
Tab. 5. Relative importance of different processes for the present bedrock forms.Scale; X minor importance; XXX – major importance.
Yosemite ValleyHetch Hetchy valleyKings Canyon
Foresta Upper surfacesHetch HetchyKings CanyonSequoia
Kern RiverValley
Horizontal sheeting X XX XXX XXVertical sheeting XXX - X XXXDeep weathering XX XXX XX XXXSurface weathering X XX XX XXFluvial erosion XXX - - XXXGlacial erosion XXX X X -Glaciofluvial erosion X X X -Glacial stripping XXX XX XXX -Fluvial stripping XX - - XXX
At first sight and just by looking at the gross morphology of the studied areas horizontal and verticalfracturing suggest that the plutons intrinsically structural control in combination with glacial erosionare the dominating parameters. However, it is hard to determine how deep the ice has eroded into
47
fresh bedrock. Glacial erosion as a cleansing agent is dominating within valleys and on higherelevations such as Tuolumne Meadows. One could say that on a macro-scale or regional scale oflandform assemblages (10-250 km2) they override the produce established by other processes. Ona meso-scale or district scale of major landforms (1-10 km2) the structural control is a dominatingfactor but it does not diminish the importance from other processes. However, when looking at theparameters on a micro-scale or local scale of slopes and minor landforms (0.1-1 km2) it is evidentthat weathering, above all deep weathering, has had a dominating influence on the studied landformsand occur at all topographic levels. Hence, it is evident that glacial erosion is of minor importancewhen compared to landforms established by deep weathering. An event that Kern River Valley mayillustrate in that the southern part of Kern River has never been glaciated and yet the deep weatheredlandforms found here may also be seen in former glaciated areas.
Fig. 48. Long term evolution of Sierra Nevada. Not to scale.
Sierran landforms are compatible with Scottish and Swedish landforms along with forms found in thehumid tropics. In north-east Scotland weathering zones on a district-scale are recognized as patternsfrom humid tropical weathering systems of Neogene to early Pleistocene age (Hall 1985, 1986).
N
Paleo-saprolites
48
Lidmar-Bergström (1988, 1989, 1996, 1997) came to the main conclusion that the major landformsin south Sweden consists of exhumed sub-Cambrian and sub-Mesozoic paleo-landforms.The long term evolution of Sierra Nevada follow a complex and intriguing pattern (fig. 48). Themajor impression is that old saprolites in deep profiles are preserved on the western low angle slopeof the tilted block. A common sight are mature and advanced weathering profiles containing largeamounts of clay pointing at subtropical weathering conditions. Profiles, having escaped glacialerosion, lay further west and are probably remnants of older saprolites preserved by the western tilt.Furthermore, they have a higher clay contents than younger gravely saprolites situated on higherelevations and/or further east. However, erosion has been more effective on the eastern side wherethe tilting is more elevated.
The great variety of tors and core stones resting on the slopes and top surfaces as well as the lightcolored clay formation within joints points at weathering in a tropical climate. The etch forms andsub-aerial weathering forms are probably the result of prolonged deep weathering and subsequentstripping during late Tertiary. This was most likely initiated during late Mesozoic to early Tertiary.The eastern slope fault scarp is characterized by slope processes due to rapid uplift. The pedimenteast of the fault scarp may be remnants of a surface developed prior to uplift and is thus possible tocorrelate to the undulating relief of the uplifted plateaus. A similar kind of surface in Norway thoughtto reflect weathering in a tropical climate is described as the ”Paleic surface” (Gjessing 1987). Thewestern uplifted part of the tilted block is a stripped etch surface formed by deep weathering andstripped by fluvial and glacial erosion during Plio-Pleistocene. The general conclusion is that a varietyof erosion processes have occurred at Sierra Nevada but the major forms have resulted from theoblique uplift of the eastern rim in combination with stripping of old saprolites and renewed rimincision along old valley systems (fig. 49). Finally a glacial reshaping of landforms has taken placeduring Pleistocene. Hence, the study points at several important steps in the geomorphic evolution oftilted block mountains.
Fig. 49. Steps in the geomorphic evolution of tilted block mountains.
Planation Tilting / Stripping Incision / Stripping
incision
49
6 Conclusions
While driving through Sierra Nevada it seems like this is a mountain range slowly being decomposedfrom solid bedrock to gruss and clay. The western parts of Sierra Nevada is covered bysedimentary cover rocks and alluvium. Hence, the transition from a landscape exposed to deepweathering and subsequent, but partial, stripping to the more elevated and stripped etch surfaces inthe east is gradual but yet overwhelming.
The gross morphology in Sierra Nevada suggest a structurally controlled landscape where horizontaland vertical sheeting have created domes. The relief on exposed etch surfaces, plateaus, proposethat the major landforms are inherited and preserved even though they have been slightly reshapedby glacial erosion. Postglacial weathering is active but of minor importance for the major landformsdepending on its slow moving process. On plateaus in the western part of the Sierra Nevadapostglacial weathering has acted over a longer time span with more accentuated forms as a result.
The Pleistocene glaciation has only played a minor role in remodeling Sierra Nevada. In the easternHigh Sierra where glaciation has been heavier it is seen in cirque glaciers, roche moutonnées,glacially polished surfaces and trough valleys. Glacial erosion in the western part of the batholith isdetectable in the magnificent U-valleys, glacially polished surfaces and in the shaping of rochemoutonnées. However, the majority of the roche moutonnées follow ancient horizontal sheetingplanes and are therefore primarily structure controlled.
In summary the gross morphology suggest a structure controlled landscape where periods of deepweathering during the Mesozoic and early Tertiary has exerted strong influence on developedlandforms. In combination with uplift and high denudation rate during late Cenozoic the surface wesee today in Sierra Nevada has been extracted. The Pleistocene glacial influence has merely giventhe landscape its ultimate appearance. This study may also be applicable to the evolution of formerlyglaciated continental margins such as the Scandinavian peninsula with fluvial incision in the west,Norway, followed by glacial over-deepening and the development of fiords and trough valleys.
50
Acknowledgments
This study was carried out at the Department of Earth Sciences, University of Göteborg, Sweden.The performed field study was supported by a grant awarded from Geografiska Föreningen iGöteborg. Financial support was also acknowledged from ICA-Allköp, Horred.
I would like to sincerely thank my supervisor Mats Olvmo1 whose great enthusiasm regarding longterm landform development inspired me in writing this thesis. He also critically read the manuscriptand gave several pieces of elucidative advice regarding its improvement.
Thanks also to Magnus Johansson1 for his enlightening comments and useful discussions while writingthis thesis. Björn Holmer1 gave general and valuable advice for which I am much obliged.
A special thanks goes to Laura Coase who was a great company while assisting me during somefield days and illuminating nights. All friendly and accommodating people I met while in Californiadeserve a special thank. I am also very grateful to my friends at the department for valuablediscussions and joyous acclamations during the progress of this work.
Above all I would like to thank my family who have put up with me and my absent-mind during thiswork. Without their love, support and patience this thesis would probably not have left its initialstage.
1 Department of Geosciences , Göteborgs Universitet
51
References
Bateman, P.C., Clark, L.D., Huber, N.K., Moore, J.G. and Rinehart, C.D. 1963: The SierraNevada batholith - A synthesis of recent work across the central part. U.S. GeologicalSurvey Professional Paper 414-D, 46 p.
Battiau-Queney, Y. 1997: Preservation of old paleosurfaces in glaciated areas, examples from theFrench western Alps. In: Widdowson, M. (ed): Palaeosurfaces: Recognition,Reconstruction and Palaeoenvironmental Interpretation. Geological Society SpecialPublication 120, 95-124.
Benn, D.I. & Evans, D.V.A. 1998: Glaciers and Glaciation. Arnold Press, London.Bird, M.I. & Chivas A.R. 1993: Geomorphic and palaeoclimatic implications of an oxygen-isotope
chronology for Australian deeply weathered profiles. Australian Journal of Earth Sciences40, 345-358.
Bouchard, M., Joliceur, S. & Pierre, G. 1995: Characteristics and significance of two pre-late-Wisconsian weathering profiles (Adirondacks, USA and Miramichi Highlands, Canada).Geomorphology 12, 75-89.
Boulton, G.S. & Jones, A.S. 1979: Processes of glacier erosion on different substrata. Journal ofglaciology 89, 15-38.
Christensen, M.N. 1966: Late Cenozoic crustal movements in the Sierra Nevada of California.Geological Society of America Bulletin 77, 163-182.
Cooke, R., Warren, A. & Goudie, A. 1993: Desert Geomorphology. University College LondonPress Limited. University College London, United Kingdom, 526 p.
Dahl, R. 1965. Plastically sculptured detail forms on rock surfaces in northern Nordland, Norway.Geografiska Annaler 47A, 83-140.
Dohrenwend, J.C., Wells, S.G., McFadden, L.D. & Turrin, B.D. 1987: Pediment dome evolution inthe eastern Mojave Desert, California. In International Geomorphology 1986 part II, editedby V. Gardiner. John Wiley & Sons Ltd, 1047-1062.
Ericsson, B. & Gembert, B. 1991: Kvartärgeologi - Kompendium för grundkurs i geovetenskap.Uppsala Universitet, 78 p.
Gjessing, J. 1967: Norway’s Paleic Surface. Norsk geografisk Tidsskrift 21, 69-132.Hall, A.M. 1985: Cenozoic weathering covers in Buchan, Scotland, and their significance. Nature
315, 392-395. 1986: Deep weathering patterns in north-east Scotland and their geomorphological
significance. Zeitschrift für Geomorphologie N. F. 30, 407-422. Sugden, D.E. 1987: Limited modification of mid-latitude landscapes by ice sheets: the case of
northeast Scotland. Earth surface processes and landforms 12, 531-542.Hill, M. 1975: Geology of the Sierra Nevada. University of California Press, Berkeley and Los
Angeles, California, 232 p.Huber, N.K. 1981: Amount and timing of late Cenozoic uplift and tilt of the central Sierra Nevada -
evidence from the upper San Joaquin River basin. U.S. Geological Survey ProfessionalPaper 1197, 28 p.
1989: The Geologic Story of Yosemite National Park. U.S. Geological Survey Bulletin1595, 64 p.
1990: The late Cenozoic evolution of the Tuolumne River, central Sierra Nevada, California.Geological Society of America Bulletin 102, 102-115.
52
Kleman, J. 1994: Preservation of landforms under ice-sheets and ice caps. Geomorphology 9, 19-32.
Lidmar-Bergström, K. 1988: Denudation surfaces of a shield area in south Sweden. GeografiskaAnnaler 70 A, 337-350.
1989: Exhumed Cretaceous landforms in south Sweden. Geomorphologie N.F., Suppl.-Bd.72, 21-40.
1996: Long term morphotectonics evolution in Sweden. Geomorphology 16, 33-59. 1997: A long-term perspective on glacial erosion. Earth Surface processes and landforms
22, 297-306. Olsson, K. & Olvmo, M. 1997: Palaeosurfaces and associated saprolites in southern Sweden.
In Widdowson, M., (ed): Palaeosurfaces: Recognition, Reconstruction andPalaeoenvironmental Interpretation. London, Geological Society Special Publication 120,95-124.
Mabbutt, J.A. 1961: ”Basal surface” or ”weathering front”. Proceedings of the GeologicalAssociation (London) 72, 357-358.
Matthes, F.E. 1930: Geologic History of the Yosemite Valley. U.S. Geological SurveyProfessional Paper 160, 137 p.
Muir, J. 1912: The Yosemite. New York, Century Co. Reprinted 1962 by Doubleday & Co., NewYork, Natural History Library N26, 225 p.
Oberlander, T.M. 1972: Morphogenesis of granitic boulder slopes in the Mojave Desert, California.Journal of Geology 80, 1-20.
1974: Landscape inheritance and the pediment problem in the Mojave Desert of SouthernCalifornia. American Journal of Science 274, 849-875.
Ollier, C. 1982: The Great Escarpment of eastern Australia: tectonic and geomorphic significance.Journal of Geology Society Australia 29, 13-23.
1988: Deep weathering, groundwater and climate. Geografiska Annaler 70A, 285-290. 1991: Ancient Landforms. Belhaven Press, London, 233 p. 1995: Tectonics and landscape evolution in southeast Australia. Geomorphology 12, 37-44.Olvmo, M., Lidmar-Bergström, K. & Lindberg, G. 1998: The glacial impact on an exhumed sub-
Mesozoic etchsurface in south-western Sweden. In press.Schaffer, J.P. 1997: The Geomorphic Evolution of the Yosemite Valley and Sierra Nevada
Landscapes - solving the riddles in the rock. Wilderness Press, 388 p.Sharpe, D.R. & Shaw, J. 1989: Erosoin of bedrock by subglacial meltwater, Cantley, Quebec.
Geological Society of America Bulletin 101, 1011-1020.Shelton, J.S. 1966: Geology Illustrated. W. H. Freeman and Company, San Fransisco and London
434 p.Sugden, D.E. 1978: Glacial erosion by the Laurentide ice sheet. Journal of Glaciology 20, 367-
391. 1989: Modification of old land surfaces by ice sheets. Geomorphologie N.F., Suppl.-Bd. 72,
163-172. John, S.J. 1976: Glaciers and Landscape.London, Edward Arnold, 376 p.Summerfield, M.A. & Thomas, M.F. 1987: Long-term landform development: editorial introduction
in Gardiner, V. (ed) International Geomorphology 1986, Part II. Wiley, Chichester. 927-933.
Thomas, M.F. 1994: Geomorphology in the Tropics. A Study of Weathering and Denudation inLow Latitudes. John Wiley & Sons Ltd, Chichester, England, 460 p.
53
1995: Models for landform development on passive margins. Some implications for reliefdevelopment in glaciated areas. Geomorphology 12, 3-15.
Twidale, C.R. 1993: The research frontier and beyond: granitic terrains. Geomorphology 7, 187-223.
1995: Bornhardts, Boulders and Inselbergs. Caderno Lab. Xeolóxico de Laxe Coruna 20,347-380.
Campbell, E.M. 1995: Pre-Quaternary landforms in the low latitude context: the example ofAustralia. Geomorphology 12, 17-35.
Vidal Romani, J.R. 1994: On the multistage development of etch forms. Geomorphology 11,107-124.
Vidal Romani, J.R., Campbell, E.M. & Centeno, J.D. 1996: Sheet fractures: response toerosional offloading or tectonic stress? Geomorphologie N.F. Suppl.-Bd. 106, 1-24.
Uppsala excursion guide to USA 1980.Young, R.W. 1983: The tempo of Geomorphological change; Evidence from Southeastern Australia.
Journal of Geology 91, 221-230.Wagner, D.L. 1991: Decomposed Granite. California Geology Nov 1991, 243-249.Wahrhaftig, C. 1965: Stepped topography of the Southern Sierra Nevada, California. Geological
Society of America Bulletin 76, 1165-1190.Whitney, J. D. 1869: Yosemite Guide Book. Cambridge University Press, Welch, Bigelow &
Co., 155 p.
Further reading
Hambrey, M. 1994: Glacial Environment. UCL Press Limited, London.Hill, M. 1984: California Landscape - Origin and Evolution. University of California Press,
Berkeley and Los Angeles, California, 262 p.Jones, W.R. 1989: Yosemite the Story Behind the Scenery. KC Publications, Inc, 48 p.
54
Appendix
The geologic time scale used in this study (Huber, 1989).
