The surface of the inland ice in the Atndalen valley, South-central Norway

BJ0RNA.FOLLESTAD NGU-BU LL 43 8 ,20 01 -PAGE 15

The surface of the inland ice in the Atndalen valley, South-central Norway

BJ0RNA.FOLLESTAD

Follestad,B.A.2001:The surfaceof theinlandicein theAtndalenvalley,Sout h-cent ralNorway, Norges geol ogiske undersekelse,Bulletin438,15-19.

On thebasisof differentfield observat ions,severalaut ho rshave discussedtheformofthe surfaceof theinlandice insouthernNorway.Inthiswork,lateralfeat uresind icating theupperlimitofzonesof meltw ater drainage marked by meltwa te rchannels areused fo r thereconstructionoftwodistinctice surfaces durin gthedeglaciatio n of theval- leyAtndalen.Through themappingof thesefeatu resadetailed image ofthe deglaciatio ncan beobtainedandalso beusedforregi on alcorrelationof theice surface.

BjemA.Foll estad,Norg esgeologiskeundersekelse,N-7491Trondhei m,Norway.

Introduction

The shapeanddistributionof theinlandice in South -centra l Norway have beenthetopic ofa long-sta nding discussion (Hansen1886,Holm sen1915,Mannerfelt 1940,Strern1956, Gjessin g 1960,Soliid& Carlson 1980,Andersen etal.1995, Andersen2000).Duringthedeglaciat ion ofthe area,the ice divide of theinlandice moved to thesouthandallowed the formation of ice-dammedlakes at successively lower eleva- tions (e.g.,Holmsen1915,1960).This view was based on the study of glacial stria ti onsand the formation ofshorelines.

Since Mannerfelt (1940) found no evidence for termi nal morain es in thecirqu evalleys oftheRondaneMountains,he concluded thatthe limit ofglaciation had been far above these mountainsandthatthe area was deglaciate d throu gh a vertica l dow nwast ing of the inland ice. Gjessing (1960) quest ioned the proposed extensionofice-dammedlakes, as he found that a shoreline could beformed in a sublateral and/orsemi-sublateralpositi onin relationto theedge ofthe inlandice.Thisview was supporte dby Foll estad(1997)and Foll estad &Thoresen(1999) who conclu ded thatsublatera l and/or semi-sub lateral erosion and accumulat ions can explain the presenceof shorelines relatedto theUpper and Low er Glornsje in the Alvdal area.

In this paper the distribution of sublateral and/orsemi- sublateral meltwater channels are used to reconstruct the surface of the inland ice duringthe deglaciation in thesur- roundi ngsof thevalleyAtndalen.The map data of Gjessing (1960)andSollid&Carlson(1980)arecombi nedwithalarge data-set,mainly of meltwater channels,collected by the author duringthelast few years.

The Atndalen valley

The topography of the area around Atndalen(Fig.1A)can be character isedas a land block, intowhichtheAtndalenvalley and its trib utaryvalleys,e.g.theLangglu pdalen valley and the Ranabekken valley(for locations;see Fig.2),havebeen eroded.The elevationof the Atndalenvalleyis710m a.s.1.in southernareas(at Stor-Elvdal,Fig.1B)and800ma.s.l,in the northern parts.As the general surfaceof thesurrou nding landblock is1100 - 1200m a.s.l,thisgives areliefamplitu de of400to500 metres for the Atndalen valleyin thenorthern and sout hern areas,respective ly.Prominent peaks, such as e.g.Hoqrond en (2115 m a.s.l.) and Eiriksrudhei (153 5 m a.s.l.), occur, respectively to the west and east of the Atn dalen valley. As the increasing reliefamplitudeis to the southin At ndalen and to the east in the valley Folld alen,a

Fig.1(A) Locationmap ofthearea.(B) Relief map of the land surface between the valleys of Gudbrandsdalen and Glarndalen(lZlsterdalen).

Theterrain mod el showsthat the valleysare tinyfingerspenet ratin qinto alandblockwit ha surface at c.1100 ma.s.l,Someadjace nt moun- tainareas,suchasRondan e,rise abovethisgen- erallevel,and reachupto c.2100ma.s.1.

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Fig2.Sim pli fiedQuate rn arymapofthe surro undi ngsof the Atn dale nvalley.taken from theNGU digitaldatabase.

BJ0RN A.FOLLESTAD

vert ical down wa stingof the inland ice will have resultedin degla ciated areas in thenorth w esterndistri ct (Hje rkinn, Fig.

1.B) which is sit uate d at up to1026ma.s.l.By co m parison, theinland ice was st illsome 300 metr esthi ckin thesouth- ern part s of theAtndalenvall ey and in theAlvdal area.

Meltwater forms in the Atndalen valley

Surficial dep osit s wit h melt wate r forms are extens ive ly develop ed alo ng thesidesof theAtndalen valley (Gjessing 1960, Sollid&Carlson 1980,Follestad 2000), inseveralareas (seeFig. 2forlocati on ).

In the area of Gravskardhogda, amarked upper zoneof melt w ater channe ls can be seen east of the mounta in Eiriksrudh oi(1535 ma.s.l).The upperlimit of these channe ls, whi chsta rtintherun-offpass of Gravskardet and run no rth - west, is at 1482ma.s.1. Theytermin ate afte rc. 2.5 km in an areawit hamarked run-off pass, at c.1396 m a.s.l.Ano t her setof lateral channe lsisseenalo ng theso ut hw esternsideof themountainEirlksrudh ei (1535 ma.s.l.).Thismelt w ater sys- tem sta rtsatc.1330 ma.s.l.,and canbe foll owedtothe no rth for c.3km.Farth erto the north,flushed surfaces along the weste rnside of Sleuk ampenmountain and lateral channel s in the mountainousterrain so ut h of Stat sbu ey l show the cont inuat io n of this later al drainag e to the north.These meltwater channels are commonly morethan 10 m wide and 2-6 m deep, and in placestheyexpo seflushed rock sur- faces.Someof theselateral channelsarehangingalo ng the valley side(Fig.3), whi chind icateaformati oncontempor a- neous wit h thepresenceof an icebodyin themain valley.

Another zo ne of later almelt w ater chan nelsis seen atc.

1140 ma.s.l,in thelow estparts of the depr ession bet w een the mountains Eiriksrudhoi (1535 m a.s.l.) and Indre Kam pe n.These chann els are direct edtoward s amarkedrun- off passfarthernorth,lying at 1130 ma.s.l.The so ut hwa rd continuation sof thesedrainage syst em s indicatethat the

Fig.3.View from thewesternsideof the mountain Eiriksrudhoi to the weste rnsideof At ndalen.Marked meltwaterchanne lsarepresent in thisarea.In the background,the Rimabek ken (R)valley.Arrowsind icate themarkedupperlimit fortheflushedareasalongthewesternside of Atndalen.Photo: B.A.Follestad,1999.

NGU-BULL43 8 , 200 1-PAGE 17

melt w aterdrain agemainl y camefrom the catchmentareas west of Gravskardh og da (1735 m a.s.l.) and from aglacier in theAtndalen valley.

In the tributary valleys of Langglupdalen and Ranabekken, promi ne nt one- and two-sid ed melt w ater channe lsareseen inanarea dominatedbya thick andcon- tinuou s coveroftil l.In Langglu pdalen,theuppermost chan- nel star tsatc.1480ma.s.l,and runsina weste rlydirecti on toward s the run-off pass bet w een the valleys of Langglupdalen and Rondvassdalen sit uated at c. 1418 m a.s.l.,The channelisup to c.10 mwideand,in mostplaces, showsa marked innerbrink.Theouter brin k ofthe channe l is less distinctandgenera llymissing.Exposedbed rock can be seenin thechann el.

Inthe Ranabekkenvalley, severalsyste ms of lateral chan- nels canbe seen(Fig2).The uppermostchann el in thisarea hasanupper limitin the areaof OksliMount ain,atc.1500 m a.s.1. It can be foll ow ed nearly continuou sly in a weste rly direct io n forc.2.5kmwhere it ends at c.1480ma.s.1.eastof the sma ll lake at 1461ma.s.1(Fig.4).Thisgivesagrad ient of c.1%fo r thechanne l.These channelsareeroded in a cov er of block-r ich till which shows well developed so lif luct ion lob es above theupper channe l.The syste m of lateralmelt-

Fig.4.(A) Marked lateral channels along thenorth ernvalleysideofthe mounta in Hogronden.

(B) Asummerpict ureof the uppermostmeltwaterchannel,atc.1520m a.s.l.The meltwa terchannels are generallymorethan5mwide and,in some parts,haveamarked outer edge. Thephoto showslateraland marginalfeatur es.Photo:B.A.Foll estad,1999.

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water channelsterminatesat c. 1300 m a.s.l., c.500m south of Neverbukol len(1345 m a.s.l.).

Inthe cirquevalley north of Hogr ond en,aset of termina l moraines canbe observed .The distal ridge (Fig.5),lyingat c.1650 m a.s.l,along the eastern side of the cirqu e,turn s north and may be followed more or less continuou sly through a small lake(1584m a.s.l.)to the western side of the cirque valley.Lessdistinct ridges are present proximall y to thedistal moraineridge.

Theup permost channelsin thevalleys of Langgl updalen and Ranabekken are dippi ng ina westward direction and show a melt waterdrainag ethrough marked run-offpasses at,respectively,c. 1410 ma.s.l,and1230 ma.s.l,Thus,it can be concluded that the hydro log ical melt water gradi ent was towards the valleys of Rondvassdalen and Deralen,when the surface of the inlandice was at c.1550 m a.s.l,in the Atnda len valley. As the terminal moraine in the nearby cirquevalley is sit uated above andoutsidetheareas which were covered by the inlandiceduringthe formation of the lateral channels,informationfromthe cirque glaciat ion can- notberelatedto thedescribed featuresleft by the ice sheet.

Theweste rnsideof theAtndalenvalley,fromthe areaof Bjornh oll iato thevalley Ranabekken,has beenmapped and described by Gjessing (1960) and later by Sollid &

Kristianse n(1983) and Fol lestad (1999).Asshow ninFig.2, meltwater channelsareonly seensporadically, down to c.

1100 ma.s.l,on the valley slopeeast of Oksli(1779 m a.s.l.) and Storsvulten(1790m a.s.l.).Belo w thisaltitude,there are lateralchann elsandlarge areasdominatedbyflushedrocks.

Aspointedout byGjessing (1960),small canyon sc. 4 metres deep and 2-3 metres wide are present.Thisbelt ,charac- terised by melt watererosion, is dominat ingthe valleyslope downto860 m a.s.l.There,the zoneterminatesinacanyon 5 to 15 metresdeep and up to 20metr eswidewhich can be followed continuously for 2.5 km.Gjes sing (1960) poin t ed out thatlater al channels in thiszonearedominatedby melt- watererosion featuresthatareslopingtow ard sthe nor th(at a gradient ofsome 2%).How ever,theuppermostdistinctive erosion featu resoccurat altitudesof 1100ma.s.1. and 990m a.s.l.,respecti vely, inthe areaof Veslkoll hoi andthe moun- tainPerskampen 8 km to thenorth.Thisgivesa gradientofc.

1%forthe upper lim it betw eenthecontinuous cover oftill and thezonesdominatedby meltwate rero siontowards the north.These lateralforms terminate in the proximal part of the Tverrgje letcanyon, whichis eroded alonga N-S trend in g fracture betw eenPerskampen (1050 m a.s.I.)and Simmelhoi (1356 m a.s.!.).This canyo nismorethan100 mdeep.As sev- eral meltw ater channels at different levels merge in the proximal parts of the canyon,it canbeconclud edthat this canyonhas beena majorlateral drainagechannel. In the dis- talparts of theTverrg j eletcanyon and farth ernorth,marked lateral melt waterchannelsoccuralong the valley side.These channels arecommonly 5-10m wide and 1-2 mdeepand can be follow eddistal lyfor c.1.5 km.

Alo ng the opposite sideofthe Atndalenvalley,marked

BJ0RNA.FOLLESTAD

Fig.5.A disti nctend-moraine(M)occursin thecirque valley northof Hog ron den.Smallerterminalmorainescanbe observe dintheproximal areaof the cirque.Phot o:B.A.Fo llestad,2000.

lateralmeltwaterchannels are seenin thearea of thestream Stor bekken.These features are slopingnorthward sfrom 870 m a.s.l, to 840 m a.s.l.At the mouth of the valley Deralen, where theriver Atni enters theAtnd alen valley,a fan formed of glaciofluvial material is present.Inits centralparts,this deposit is eroded andthree, large, later alterraces occuron bot h sidesof the Atniriverfrom 860m a.s.lto800m a.s.1.On the uppermostterraces,one-ortwo-sid ed meltwat erchan- nels and small kettle holes are seen,indi cating a sub lat- eral/subgl acial form ation in connectionwit h the presence of inland ice in At ndalen. These lateral terracescan be fol- low ed nort heastw ards towards the Sagtj orni Nature Reserve,wheretheygraduallydisappear.Eastof thefarms at Statsbuoyi,marked lateral meltwater channels arepresentat analti tudeof c.900 ma.s.l,These channels,and theterraces north of the stream Stodsbubekken,are thought to have formedmore orless sim ul taneous lywit htheterracesatthe mouth of the Deralen valley,as poi nt ed out by Sollid &

Kristia nsen(1983).

The meltwater deposits in the low er partsof Atndalen are of minor interestin assessing the validi ty of using the meltwater channels for reconstructing the inland ice sur- face.However,theyare shown on the map(Fig.2)in orderto complete the overall pictu re of deglaciationfeatures.

Reconstruction of the ice surface

Asdescribed above,the systems of meltwaterchannels have awide occurrencein the Atndalen region.A reconstruction based onthe upper limitfor these melt watersyste ms(fig.6) showsthe occurrenceof two,clearly separated, icesurfaces, some400met resapart.Inthe valleys of Ranabe kkenand Langglupdalen,theupper system of channels corresponds to aninland ice thicknessof c. 4-500metresinthese hang- ingvalleys abovethe mainAtndalen valley. In the Atndalen valley itself,the thicknessof the inlandicewas c. 800 metres.The lowe rsystem of lateralmeltwaterchann els (along the valley sidesofthe Atndalenvalley)indicate s that the inlandicein the main valley was c.400 metres thick in the areaof Stormyld ingi,anddecreased northwards.

Glaciersof thisthickness inthe tributaryvalleysandin the

BJ0RNA.FOLLESTAD

Fig.6.Atent ati vereconstructionof thesu rfaceof theinlan dicewhen theicesurfacewas sit uated atc.20 m a.s.l inthenorth ern partsof At ndalenvalley.Analt it udeof720 ma.s.1here,in thenorth ernpartsof the At ndalenvalley,will give ice-freeareasin the Follda lenareatowards Hj erkinn.Thereconstruction isbased on aNdipping ice su rfaceofc.l%.

main valley can probably becharacterised as temperate glaciers (Ben & Evans 1998).

In a theoretical model for evaluating the meltwater run- off in the distal parts of a glacier, Piotrowski (1997) and Piotrowski & Talaczyk (1999) have shown that meltwater will haveto drainsubglacially if thesubst rat um drainagecapac- it y is exceeded .A similar view was invoked by Gjessing (1960)in his concept of sheet drainage,when the ice body started to float in a glacially dammed lake. In both models, additional meltwater coming into the subglacial drainage system will have to be drained laterally when the hydraulic transmissivity in the subst rat um and in the fractured ice body is exceeded. Thus, the lateral channels will have formedclose to the surface of theinlandice and will give us thepositionof the correspondingicesurface when usedin a regional model.

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Conclusion

Itis concluded that the regional patternof meltwaterchan- nelscan be usedas a tool for reconstructionof the surface of the inlandice during thedeglaciation.

Acknowledgemen ts

TheGeolog ical Sur veyofNorway andNORPAST research programme su p porte dandmetthe costsofthis st udy.I than kJanneGreteWesche andJohnDelsfortheirassista nceincreat ingFig.2 and Fig.6.1also thank Bjo rn Bergstromand EilevLarsenof theGeologicalSurveyof Norw ay, Trondheim,for their interest and for theirvaluab le sugges tio nsand com me ntsonthe man uscript.Constru ctivecom m ents by thereview - ers,RolfSerensenandAtleNesje,aregreatlyapp reciated.

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