Development of minor late-glacial ice domes east of Oppdal, Central Norway

BJ0RNA.FOLLESTAD NGU-BUL L 441,20 0 3 - PAG E39

Development of minor late-glacial ice domes east of Oppdal, Central Norway

BJ0RNA.FOLLESTAD

Foll estad,B.A.2003:Developm ent of minorlate-glacial icedom es eastofOppdal,Cent ral Norway.Norgesgeologiske undersekelseBulletin441,39-49.

Glacialst riat ionsandice-marginalformssuchas lateralmorainesand melt w ater channelsshowthatamajornorth - westerly -d irecte diceflowinvaded the Oppdal area priortothe Young er Dryas(YD) Chronozone.Throu ghthe main valleyfromOppdal toFagerhaug and Berkak,a northerlyiceflowfollowedthe majornorthw esterly-dir ectedflow andis correlated with the early YDmargi naldeposits in the Sto resarea.A marked,younger,westerly-di rected ice flowfrom alate-glacialdome eastof the Oppdalareaisthought to correspo ndwit h theHoklingenice-margin al depositsdatedtothelate YDChronozoneinthe Trond heimsfjo rddistrict.In the main Oppd al-Faqerhauq-Berkak valley,this younger ice flow turnedtothe southwes t and canbetraced southwards totheOppdal areawhereit joined therem nantsofaglacierin theDrivdalen valley.Alongthe westernsideof themountainAllmannb erget, a prominen tsetof lateral,glacial meltw ater channelsindi catesadrainage which turned westw ardasit met andcoa- lescedwit h the N-S orientated glacierin Drivdalen.Themountainridge linking Allmann ber get(1342ma.s.l.)and Slssih oa(1621m a.s.I.) was anunatak standing up abovethesetwo merging valleyglaciers.

The surfaceoftheinlandicerepresent edbytheKnutsho moraine syste msatc.1300m a.s.l, in theHjer kinn area did not covertheStororkelsjoenlake(1058m a.s.l.),as it term inatedinthe run-offpassbetweentheFolld alen and Einnund alenvalleys, sout heastof theHjerkinn area. Itisthereforeconcludedthatthe Knutsho event , whi chis thought to belongtothe PreborealChronozone,is younger than thiseasterly-situated glacialdomeand thus sup- portsalate YDage for the glacialdomeeast of Oppdal.The lower-lying,lateral, glacial melt waterchannel s ind icate the presenceof ratherpassivelymelt ingvalleyglaciersin theOppdal area.Thus,it isthoughtthatthe deglaciat ionof the mainOppdal-Faqerhauq-Berkak valleyareawasmoreor lesscompl etedbeforethe Knutshe eventoccurred in theHjer kinn area. The glaciofluvialdrain agepatt ernsdemon stratedin this st udy bythemarginal flushedareasand meltwat erchannelshave provided impor tantdata for a detailedmodelof the deglaciati onandthedist ribut ionof glacial dom es inthispartof Nor w ay.

BjemA.Follestad,Norgesgeologiskeundersekeise,N-7491Trondheim,Norway.

Introduction

TheOppdal area(Fig.1)hasa key positioninconnectingthe west ern, northern and eastern parts of cent ral South Norwayduring theWeichselianglaciation.However,only a few authors,e.g.,Lekas(1955),Sollid(1964,1968),Sollid et al.

(198 0),Reite (1990, 1994 )and Sveian etal.(200 1)have dis- cussed the deglacialtion history of the area.The most detaileddescription isthat of Lekas(1955)for thevalleyof Jernbanedalen,which isthe valley between Oppdal and Berkak.Revisionsof the existing mapsat scales of1:100,000 (Sollidet al.1980 )and 1:250,000 (Reit e1990)were carried out duringthe years2000 -20 02.In this paper thenew result s are combined withthe existing map data and used for the esta blishment of a deglaciation model for the Oppdal area.

The followingissueswillbe considered in this contribution:

(1)the extensionand ice-surfacegradient of the inland ice sheet during different stagesof deglaciation,discussed on the basisof the distribution of lateral morainesand lat- eral-/sublateral glacial meltwater forms and glaciofluvial deposits foundat different alt it udes;

(2) attemptsto relatetheseglacialsurfacesto the gener- ally accepted deglaciationmodel inSor-Trendelaq and More

& Romsdalcounties,as described e.g.,by Follestad (1994), Reite(1994)and Sveian et al.(2001).

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Fig. 1.Out line map ofSer-Tronde laqcountyshowingthe location(box) oftheOppdalarea.

NGU-BULL 441,2003 -PAGE 40 BJ0RNA.FOLLESTAD

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Fig.2.A: Thefor mat io n of meltwaterchannels at the margins of a sub- polar glacier according to Em b leto n & King 1975 in Benn & Evans (2002), slightlymodifie dby the author.Asthe marginal stream under- cutstheice edge,the ice margin collapses and sub-marginal drainage willtake place.B:Marked drainage features formed in the area of a run- off pass duringthe deglaciationof an area where nunataks stand up in a c1imato logicallypassiveinland ice.The features are modifiedslightly fro mthe firstversion ,presented in Follestad & Thoresen(1999).Eskers are formedagainst the terrain in the proximal areas of the run-off pass.

Slukaserare formedanddepo sited onthe distal side of the run-offpass.

Here,accumulationwilltake place inthe unsaturated subglacialwater zo ne of the passive inland ice as the drainage velocity gradually decreases and appro acheszero in the saturatedparts ofthe glacier.

Flushed zones and marginal channels will be formed aboveandinto the run-off pass.The lowest possible marginal channel willbe determined by the alt itu de of the run-off pass and will look like a shoreline.

However, sorted materialascribed to wave washi ng in an open ice- dammed lake is missing .

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ier or inland ice inthe lowe r partsof theterrain.Follestad (1997) described marked erosion gulliesand glaciofluvial accumu lations on the distalsideof run-offpasses,and con- cluded that onlya subglacialdrainagethrou ghthe run-off passescouldexplain these formations.In someofthe run- off passeshedescribedglacio fluvial accum ulationsat differ- entaltitudesdownhill.Asthe altitu deof thesedepositsin a distalposit ionfrom a run-off passwill bedet erminedbythe subglacialwater tablein thedistal areas,afallingwatertab le will create a new, lower lying ,accumulationbase(Fig.2B).

Glacialstriation sand erratic blocks have been used for the locatio n ofice dividessinceHansen (1895)int roduced the term.In com bi nationwit hreconstru ctionsbasedon the marginal moraines,glacial striations have been used for establishing the positions of ice divide sand mig ration of

Deglaciation models based on ice- mar9inal moraines and systems of

qlaclofluvial meltwater channels

Ice-marginal tnoraines,such as lateralmoraines and end tnoraines,havebeenusedfor therecon struct ion of the ice recession in Norway,since Andersen (1954) made the first reconst ruct ionof the surfa ce of the glacierin Lysefjord en in Sout hw est Norway based onlateral moraines.Later investi- gationshave usedthis method and shownitsvalueformod- elling the exte nsionof the glaciersand the expected posi- tions of the ice margins in southern,western and northern Nor w ay;e.g., Andersen (1960,1968, 1975),Soliid (1964), Anundsen & Simonsen (1968), Folle stad (1972), Faret h (1987),Reite(1990,1994),and Sveian et al.(2001).

Patternsofmarginalsystemsofmeltwaterchannelsare, togetherwit h glacial striationsthe majorsou rces ofinforma- tion for reconst ruction of icedivide s and run-offmodel s in the Swedishparts of theScandinavian mountainchain(e.g., Kleman&Borgstrom 1995).

In Nor w ay,Hansen(1886)described the distrib ution of 'shorelines' in ice-dammed lakes and established the first model of 'glacial lakes' formed sout h of the present-day water divide. Later geoscientists have used this concept (e.g., Hol msen 1915).The value of meltwa ter channels (Sw edish: skvalren nor)was first recognised andadd edto the deglaciation model by Mannerfelt(1940).He found that lat- eralchann els and'nunatak lines'proved thatthepeaks in the Rondane Mountains were real nunataks and stoo d up above a clima tologically passive inland ice.Later,Gjessing (1960) introducedthe concept of subglacialdrainage and a corresponding water tablein the ice/ice-tunnelsystems,and concluded that subglacial run- off passeshave a geomor- phologicaleffect on the landformsin a proximal direction. He though t thatthis concept could beused to explain the marked geomorphologicalformsin SoutheastNorw ay.This idea precipitatedarath er violentdebate and his model was rej ect edby,e.g.,Hopp e(1960)andHol m sen(1967).

Piotrowski (1997) and Piotro wski et al.(1999) reported thatsubg lacial drain ageisthemain factor in theform at ion oftunnel valleys and the distribution of fine-gra ined sedi- ments inareasproximal to the glacier front.Benn & Evans (1998)discussed the formation of latera l meltwater chan- nelsat the margins of a subpolar glacier.They concluded thattheseformation scould develop inamarginalor even sub-marginalposition(Fig.2A).

Follestad (1997, 2000) found that meltwater channels descri bed as 'shorelines' and formed in open glacially dammed lakes(e.g., Holmsen 1915,Sollid &Carlson 1980), generallycould be explainedasmelt w at er channels form ed in the proximalareas to run-off passes.These'shoreli nes'are, inmanyareas,foundtorep resentthe lowestlimi tforflushed zones alo ng the valley as described by Follest ad (2000).

These flushedzones reach up to 10-50m above the'shore- line' and can only be explai ned by vio lent glacio fluvial drainage betweenthehillsideand a passiveremainingglac-

BJ0RN A.FOLLESTAD NGU-BU LL441, 2003- PAGE 41

youngerglacial domes and icedividesduringthe deglacia- tion in Norway,e.g.,Rye & Follestad (1972),Vorren (1973), Bergersen &Garnes(1981),Olsen (1983,1985)and Nesjeet al.(1 988).

Topography of the Oppdal area

Barrett (1900) and Reusch (1905)recognised tributary val- leys ofthe'fishhooktype'andnot iceable ledgesin the valley sides in an olderpreglacialrelief.Holtedahl(1949)measured and describedtheseledges westof Oppd alup to c.530m a.s.l.:and hecon sidered they were ofa fluvial originfrom beforethe beginningof the IceAge.Duringthe glacial cycle of the IceAge a youngervalleysyst em,know nas theDriva- Sunndalen valley, was erodedinto theoldpreglaci al valley.

Thisvalleybecame an important topographicalfeat urefor drainage ofthe inlandiceina westwarddirection(Holtedahl 1953,1960).

From this description of the terrain it might be con- cluded that the old terrain,consisti ng ofthe high plateau 1100-1300 m a.s.l. and outstanding peaks such as, e.g., Stororkelhoa 1524 m a.s.1. and Snohetta 2286 m a.s.l.,cut through by an old major valley system of fluvial origin before thebeginning of the IceAge(HoltedahI 1949),wasan import ant facto rin the development of icesurfacesduring thedifferent phasesof glaciation.Thus, this old landscape andthe youngerDriva-Sunndalen valley, togetherwiththe remaining surface of the inland ice at the end of the Weichselianglaciation, and the main routes of the atmos- phericcyclones with their associatedheat flowand precipi- tation, would have hadto haveacted togethe rto determine the developmentof deglaciation in theOppdal area.

Marginal moraines and meltwater channels in the Oppdal area

As noted above,l.ekas (1955) hasgivena detaileddescrip- tion of the surficial depo sits in valley of Jernbanedalen, whichinthispaper iscalled theOppdal-Faqerhauq-Berkak valley.He found thata number of Quaternary ice deposits such as the marginalterracesalong the southwestern side of the valley, the nearlyhorizontalterraceinthe lower part

of thevalley,and thegreat eskerin the bottom ofthevalley, are all formations which show evidence of a westerly - directed glaciofluvialdrainage.Theseforms and deposit s,as wellasthose on thehigh plateau and in thevalley eastof the Oppdal-Faqerhauq-Berk akvalley, seemto fit into apat- tern whichwill be further document edhere and used for a tentativereconstruction of themarkedice surfaces present inthe area.

Marginal moraines in the Oppdal area

The areawest of Oppd al-Berkak can be characterisedas a glacially sculptured landscape wit h the pron oun ced Oppdal-Faqerhauq-Berkak valley cutting through an area surrounded by mountains,the highest reaching up to c.

1560 ma.s.l,

During the glacial cycles,cirques and cirques valleys were formed along the sides of the main Sunndalen- Oppdal-Drivd alen valley. In the Oppdal area, Holtedahl (1949) found no evidence for a younger cirque glaciation and he conclud edthat the glaciation limit wasfar above these cirquesduringtheperiod of deglaciation.On the map compiled by Sollid et al. (1980), distinct younger cirque morainesare show n in the west ern part s of theOppdal area.

Such feature shave also beendescribedby Foll estad(1994) from theadj acent areasofTro llheime n(Fig.1).

On the west ern side of the Oppdal-Faqerhau q-Berkak valley,lateralmoraines and lateralmeltwater channels have been recordedby Sollidetal.(1980) andremappedby NGU in 2002.The highest and most westerly sit uated lateral moraine ridge(Figs.3and4)is very distinc tand reachesan altitudeof c.1320 ma.s.l. in the sout hern part ofthevalley side westof the lakeStavsjoen(1114 m a.s.l), Here,marked later almorainesoccuras abelt of ridges,whichcanbe fol- lowed northwards for c. 3 km.These forms termin ate at c.1220 m a.s.l.in the lakeStavsjoen areawheresomeofthe moraineridgesareup to 5 m high.Farth er downhill,marked flushed areasandlateralchannels occur.Theyallslope ina northward direction towards a marked run-off pass at c.

1100 m a.s.1.west of the mount ainStorhea(1167 m a.s.I.).A small,but ratherdistinctive,terrace-shaped accumulat ionof

Fig.3.Aglacierprofile istent atively recon- struc ted through the main Oppdal- Fagerhaug-Berkak valley and ext rapolated totheStorasareain thenor th ernpartof the Meldalen valley.On the basis of lateral moraines and lateral meltwater channels, the early/middleVD-icesurface(F1)and the correlatedice surface(F2)in theOlmdalen valleyand theLate YDice surface(F3)are indicated. It can be noted that the final drainageshownby'arrows'fromthebottom ofInnerdalenwastowardstheOppdal area.

Thealtit udeof thisfeature willhavebeen determined bythe botto m of therun-off passinthemainOppdal- Fagerhaug valley (570ma.s.l),

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NGU-BULL 441,2003 - PAGE 42 BJ0RNA.FOLLESTAD

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Fig. 4.Distribu tion of ice-marginal forma- tionssuchas lateral moraine s, lateral mar- ginal/submarginalmeltwaterchannels and flushed areas of bedrock in the Oppdal area.

glaciofl uvialmaterial islocatedonthe nort hernside ofthis run-offpass(Fig.4),proving that meltwater drainagewas towards the nor t h.These features, togeth er wit h lat eral morainesfarthe r nor t h,areplott ed in aprofilefollowing the main Oppdal-Fagerhaug- Berkak valley to the Storasarea (Fig.3).

As show n inFig.3,these high-lyingmoraine ridge smay represent a correspo nd ing surface for a glacier in the Oppdal-Fagerhaug area which may have reached as far northwestas Storas,as suggested by Reite(1990).As thedis-

tancetothe Storasdeposit is some 45 km from Oppdal vil- lage.this indicatesa meanice-surfacegradien tof 28 m/km . Theterraingradientover the same distanceisc.11 m/km.

giving a relative gradient of c. 17 m/km.This is a steep.

thoug hacceptable,terrain-guided gradient for glaciersur- facesofYD age(e.g.•Faret h 1987,Follestad 1972).

The markedtrim line(Fig.SA-B) seenalong the south- weste rnsideof theOlmd alen valley rises from c. 1300 m a.s.l.

along the northern valley side of the mountain Sissihoa (1570 m a.s.1) to c. 1400 ma.s.l,nearKrinqsalen(Fig.4).This

BJ0RNA.FOLLESTAD

indicates that tribut ary glaciersentered the mainOppdal- Fagerhaug-Berkakvalley from the Olmdalen,Langveldalen and Innerdalen areas.Asindicated by the above-mentioned trimline, theice flowed ina northerly direct ion and origi- nated from aninland icesheet with a surface whichreached up to c. 1400 m a.s.1 in the easte rn plateau areas above the Veslnesa mountain (Fig.4).The lateral moraine (c.1020 m a.s.l.)at themouth of Langveldalen(Fig.6,seeFig.4for loca- tion),and morainesand margin almeltwaterchannelsalong the western side of themain Oppdal-Faqerhauq-Berkakval- ley(Figs.3 and 4), ind icat e a northerly-slopingglacier in the main valley as long asthe surface of the inland ice sti ll reached above c. 1000 m a.s.1.Below an altitudeof c.l 000 m a.s.l.,a marked change in thedrainage directionof the melt- waterchannels can beobserved, e.g.,in thelakeStavsjoen area (Figs. 3 and 4).These sout herly and sout hw este rly

NGU-B ULL 441,200 3 - PA GE43

c

Fig.5.A:A SPOT sate llite image of the studied area inthe Oppdal- Stororkelsjoendistrict.Marked northerly-d irecteddrumlinfeatures can beseenin thecentreof the image.Tothe right there are youngerand westerly-directed drumlin features.Fo rnames,seeFig.4.The SPOT image isprovidedby St ate nskartverk,Arenda l,and has been colour- coded byJohn Dehls (NGU). B:A markedtrimlineisfalling fromc.1400 ma.s.1. in the sout h(leftKringsalen,1850ma.s.l.)toc.1300m a.s.1.in the easte rnvalley slope ofSissihoa mou nt ain(cent ral partsof the photo).ln theforegro undamarkederosionalcanyonhascut through a glacioflu- vialdeposit.Theupperlimit forthisdeposit is atc.1020 ma.s.1.Photo- BA Follestad,2001.C:Marginal lateralmeltw aterchannels along the westernvalleyslopeof Olmdalen indicatea vertica l downmelti ngofthe inland iceduring alaterphaseofthedeglaciation.The photoistaken fromStororkelhea (1524ma.s.I.).TheStoror kelsj eenlake canbe seenin the middlepartofthephoto.Phot o-BAFollestad,2001.

drainage feature s,seen along both the western and the eastern valley sides,will bedescribe danddiscussedbelow.

Marginal channels in the

Stororkelsjeen

area The Storork elsjoen area (Fig. 4) east of the Oppdal- Fagerhaug-Berkak valley is dominated by a high plateau some 1100 m a.s.l.,wit h outstanding mountain s such as Stororkelhoa(1524 m a.s.I) andKrinqsalen (1580 ma.s.l.).

The basin between these mountai ns is occupied by the Stororkelsjoenlake.To the sout h,amarkedvalleyrunsinto thelakeFundin areain the countyofHedmark.Northof the lake Stororkelsjoentheterraingenerally slopes towards the north.

Thisgeomorphology expo ses several, moreor lessdis- tinct,run-off passes where marginal lateral channel sand related features, such asflushed surfaces of bedrock,are

NGU-BULL 441 ,2003 - PAGE 44 BJ0RNA.FOLLESTA0

Fig.6.A markedlateral moraine at1020 m a.s.l.east of the Langveldalen valley.The lateral moraine shows a falling gradie nt into the valley of Langveldalen.Photo -BA Follestad,2000.

quite common.An outline of these lateral systems is pre- sented below.More comprehen sivedescriptionsare given inanAppendixfile whichcan be obtai nedfrom the aut hor upon request.

Inthe Veslnosa Mountain(1382ma.s.l.)area ,two sets of lateralmeltwater drainagesystems have been recorded (Fig.4).Theseshow a northerly-directed,marginalglacioflu- vialdrainagealong the easternside of Veslnosathrough a run-off pass sit uated at 1360 m a.s.!. (Fig.9 and Append ix file).These drainage systems can be followedto the western sideof Veslnosa where markedglaciofluvialdeposits occur at c. 1220 m a.s.!.lnthe textthatfollows,these format ions are referred to as drainage systems(1a)and(1b)(Fig.4).

Inthe Olmdalen area marked meltwater chann els are present along the western sideof the valley(Fig. SA and C).

These lateralchannels reach up to c. 1300 ma.s.l,and can be followedmoreor less continuously into northwesternpart s of the Olmdalen valley.Thegradient of thesechannelsis c.13 m/km.Inthe furthe r text below,thesemarginalforms are referredto as (2a)(Fig.4).Moredetaileddescripti on s ofthese drainagesystem saregiven in the Appendix.

Along the western side of the Unndalen valley(Fig.4), prominentmarginal channelsareseenwhich are point ing into the marked run-off pass situatedat 1192 m a.s.1.(Fig. 7).

The run-off pass, which is 3-400 m wide in its upp er parts, haslarge areasof flushed rocks and welldevelope d mar- ginal channels. In the bottomof this run-off pass amarked 10-20 m-broad and 5-10 m-deep canyon can be follo wed downhill in a distal direct ion for some4 km. Tw o distinct accumulations are seenalong the edges of the canyon at altitudesof 1120ma.s.land 880 ma.s.l,Theupper one can be followedcontinuou slydownto 1040 ma.s.l,whereit ter- minate s(Figs.8and SA).Both depositshaverather irregul ar surfaces wit h severaldepressions,hereinterpretedaskettle holes.Minor sectionsexpo sepoorly sorted gravelly sand.In

the textthat follows,these forms arereferred to as drainage systems(2a) and (2b)(Fig.4) .

Fromthedrainage systems (1a),(1b),(2a)and(2b) it can be concluded that the marginal channels and melt water deposits,characterised by irregulariti esin the surfaceand poorly stratifiedglaciofluvialmaterial,were formed inclose cont act with the remaining inland icein the plateau area around Storor kelsj oen and in the main valley of Oppdal- Faqerhauq-Berkak. This indicates that the correspondi ng surfa ce of the inland ice on the plateau area had to be situ- ated atleast 1360 m a.s.1for meltwate rto be ableto enter the mainvalley throughthisrun-off pass on the mountain Veslnosa(1382m a.s.l).Theposition and formationof the lat- eral or sub-lateral kame terraces(at c.1080 m a.s.l.) in the Oppdal-Faqerhau q-Berkak areain a subglacialwater table in the mainvalley glacieror in an ice-dammed lake givesthe approximate altitude ofthe surface of the mainvalley glac- ier.Fromthe irregulariti esin the surface and the poorlystrat- ified glacioflu vial depositsat,respectively,1120 m a.s.l.and 880ma.s.l,(drainage systems (2a)and (2b)),itcan be con- cluded that the associated marginal channels and the weakly strat ified and poorly sorted glaciofluvia l deposits accum ulatedin a subglacial water table in contactwit h the margi n of a glacier.Asthe prominent upper deposit corre- spond ing to the above-described feat ures formed by drainage along the easte rnside ofVeslnesa,this accumula- tion isthoug ht to have been depositedin approximatelythe same wate rtable.Thisindicatesthat the plateau glacier in the Stororkelsj oen basin had an outlet 'glacier' flowing through the valleys of Olmdalen and Langfallsbekkvelvet (Fig.4).Wherethese outlet glaciersreachedthe valley glacier in the Oppdal-Faqerhauq- Berkak valley and the general water table in thisglacier,subglacialaccumulations would have formed.Thelowermost deposit in the Olmda len valley shows,furt her,thatthe plateauglacierand the outletglacier in Olmdalen existed evenwhenthe generalwatertable in

BJ0RNA.FOLLESTAD

--_.-

^NGU-BULL441, 2003 - PAGE45

Fig.7.The marked run-offpass 1192ma.s.l,betweenthe valleysofUnndalenand Olmdalen.The photoistaken from the passarea,looking towards southeast.Photo-BA Follestad, 2000.

themainvalleyglacierof theOppdal-Faqerhauq -Berkakval- leyhad beenlow ered toc.800 m a.s.1.

Thethreerun-offpasses west ofStororkelsjoen (7058m a.s.l.) have inletssit uate dat c.1180ma.s.1.alo ng the ridg e bet w eentheUnndalenvalley and Storo rkelsj een(Fig.4).This altitudeisonly a few metres lowerthanthe run-off pass at 1192 ma.s.l,nor thward tow ard sOlmd alen.Thus,we might conclud ethatglaciofluvial melt water in theproxim al areas of the run-offpassat 1192 ma.s.l,turned tothesout hand east befo rethe run-off passwasdeg laciated,andfound its wayalong theeasternsideoftheVeslne sa(1382 ma.s.l.),In the text below,theseform s are referredto asdrainage sys- tem (2c).

Alo ng thevalleyside northeastof Rundhea mountain (7374ma.s.l.),markedflushed rocksurfaces and margin al channels are present (Fig. 4).These system scan befollowed continuouslyfor 2-2.5 kmtothenort hwhereamarkedrun- off pass (1090 m a.s.l.)isrecor de d between Skaret (1178m

a.s.1) andVeslhoa(1092ma.s.l.).Themarkedcanyonon the distalside ofthe run-offpass is6-10 m deep andcanbefol- low ed downhill over several kilom et res. Broad zones of flu shed bedrock surface s occur alo ng the sides of the canyon,indi cati ng thatthere was a rath erviolent subglacial drainage in this area.An esker (slukas) up to 10 m high is seenat980 ma.s.l.andcanbefo llowed 300mdownhillin a so ut hw esterlydirect ion ,term in at in gat940 ma.s.l,Fro m this we can conclude that the accum ulation of theslukas took placein the unsaturated zoneof theicebod y(cf.Fig 2B)in themainOppdal-Faqerhauq-Berkakvalley.

Farth erto the southwest,the marked and nearly hori- zontal marg inal chann el systems conti nue. The distin ct flushed zo ne east ofVeslhea(1092 m a.s.l.) bends north- westwa rds and later sout hwestwards and than continues, togeth er wit h the above- d escribed marginalsystems,to the southwe st. Asdescribed by l.o ka s(1955), marked marg inal meltwa te rforms ,suc has lateral channels andterraces, are

Fig.8.lnthedistalparts of the run-off passat 1192ma.s.l.twomarked terrace-shapedaccumulations areseen.Smallsectionsthroughtheflanksof thesedepositsshow poorlysorted sandy gravel. Pitsandsmallerdepressionsonthesurfacesare interpretedas kett le holes.Photo-BA Follestad, 2001.

NGU-BULL441,2003 - PAGE 46

seen along thewesternsideofthe valleyoftheriverTinnia.

These lateral channelsand terraces occur from the upper parts of thevalleysid edown to the botto mofthevalley.It seems reasonableto relat ethe highest-lying formsto sub- glacial or subaerial melt water drainage, which came throughtherun-off pass at 1090 ma.s.l.intheSkaret area.

Thesedrainagefeatures can befollowedmore orlesscon- tinuou slyina southwesterlydirection,asa set of marginal channels,tothe areawest ofOlmdalen (Fig.4).There,these channels disapp ear in the steepvalley sideatc.800m a.s.l.

These formsarereferred to below asdrainage systems(3a) and(3b-c)(Fig. 4).

Inthe areas northofthevestorketheamountain,adis- tinct flushed zonemorethan 200 mwide occursalong the nort hernsideofthemountain.This flushedzonecontin ues into an areaof ridgesand terraces.Small sectionsthrough thesedeposit sexpose strat ifiedand poorl ysorted glacioflu- vial materia l which, toget her wit h the irregulariti esin the surfaces,isconsidered to favoura subglacialorigin.

Fart her to the west, north of the road to the farm Pikhaugsetrin,a marked belt of ridges andterraces is seen.

These featurescan be follo wedmoreor lesscontinuous lyto Pikhaugsetrin wherethe continuation of the belt is shown aspronounced ridgesalong the slope down to the farm.

Sect ionsthrough some ofthe ridgesshow sortedglacioflu- vialmaterial.On the valley sidewest of Pikhaugsetrin, ridges more than 5 m high,interpreted as eskers,continue upthe hilltowardsthewest. These ridges arepointingdirectlyinto the run-offpassareawest ofVeslhoa (1092ma.s.l).lnthe dis- cussion below,theseformsarereferred to asdrainage sys- tem(4a)(Fig. 4).Themarked eskersystem along the north - western side of Stororkelhoa can be follow ed over a dis- tanceofsome2 km to the sout hwest.There,marked lateral terraces with kettle holesareseen.Theform of theseter- racesshow s that theyweredeposited bymeltwater enter- ingtheareaalongthe southwesternside of Sto rorkelsjoen.

Northwestof theseterraces asetoflatera lchanne ls begins to takeshape.Thesechannelsbendto the westand nort h-

Fig.9.Flushed zone(left)and a markederosionridg einanareawitha cont inuo us tillcover,inthebott om oftheLangveldalen valley.Phot o- BA Foll estad,2001.

BHoRNA.FOLLESTAD

west towards the Pikhaugset rin farm.In the text below, theseformsarereferred toas drainagesyste m(4b) (Fig. 4).

Inthe valleybetween Veslorkelhoa (1291 m a.s.l) and Stororkelhoa(1524 m a.s.l.),markedterracesare present.The elevati on ofthe highe stterraceis atc.1030 m a.s.l.This alti- tudecorrespon ds with the altitude of a marked run-offpass westof theBjorkset ra farms(Fig.4),which drained the inland ice towardsthe Langveldalenvalley.These deposits,and the markedridges fart herup the valley,were formedin a distal posit ion to a marked run-offpass at 1095 m a.s.l,abou t 1.5 km nort hwestofVeslorkelsjoen(1010 ma.s.l.),Thelowerter- racesinthis valleytow ards Bjorksetrawereformedwhenthe Innerdalenvalleystartedto drainthe area.Theseforms are referredto asdrainagesystem(Sa) (Fig. 4).

Themeltwaterformsin thevalleyof Langveldalenarerep- resented mainly bythemarked flushedareas(Fig.9)inthe southern partsof thevalleyandthe promin entfan-shaped depo sitsof glaciofluvial materialin the mouthofthe valley.

Theseformsindicate thatthe last drainage of the inland ice in the Stororkelsj oen basin took place through this valley befor ethe ice became so thin that it broke up and waseven- tually drained out through the Kvikne area(Fig.1). In the nor th ern part s of the Langvella valley,the highest-lyingter- races wit hchannelsandkettleholes spread out from some 570 m a.s.l.in a westerly direction.Inthecent ral parts the Langvellariver has erode d this deposit,and severallower- lyingterraces havebeen formed.A pit in the lowerpart of the deposit exposes more than 30 m of inclined layers of sand and gravel.

Twomarkedterrace-shapeddeposit s are seen fart herto thenort heast atc.560m a.s.l.These deposits havehorizontal surfacesup to 50 m wide wit h small drainage channels pointin g to the southwest, asdescrib ed by l.ekas(1955).

Depressionsofdifferentsizeare presenton these surfaces and theseareinterpreted askettleholes.Theseform at ion s, andthe largeareasofsiltand finesandinthe valley bottom in the proximal areas of the run-off pass (560 m a.s.l.), demonstrateanice damming anda late branch of a glacier flow ing into this area from the Berkak valley.In the text below,these formson the eastern side of the main Oppdal- Fagerhaug-Berkak valleyare referredto as drainagesystems (6a) and(6b) (Fig 4).

Fromthedrainage systems(l a-c),(4a-b) and(Sa-b),it can be concluded that largeamounts of glaciofluvialmeltwater from areasto the southeast drained through the run-off passes intheStororkelsj oen area.Thewesterly drainagecan, in mostcases,onlybeexplainedby a subglacial and/orsub- marginal drainage determin edby a higher-lyingice surface tothenorth andnort heast. The marked margin al channels and terraces along the eastern side of the Oppdal- Fagerhaug-Berkak valley were formedduringthis period of glaciofluvialdrainage,as alreadysuggested byl.ekas(1955).

From theselate ralforms it canfurtherbeconcludedthatthe surface of the inland icein the Oppdal-Faqerhauq-Berkak areawas atleastatc.800 m a.s.!. in the areaeast of Oppdal

BJ0RNA.FOLLESTAO

when this sout hwest erly drainag eoftheinland ice east of the mainvalley st arte d.Asillust rat ed in Fig 4,this marked drainage conti nued throu gh the valley of Langveldalen evenwhenthesurface of the inland ice was lowered byc.

100 metres in themain Oppdal-Faqerhauq-B erkakvalley(cf.

(6a),Fig.4).

From the draina ge systems(6a) and(6b),it can becon- cluded that during a final phasemeltwater drained in a westerly direct ion , and the altitude for this drainage was determined by therun-offpass atc.560 m a.s.l. This is also the present watershedin themain valleytoday.Finally,the lastrem nant of the inland icebroke up in the Berkak area and,as the ice dam was broken,the present-daydrain age system wasesta b lished(6b).

Discussion

Foll estad(1994), Reite(1994),Andersen (2000)and Sveian&

Re(2001)have discussed the extensionand distribut ion of the inland ice in the areas around Oppdal.Reite (1994) and Andersen (2000) indicated that the inland ice was rat her extensive in the northeastern parts of Trollheimen (Fig. 1) and the Meldal valley, northwest of the main Oppdal- Faqerhauq-Berkak valley,during the YD event,whic h was rep resente dby theTautra-Tlllet-Storasice-marginaldeposits inSer-Trende laq. They concludedthat the ice advancedur- ingthis event reached as far nort has theStoras depositsin the northern partsof the Meldal valley.These deposits,and a possible further westward ext ension of comparable deposits which were mentioned but not described for the northern valleysin the Trollheimen mountains,were related tothe late Allerod/early YD chronozone.According to Reite (1994), the ice-ma rgi nal deposits in the Stere n-Budalen- Berkak area were related to the foll ow ing event, the Hokl in gen Substage,whichhas previously been datedto the late YD Chronozone,c.10,300- 10AOO14C yearsB.P.(Sollid&

Reite 1983).

In the western parts of Trollheimen and in the Todalen valley (Fig. 1),lateral andterminalmoraines related to valley andcirqueglaciers have been describedbyFoll estad (1994).

Theseformat io nsare not dated,but have beenassigned a YD age.In the northeastern partsof Trollhei men,Foll estad (1994) found no evidence for thepresence of valley glaciers, e.g., in the 0vstebodal en,Vinddoldal enand Folldalenvalleys (Fig.1).In the upper, western partsof 0vstebodalen and Vinddoldalen, terminalmorain eshave beendescribed and are interpreted to relate to an old cirque glaciatio n (Follestad1994).Asthis took place most probablyduringVD, this is taken as evidence for a more or less complete deglaciation of the main 0vstebodalen and Vinddolda len valleys.

Noevidence for the presenceof a huge valleyglacierhas sofar been found eit herin Folldalen or east of theMeldal valley, assuggested forthe continuation ofthe Tautra-Tiller- Srordsicemarginal deposits(Reit e1994).Thus,it is concluded that the most probable continuation of the Taut ra-Tlll er-

NGU -B UL L 441,2003 - PA G E47

\

Mere S Romsdal

A

N

1

^M0re&

-.

Romsdal

0, 1I5 km

Fig.10.A tentativereconstructionof thetwomainice-stream phases during thedeglaciationofthe area:A:The early/middleYDicesurface (F1andF2j, whichreachedthe Sto ras marginaldeposits in the northern part s of the Meldalenvalley.B:Thelate YDice surface (F3)dominatedby a westerly-moving ice-stream from an easterly -situated dome.This ice st reamin themain Oppdal-Berkakvalleywasdivided intotwo major stream s,whichcan be demon strated intheOppd alandMeldalareas, respecti vely.

Storas ice-marginal deposits has to befound in the areas east of the Trollheimen mountains(Fig.1GA).Thegradient used to correlate the Storas deposits in the Meldal valley with the lateral moraines at c. 1200-1300 m a.s.l.in the Oppda larea is rat hersteepand,therefore, margina ldeposits occurring farth ernorth in the Orkdalenvalleymightbe con- sidered as alternativemarginal positions for thisvalleyglac- ier. Carbon datings recentlycarried out on shellfragments from glaciofluvialmarginal depositsat Orkland(12km nort h of Storas)and Kvale(17km north of Storas) (Lars Olsen,pers.

comm. 2003), dated respectively to 11,540 +/- 70 and 11,280+/- 6014C years Bp,indicate a late Allered - early YD age forthesedeposits.In thiscase thegradientforthesu r- face of the valleyglacierwouldbec.20 m/kmor,reduced for

NGU-BULL 441, 2003 - PAGE 48

the terrain gradient c. 10 m/km, which is consideredaccept- able fora YDvalley glacier inthis area.How ever,the magni- tudeof the Storasmarginal depositandthefact thatpossi- blecorrelative margi nal deposit sare notfoun d eit her to the eastor to the west of Storasfor a weste rly-d irecte d branch ofaglacierintheMeld alarea,favour theinterpr etat ionthat the Storas margina l deposits are representative of the Tautra-Tiller-Storas ice-marginal event in the Meldalen- Orkdalen area, as alsosuggested by Mangerud(2003). This will also help to explain the described lack of marginal deposits in the main northerly-d irected valleys of Trollheimen (Fig.10).Even though the lateral moraines are few and rather discontinuously distributed in the st udied part of theOppdal area, a reconstructionbased on the gra- dientused in Fig.3 is considere daccepta ble. Thus,anice sur- face atc.1300m a.s.l.in the central part s of the mainvalleyin theFagerh aug areaisthou ghtto correlatewit h the Storas margin al depositsinthe nort hern part s ofthe Melda lenval- ley.

The marked trim line(Fig. 5A-B)betweena relativelycon- tinuous coverof till and the above-lyingareas dom inatedby block fieldsin Olmdalen might,furthermore,be roughlysyn - chronous wit h this c. 1300m surface in the main Oppdal- Faqerhauq-Berkakvalley.This imp lies that the comparable surface ofthe inland ice in the Stororkelsjeen area was at c.1400ma.s.l,and the peaks of themountains Sisslhoa(1570 ma.s.l.),Krinqselen(1580 ma.s.1)andStororkelhea (1524 m a.s.l.)were nunatak s(Fig. 10A).

Sveian &R0(2001)describ ed marginal deposits charac- terising the younger Vuku event in the Alen area.These deposit s were formed during the early Preboreal Chronozone (Reite 1994).at a time when the ice surface reached more than 1000ma.s.l.in thecentra leastern areas of Ser-Trondelaqcounty(Sveian &R02001).In this connec- tionit can be noted that the west erly oriented striationson the mounta inat1160 ma.s.l,andthedrumlins in the areas westofBj orksetrafarm suggestthat a youngerwesterly ice flow affec te d this mounta in area in a later phase of the deglaciati on (seeFig. 3).This ice moveme ntisrepr esented by younge r westerly and southw esterly stria tions on the Vorra mountain (928 m a.s.l). This suggests that the ice movement through the main valley of Oppdal-Fagerhaug- Berkak valley turned to the southwest, as show n by the sout hw est erly trending lateral moraines in the Stavsj oen lake area and the lateral,sout hw est -directed,margi nal chan- nelsbelo wc. 1000 m a.s.l.(Fig s.3-4).This givesan ice thick- ness in the mainvalley of c. 500mwhen the ice flowturn ed to thesout hwestin theFag erhaug area.Moreover, the alt i- tude of theice surfaceofthe contempora ry northw esterly- directed iceflow in the Berkak areawasclo seto c.1000 m a.s.l,Asthealt itu de forthis ice surface at Berkakis approxi- matelythe same as for the YD glaciersurface,thisyounger ice flowmightalsohavereachedto Storas(Fig.10B).

The surfaceof the inland icerep resented bythe Knutshe moraine system, which is falling in a northerlydirect ion,isat

BJ0 RN A. FO LLESTAD

c.1300 m a.s.l,in the Hjerkinn area.This shows that the Stororkelsj c en(1058 m a.s.l.)wasmore or less deglaciated during the form ationof this moraine system. It might there - forebeconcluded thatthe Knut sheevent,which isthought to belong tothe Preboreal Chronozone,is younge r thanthe described,east erly-situ ate d,glacialdome and thus sup por ts alateYD agefor the glacialdom e eastof Opp daL

Conclusions

It can be shown that the northerly-directed ice flowthrough the main valley of Oppdal-Faqerhauq-Berkak during the finalphase of the Weichselianglaciation was follo wed by a you ng er, westerly iceflow from aglacial dome inthe area east ofOppdaLWhen the surfa ceof the inland icewaslow - ered to som e 1000m a.s.l.in themain Oppdal-Fa gerhaug- Berkakvalley,thislatericeflowhad a compl exflowpattern asitturnedsout hwes twards in the southernandnorthwest- wards in thenort hern part of the mainvalley.This flowpat- tern is here tentatively correlated wit h the Hoklingen Sub stage inSer-Tre nd elaq county(Reite 1994),and is older thanthe Knutshe event(thought to belong tothe Preboreal Chron ozone).Ayounger dome in the areas east of Oppdal cont radict sthe commonviewof therehavingbeen a south- and southea sterly located ice-divide south of the present water divid e that dominated the deglaciat ion durin g the ent ireYDin southeastCentral Norway(Soliidetal. 1980).On the contrary,theglacial domest ronglysupports the earlier view of, e.g.,l.oka s (1955) and Soliid (1964,1968),which reported a subglacial drainagefrom eastto westover the Kvikne run-offpass witha sub sequ entswing ingof themelt - water drainagesout hwestward s towardsthe Oppdalarea.

Acknowledgements

Financialsupport for this study fromthe Geolog icalSurvey of Norway and OppdalCommune is acknowledged withthanks.SivToneBardsnes carried outthemapp ing along the western sid e of the main Oppdal- Fag erhaug valley in 2002.1thank Lars Olsen,Arne Reite and EilivLarsen fortheircriticaland constructivecommentson the orig inalmanuscript.

Iren eLundqui sthas pro cessedsomeof the figures.The revisedmanu- scrip twasreview edbyLarsOlsen,andtheEng li sh corrected by David Ro ber t s.The aut horwouldliketo express his gratitudeto all these col- leag uesand to othe rpersons who have contributedin different ways.

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Andersen,RG. 1968: Glacial geology of Western Troms,North Norway.

Norges qeoloqiskeundersekelse 2S6, 1-160.

Andersen, B.G. 1975: Glacial geology of Northern Nordland,North No rw ay.Norges qeotoqiskeundersekelse 320,1-74.

Ander sen,B.G.2000: Istid eri Nor g e.UniversitetsforJaget,1-216. Anund sen, K. & Simonsen, A. 1968: Et preboria lt breframstet pa

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Barret t,R.L.1900:TheSundal drainagesystemin CentralNorwa y.Bulletin of the Geographic Society ofAmerica3,21pp.

Benn, D.I.1996:Subglacial and subaqueous processe snear a glacier