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Abrasion by Glaciers, Rivers, and WavesAuthor(s): Lewis G. WestgateReviewed work(s):Source: The Journal of Geology, Vol. 15, No. 2 (Feb. - Mar., 1907), pp. 113-120Published by: The University of Chicago PressStable URL: http://www.jstor.org/stable/30056365 .

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ABRASION BY GLACIERS, RIVERS, AND WAVESLEWIS G. WESTGATE

Ohio Wesleyan University, Delaware, O.INTRODUCTION

Erosion'or land sculpturencludesrockdisintegration,wheretherock is coherent,and transportation f the materialdisintegrated.Disintegrationhas been divided into weathering and corrasion.Stream-andwave-wearmaybe eitherchemicalormechanical,houghin all ordinary ircumstanceshe mechanicalwearis so muchgreaterthan chemicalsolution that the latter may be neglected. Ice-wearis purely mechanical. Omitting chemical solution, which is anentirelydistinctprocess,corrasionmay be definedas the mechanicalwearperformedby wind, streams,waves or glaciers. Gilbert2usesthe term"corrasion," xcluding hemicalcorrasion,or the "mechani-cal wear . . . . performed by the aid of hard mineral fragmentswhich arecarriedalongby the current." Chamberlin nd Salisbury3define stream-corrasion s "the wear effectedby runningwater."This use of the term is wider thanthat of Gilbert, or it includesnotonly wear by tools, but also the processof sweeping away materialwhich has alwaysbeen incoherentand "material oosened n advanceby the processof weathering." To distinguishthese radicallydif-ferentprocessesof stream-action,n this paperthe term"abrasion"will be used forthe mechanicalwearperformed y tools,and"pluck-ing" for the removalof rockfragments. "Corrasion,"ncludingallmechanicalwearby streams,will includeboth abrasionandplucking.None of the termsare new; they have more or less overlapped nuse, and it is believedthat the meaninghereassigned o them is thatgenerallyunderstood oday. The terms can be applied to glacier-and wave-actionwith the same significancewith which they are

1Gilbert, Geologyof the Henry Mountains, pp. 99-102.2 Ibid., p. 101.3 Chamberlin and Salisbury, Geology, Vol. I, p. 113.

113


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114 LEWIS G. WESTGATEappliedto stream-action. The analysisof erosionby waves,glaciers,and streams,wouldbe arrangedas follows:

R o c k - W e a t h e r in g AbrasionDisintegrationCorrasionErosion PluckingSolutionTransportationIn geological iteratureabrasion s ordinarily onsidered n importantfactorin land sculpture. It is the purposeof this article to suggestthat abrasionby glaciers, streams, and waves is in most cases anegligible factor in erosion, and to emphasizethe importanceofweathering n the work of erosionby streamsand waves. Wind-erosion is not considered.

GLACIAL ABRASIONThe commonunderstandingof glacial erosion has been that itis accomplishedby the wear of solid particlesheld in the bottomofthe ice against the rock surface over which the ice moved. This

processwould be favoredby the weightof the glacierand by the factthat particlesso held are often in continuouscontactwith the bed-rock for long distances. If this processwere the only or the chieffactor involvedin glacial erosion, the rasplikeaction.of the broadglacierbottomshouldproducea smoothed, ub-even urface. Withinthe glaciatedarea of North Americathere are many nearly level,glaciallysmoothed urfaces,but theseare in regionswhichwerelevelin preglacialtimes, and are areas which there is reason to believewere not deeply eroded. In the hilly regions of glaciated NorthAmerica and in glaciatedalpine valleys the detail of such surfacesis controlled by rock jointing, and glacial abrasion is limited tosmoothing he surfacesand rounding he cornersof the joint blocks.This hackly characterof the topographicdetail of surfaces coveredby the Pleistocene ce-sheetmay be in partan inheritance rompre-Pleistocene ime, but in glaciated alpinevalleys (see Fig. I), havinglateralhangingvalleys,the rounded-hacklyurfaceof the lowerpartof the mainvalleyhas been producedby the normalaction of glacialerosion n live rock, scoresand perhapshundredsof feet below theoriginal surface. Here plucking, or the removal of large blocksboundedby joint planes,has been the importantelementin erosion.


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ABRASION BY GLACIERS, RIVERS, AND WAVES 115Beforeglacialabrasionhas beenable to smoothawaythe inequalitiesproducedby plucking,the processof pluckinghas producednewinequalities. The effectof abrasion n wearingdown the valleys isneutralizedby the removal of the joint blocks when only partlyabraded. It has been a pluck-and-heal process, with pluckingalways ahead. It isnot a questionherewhether the blockspluckedareremovedmechanicallyby theice,orare oosenedbysubglacialweather-ing; the point em-phasized is thatvalley- deepeningdoes not take placethrough scratchingby material carriedin the bottomof theice.

There is nothingnew in this state-ment of the process

FIG. I.-Joint-controlled glacially eroded surface.Direction of ice-movement was to left. The viewshows the inabilityof abrasion o obliterate he control,by jointing and plucking, of the surface form. LakeCreek,aboveTwinLakes,Colo.

of glacial erosion. Plucking is recognizedmore and more. Therelativeincompetency f glacial abrasion s mentioned herebecauseit leadsup to the consideration f the inadequacy f stream-abrasion.The sameclass of facts is appealed o forevidence n both cases,andthese facts have been recognizedmuch morewidely in the case ofglacialerosion han in that of streamerosion.STREAM-ABRASION

Stream-abrasion as generallybeen consideredan importantele-ment in valley-cutting. It was clearly distinguishedby Gilbert'and recenttextsusuallyconsider t, thoughno attempthas ordinarilybeen madeto indicate the relative mportanceof abrasion,plucking,I Gilbert, Geologyof the Henry Mountains, p. 101.


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116 LEWIS G. WESTGATEand weathering. Chamberlin nd Salisbury'go as far as any recenttext in emphasizing he importanceof weatheringand plucking,andso in limiting the relativeimportanceof abrasionin valley-cutting.They say that in any valley cross-section he amount removedbycorrasion may be measured by a rectangle the width of whichis the width of the stream,and the height of which is the depthof the valley. It seems, however,that even this relativelysmallproportionate amount, while allowed to the stream, must bedenied to stream-abrasion,and divided between plucking andweathering.

Theoreticallystream-abrasions less probablethan abrasionbyglaciers. The cuttingparticlesarenot held againstthe rockbottomby any overlyingmass of ice; indeed,the weightof the particlesislessened by their immersionin watei. The smaller particles arelargely carried n suspension,strikingthe bottomonly at intervals.Fragments oo largeforsuspensionmove over the bottomwithrollingand not with slidingfriction.The form usuallyshownby the rock-bedover which the streamflowsbearsevidence o the inadequacyof mechanicalwearof detritusin shapingit and in loweringthe bed. The OlentangyRiverbelowDelaware, 0., for example, is flowingover nearly horizontalbedsof Devonian imestone. The bedof theriver,whichhas sinceglacialtime been cut a dozenfeet into the hardrock,consistsof a successionof very broad, low steps, each step being a limestonestratum, itsdown-streamlimit determined by vertical joint-faces. In someplaces the edges,and in a few placesthe surfaces,of these steps areslightlyrounded,as if by mechanicalwear; but this in no way affectsthe largefact that the rock in the stream-beds boundedby stratifi-cation- and torsion-jointplanes. The agency effectivein removingthe rockfromthe stream-bedhas takenit away in largeblocks; therock has not been scratchedaway by the mechanicalrubbing offragmentsswept down by the stream. The ordinaryprocessesofweatheringare believedto have loosened the jointedlimestone,andthe blocks were later swept away by the stream. As in the case ofglacial erosion,before abrasioncould reduce a joint-block,weather-ing processes solated the partlywornblock, and delivered t to the

1Chamberlin and Salisbury, Geology,Vol. I, p. 108.


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ABRASION BY GLACIERS, RIVERS, AND WAVES 117streamfor removal. In each case the characterof the bottombearsevidenceto the ineffectiveness f abrasion.This fact in regard o the formof the stream-bedhas been noticedby the writer in the streamscuttingthe Ohio shale and Cincinnatilimestone (see Fig. 2) in Ohio, in the sandy shales near Ithaca, inthe Berea sandstoneof Ohio, in the Tri-assic sandstone alongthe lower WestfieldRiver and the Con-necticut River, andin the crystallinerocksalong the upperWestfield River.Views showingstream-beds, notablythe collection inTarr'sNew PhysicalGeography, give evi-dence in the samedirection.

In qualification of

FIG.2.-Stream in thin-bedded Cincinnati limestone,in which the characteristic irregular surface of thestream-bed, the result of plucking rather than abrasion,is shown. Near Camden, O.

what has just been said in reference to stream-abrasion, two thingsmay be mentioned. First, reference should be made to potholeaction. It is abrasion, and where numerous potholes are formingand connect, they may decidedly aid downward erosion. Thisaction, however, is believed to be exceptional; the great majorityof streams are without it. Secondly, and forming a really importantexception, in certain cases streams are flowing over rock-beds whichare thoroughly smoothed, and appear to have been deepened by wearof stream-swept detritus. In these cases it is believed that it will befound that the rock is nearly jointless. This is especially the casewith crystalline rocks, particularly the more massive granites. Thisprocess of wear by abrasion is most common in swift streams inmountainous areas, but even here it is exceptional, and in the con-sideration of stream-erosiongenerally it is insignificant.In conclusion, the joint-controlled form of the rock sides and


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118 LEWIS G. WESTGATEbottoms of stream-beds hows that abrasionhas not been a deter-miningelementin valley-deepening, nd that the stream is a trans-portingand not anabradingagent,removingmaterialsdislodged romits bed or sweptinto it from its valleysides.

WAVE-EROSIONAs in river-erosion,o in wave-erosion,abrasion,or the wear bymaterial thrown against the base of the cliff, has been generally

emphasizedn the texts. Chamberlin nd Salisbury'makecorrasionby the impact of detritusan importantelementin wave-erosiononhardrocks,at the same timeemphasizinghe co-operation f weather-ing along joint planes. Geikie2 says: "The waves make use ofloose detritus within their reach to break down cliffs exposed totheir ury. Probably

by far the largestamount of erosionis thus accom-plished." Le Conte3says that "fragmentshurled against theshore are the princi-pal agent of wave-erosion." But ifabrasion has beenthe determining ac-tor in wave-cutting,the shore n the verti-cal zone of breakers

FIG.3.-Shore on the east side of Easton's Point,Newport, R. I.should bear evidence of this by its roundedand worn character.The only chance which the writer had to study rock shores withthis consideration n mind was at Newport. In the hard con-glomerateand sandstone at Easton's Point (see Fig. 3), on thesouthside of the island,no evidenceof abrasionwas found. To besure,in some protectedpockets, nto which gravel had been swept

1Chamberlin and Salisbury, Geology,Vol. I, pp. 327-29.2 Geikie, Text-Book of Geology, Vol. I, p. 569.3 Le Conte, Elements of Geology, 5th ed., p. 34-


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ABRASION BY GLACIERS, RIVERS, AND WAVES 119by the waves,the roll of the gravelhad smoothed he solid rock;butelsewherehewavesarebreaking itheron surfaceswhichhavenotbeenappreciably roded,or on a rockyshorecomposedof angularmassesof rockofallsizes,whichhavebeen oosenedbyweathering.Whereverthe shoreis beingworn,it is by combinedweatheringand plucking,and not by abrasion. Easton'sPoint is not yet markedby a wave-cut cliff, and so is not the best place to showthe processof erosion;but along the CliffWalk,at the west end of Easton'sBeach, wherethe rockis prevailinglyCarboniferouschists,a distinct cliff faces theocean; the base of the cliff, however, s not roundedand smoothedas wouldbe the case were it beingwornback by abrasion. A verysuggestive photographof a raised wave-cut bench on PrudenceIsland has been published,'in whichboth cliffand benchare roughand angular, he detaildeterminedby the jointingof the shales.It will be easy,of course, or anyoneto test the matterfor himself.Detritus protectsratherthan endangersthe cliff. Except at timesof high storm,the beachmaterialprotects he cliffand acts on itself.It doesnot seemprobable hat the bombardment f the cliff at timesof heavy storm would seriouslyaffect the cliff. Certainlythere islittle evidenceof such action in the detail of the cliff base. Wavesacting by hydrostaticpressurealong joint planes may be effective;apartfromthat actiontheirworkwouldappear o consist

in reducingand removingmaterialssuppliedthem by the processesof weather-ing. They, like streams,are transporting nd not abradingagents.RELATIVE IMPORTANCE OF PLUCKING AND WEATHERING

The mechanicalwork of corrasionhas been divided betweenabrasionandplucking. Abrasiondoes notappear o be an importantfactor. It is furthera delicatequestionas to how far pluckingcanbe considered separateprocess, o be distinguishedromweatheringon the one hand and transportation n the other. In the case ofglacial erosion t is easy to believe that the pressureof the ice maydislodgeblocks from a jointedfloorwhichhas not been affectedbyweathering,hough t wouldbe difficult o showthat frost-weatheringhad not had a share n loosening hese blocks. It is possible that a

1Geology of the Narragansett Basin, Monograph 33, U. S. Geological Survey,Plate XXIII.


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120 LEWIS G. WESTGATEsimilareffectmaybe produced n the sea-cliff n time of heavystorms.But theimpactofstorm-waves venwouldseemtobe lesseffective hanthe pressureof glacial ice, and the fact that the cliff base is exposedto the air, and is often, and in some cases always, water-soaked,indicates that the looseningof the rock fragmentswhich are finallydislodgedby the wavesis in realitythe result of weathering. In thecase of streamsflowingover jointed coherentrock, it is difficult tobelievethat even the swift currentsof flood seasons are able to dis-lodge rocks from the stream-beds. The impactof the water is tooslight, and is exertedon the nearlyflat stream-bedat a greatdisad-vantage. The stream is able to sweep away blocks,not too large,which have been loosenedand partlydislodgedby weathering; butit is not easy to believethat the streamis the dislodgingagent.If, as seems certainin the case of stream-erosion,ands seemsprobable n case of wave-erosion,he looseningof the rockfragmentsis the result of weathering, hen pluckingbecomesmerely the firststep in the transportation f d6bris,and is reduced to a vanishingquantitybetweenweatheringand transportation. In that case thestream becomesa transporting nd not a corradingagent; weather-ing becomes the importantfactor in valley-deepening s in valley-widening; while the streamacting as a transporting gent preventsthe processfromclogging. To the extent thatweatheringreplacesplucking by wave-action, he same thing is true of shore erosion.In glacialerosiononlyis plucking eft as a largefactor,and even hereit is not certainthat it is the only factorin joint-blockremoval.