aretes and py pks.pdf - Richmond School District No. 38

GeofileOnlineIntroductionActual location on the planet candetermine whether a mountainousarea is or may have been subject tothe effects of glaciation. Onmountains closer to the Equator, thealtitude at which the effects of icewill become apparent will be greaterthan on mountains at higherlatitudes where, with lower basetemperatures, snow accumulationoccurs at lower levels. Consequently,there are areas of permanent snowand ice at the Equator, wherealtitude permits, but greateraccumulations are found with closerproximity to the Poles.Corries, arêtes and pyramidal peaksare features associated with uplandglaciation and are the subject of thisGeofile.Definition of a corrie andthe location of corries‘Corrie’ is a Scottish term that refersto the armchair- or amphitheatreshapedhollows that are located onthe sheltered upland slopes ofmountainsides affected byglaciation. The word corrie is ananglicisation of the Gaelic word‘coire’, meaning hollow. A corrie isalso known as either a cwm (Welsh)or a cirque (French). The corrie andcirque are similar but a differenceexists between them. A corrie is analmost complete bowl, from whichthe hill steeply slopes away while acirque can also be found at the headof a deep U-shaped valley with arelatively flat floor and shallow, lessdefined hollow.In the Northern Hemisphere theytend to be found predominantlybetween the north-west and southeasternfacing slopes where shadefavours snow accumulation ratherthan snow melt. The most frequentorientation of these corries is anorth-easterly direction, directlyopposite to the dominant winddirection in the northernhemisphere, which is from thesouth-west. This means that they arein sheltered locations and least likelyto experience snow melt from thesemilder winds which bring highertemperatures from their southwesterlyorigin.Snow accumulationWhen glacial conditions dominate,snow accumulation on these slopes isable to exceed annual snow melt andas this allows a snow patch toAPRIL 2007545Corries, arêtes and pyramidal peaksFigure 1: Predominant orientation ofcorriesNFigure 2: Types of glacial erosion and weatheringEROSIONAbrasionPluckingRotationalmovementPaul Shepparddevelop from which a glacier cangrow. As the flakes of snowaccumulate, a process calledalimentation operates, so the snowpatch becomes greater in size. Withfurther alimentation the new snow,with a density of 0.06–0.55, istransformed by compaction, andrepeated melting and re-freezinginto a mass called névé or firn whichhas a density of between 0.72–0.84.Further metamorphism, mostlythrough pressure, transforms thisfirn into glacier ice by furthercompaction and crystallisation witha density of between 0.85–0.91. Thisfinal change into glacier ice occurswhen the névé reaches 30m inthickness. Nagle states that ‘glacierice in bulk is a “granular aggregateof interlocking grains” each being anice crystal’. Between neighbouringcrystals a very thin solution ofchlorides and other salts exists,which lowers the freezing pointaround the crystals and keeps thesolution plastic and able to mouldand manoeuvre over the landscape.This enables the ice to flow.Rocks become embedded in the ice. As the rock laden icemoves under the force of gravity, it acts like sandpaper as itrubs against the base or side of the valley. It can leave striationsor gouge-like indentations in the rocks. These striations can bestudied to work out the actual direction of flow.Ice freezes onto bare rock and the subsequent movementof the ice pulls away the bedrock. It is likely that thisplucking of rocks occurs on those that have already beenloosened by other agents of erosion, such as freeze-thawaction. (See frost shattering).Occurs in a corrie with the downhill movement of icepivoting around a point and overdeepening at that point. Thismovement enables abrasion and plucking to take place.WNWSWKey10403020302010SNo. of corries in agiven directionNESEEWEATHERINGFrost shattering This is mechanical action which occurs in rocks containingcrevices and joints. Once water enters these joints and laterfreezes, an expansion of 9% occurs. Repeated freeze andthaw causes the joints to weaken and widen, which willsubsequently cause pieces of rock to break away or shatterfrom the main body of rock.Solifluction Meaning ‘flowing soil’, refers to action at the base of the(gelifluction orcongelifluction)glacier where ice melt through friction occurs and, due tothe permanently frozen ground beneath it, eroded materialis carried along under the glacier. These lobes of detachedand segregated particles of soil and rock lead to theaccumulation and later deposition of material called boulderclay.Geofile Online © Nelson Thornes 2007GeoFile Series 25 Issue 3

April 2007 no.545 Corries, arêtes and pyramidal peaksLocation determines the speed ofthis transformation into ice whichcan take up to 200 years in dryenvironments such as Greenlandwhere snowfall may only be 10mmper annum, to a shorter time framein warmer and moister climaticzones.Figure 3: Erosional activity in a corrieBergschrund – allows meltwater andmaterial weathered by freeze-thaw toenter body of iceCrevasses (olderbergschrunds)Pivot of rotationTogether with the unique terrain onwhich each individual snow patch isfound, the snow patch graduallyetches itself into the hillside by aprocess called nivation, wherebyfreeze-thaw action, solifluction androtational movement erode back intothe hollow, creating a steepheadwall.ExtendingflowRotationalmovementCrevasses dueto bendingof iceFor a glacier to develop, a prolongedperiod of ice accumulation is needed.This extends into thousands ofyears. Any shorter period would notallow sufficient accumulation of iceto occur or for the ice, which flowsunder gravity, to be replaced at itssource and enable a glacier to flow.Geologists and geomorphologistsstudying mountainous areascontaining corries, arêtes andpyramidal peaks believe that thesefeatures are not the result of oneglacial period but a whole series ofglacial activities. In the British Islesand Northern Europe, over 40 iceadvances have been identified; themost recent glaciation coveringNorthern Britain and NorthernEurope ended only 10,000 years ago.The formation of a corrieA corrie develops on the highermountain slopes where snowaccumulates, compacts, becomes iceand then moves under gravity and isreplaced by new snow and later iceensuring a continuous glacial flow(Figure 3). The former pre-glacialhollow is enlarged and deepened asa result of nivation. Once the iceaccumulated in the glacier hasachieved an optimum weight anddepth and it begins to flowdownhill, rotational movement willenable plucking and abrasion to acton the base of the hollow. Thisaction will cause the deepening ofthe hollow. Material eroded by theice will be transported by the iceeither within its bulk or bymeltwater caused by frictionbetween the solid rock and the iceitself. This will allow the flow ofsome material acting as a protectivelayer and allow further deepening ofthe corrie to take place.Rate of deepeningdepends upon theresistance of bedrockThe surface of the glacier will bemarked with bergschrunds, orcrevasses, which allow meltwaterand rock debris, the result of freezethaw on the exposed mountainsides,to enter the mass of ice. Thebergschrund forms wherecompacted firn pulls away from theice frozen to the rock of theheadwall, leading to headwarderosion and the subsequent erosionand retreat of the mountainside.Similarly, the meltwater aids theprocess of freeze-thaw, while thedebris may be carried in the mass ofthe ice or make its way to the base ofthe glacier where it can act as anabrasive tool.Most of this deepening occurs onthe back wall and the rear of thehollow where the gradient is steeper.Where the glacier levels out and thegradient is less, the erosive powersof the glacier diminish anddeepening of the hollow becomesless. This leads to the developmentof a small lip over which the ice willflow before following the formerriver course or earlier glacial coursedown its valley.These processes are happeningtoday in glacial regions such asSweden where the Kebnekaisecirque glacier occupies a hollow andis acting in the manner that hasbeen described. Other areas of theworld display corries after the icehas disappeared andgeomorphologists and geologistsneed to be able to study both stagesof development to understand theintricacies involved in corrieformation.GeoFile Series 25 Issue 3Fig 545_03 Mac/eps/illustrator 11 s/sNELSON THORNES PUBLISHINGArtist: David Russell IllustrationlipCompressiveflowFlow toU-shapedvalleyThe corries in the ScottishCairngorms have been closely studiedand their formation in the Quaternaryperiod, which in geological termsrelates to the last two million years,suggests that they were a feature oflocalised mountain glaciation. Manyhollows were wholly or partly filledbut their flow away from their sourceenabled larger valley glaciers to form.The corries display signs of multipleglaciation. The glacial deposits ormoraines found in examples of corriessuch as Coire an t-Sneacdha are smallcompared to the size of the corrie andindicate that transportation by the iceand post glacial ice melt haveremoved much of this material. Forin-depth case study of Coire an t-Sneachda visit www.fettes.co./cairngorms/coire%20Sneachda.Studies in the Cairngorms have foundthat 70 to 90% of corries\wereorientated between north and east,matching similar studies relating tocorrie orientation. It was alsosuggested that the current size of thecorries investigated was reached aboutone million years ago and with theonset of later glaciations the valleyglaciers were over-ridden by ice sheetswhich covered the whole area.Evidence suggested for this is theremoval of pinnacles of rock andridges associated with the back wall ofa corrie such as that as seen at Fiacailla’Choire Chais.Geofile Online © Nelson Thornes 2007

April 2007 no.545 Corries, arêtes and pyramidal peaksFigure 4: Extent of ice cover in Europe in the Quaternary Ice AgeLake DistrictSnowdoniaThe Thames–Severn lineCairngormsIcelandwhich separated one river systemfrom another in pre-glacial times.The best examples of arêtes in theUnited Kingdom are Crib Goch inSnowdonia (Grid Reference 605545,O.S. Sheet 115) and Striding Edge onHelvellyn in the English LakeDistrict (Figure 5). At Striding Edge(Grid Reference 345149, OS Sheet 90)the serrated ridge is only a footpathwidth in places and as furtherheadward erosion occurs the ridgeitself will theoretically become lowerand wider in time. One of the twocorries making up the arête contains atypical corrie lake otherwise known asa tarn. This is Red Tarn (GridReference 348153, O.S. sheet 90)which occupies the over-deepenedhollow created by the ice.PyreneesAs can be seen from Figure 4,glaciation in the British Islesextended northwards of a line roughlystretching west-east from the Severnestuary to just north of the Thamesestuary. The mountainous areascontained north of this line includeSnowdonia, the English Lake Districtand the Scottish Cairngorms.Evidence of mountain glaciation andsubsequent corrie development isevident in all three of these regions.Alpserosion and over-deepeningassociated with such activity will leadto the formation of a ridge betweenthe two headwalls of the corries. Thissharp serrated ridge is called an arêteand marks the remains of the formerinterfluve, or broad upland ridge,GeoFile Series 25 Issue 3Fig 545_04 Mac/eps/illustrator 11 s/sNELSON THORNES PUBLISHINGArtist: David Russell IllustrationKeyIce sheetsPyramidal peaksA pyramidal peak is formed whenthree or more glaciers radiateoutwards from one mountain and theheadward erosion and all the erosiveactivities associated with corrieformation are replicated on each sideof the mountain. This leads to theclassic pyramid shape with a sharppeak. The classic pyramidal peak isthe Matterhorn, in the Swiss Alps.Figure 5: Striding Edge, an example of an arête and a corrie lakeWhen studying an Ordnance Surveymap extract of such an area adistinctive pattern of contour linesindicates the presence of a corrie. Aclose pattern of contour lines in analmost crescent shape indicates thesteep back wall of the corrie and anarea lacking in contours indicates thehollow where over-deepeningoccurred. This is often filled with alake or tarn, the alternative name forthe corrie lake. The ‘front’ of thecorrie is marked by a further contourwhere the land rises (the lip),indicating where erosion was less andbehind which the lake can accumulatein post-glacial times.ArêtesIf an upland area supported two rivervalleys flowing in different directionsin pre-glacial times, the onset ofglaciation may lead to thedevelopment of two glaciers. If this isthe case, the actions associated withthe formation of a corrie will beoccurring in more than one locationat the same time. The headwardFigure 6: World Examples of Pyramidal PeaksNameMount AssiniboineInnerdalstarnetShivlingAma DablanCnicht *Location of ‘Pyramidal Peak’‘Matterhorn’ of the Rockies‘Matterhorn’ of Norway‘Matterhorn’ of India‘Matterhorn’ of the Himalayas‘Matterhorn’ of Wales* Perhaps the least convincing example due to its low elevation (2265ft/697m)and lack of steepness and classic pyramid shape.Geofile Online © Nelson Thornes 2007

April 2007 no.545 Corries, arêtes and pyramidal peaksFigure 7: OS location of Cnicht Peak, Snowdonia (sheet 124)Reproduced by permission of Ordnance Survey on behalf of HMSO © Crown copyright 2007. All rights reserved. Ordnance SurveyLicence number 10017284.No definitive examples of pyramidalpeaks exist in the United Kingdomand only four other true pyramidalpeaks have been identified aroundthe world (Figure 6) .The United Kingdom, with itscurrent post-glacial environment,displays in its mountainous regionsexamples of glacial erosion. Corriesin particular are commonplace inSnowdonia, the English LakeDistrict and all of the upland areasfound in Scotland, while arêtes areless common and pyramidal peaksare a rarity, even on a world scale.The mostly resistant igneousmaterials, which dominate theseareas of the United Kingdom wereformed at the plate boundarybetween the Atlantic and Europeanplates and has been affected bymultiple glaciations. The currentsupposed inter-glacial period hasenabled these features to becomevisible and geographers have beenable to interpret how ice acts upon alandscape in upland environmentsand the features produced by suchglacial activity.Further informationOrdnance Survey 1:50,000 mapsheet 115 (Snowdonia)Ordnance Survey 1:25,000 ExplorerMap OL 7 (the English LakesSouth-eastern area)D. Waugh, Geography, An IntegratedApproach, Nelson.J.G.Wilson, Landscape andAtmosphere,Schofield & Sims.G. Nagle, Advanced Geography, OUP.K. Hilton, Process and Pattern inPhysical Geography, Unwin Hyman.FocusQuestions1. How do the different agents of erosion associated with glaciation actin the formation of corries, arêtes and pyramidal peaks?2. Either:a). Using the O.S.map of The English Lake District, locate Hellvellyn andStriding Edge.Make a sketch map of this area and label on your sketch map the arête, RedTarn, a second corrie and indicate the direction of ice flow and thedirection the two corries face.Or,b). Using an O.S. map of an area of glaciation in the UK you have access to,locate and identify examples of corries and arêtes and make a sketch mapindicating the features and adding arrows to indicate the direction of iceflow and the direction the features identified face.3. Explain why arêtes are less common than corries and pyramidal peaksless common than arêtes.Geofile Online © Nelson Thornes 2007