Humlum, O.

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Po/arjorschung 55 (2): 1985MitteilungenGenesis of the Push Moraine at Kötlujökull, Ieeland:A CommentaryBy OIe Humlum*INTRODUCTIONIn arecent paper in this journal, HEIM (1984) describes the formation of a push moraine, which at pre-sent is being formed along the margin of the glacier Kötlujökull (also called Höfdabrekkujökull), Ice-land. In this paper HEIM introduces the concept of a "glacier-foot" (Gletscherfuß), derived from thelowermost part of the steep glacier terminus by small-scale extrusion-fIow, and supposed to be of consi-derable importance for the formation of the push moraine described.As a member of the Danish geomorphological Myrdalsjökull research group (lead by J. Krüger, Univ.Copenhagen), I have myself studied the formation of the above mo raine during the summers of 1977,1979 and 1982 (KRÜGER & HUMLUM, 1981; HUMLUM, 1983, 1985). Our investigations were donealong the northern half of the almost 12 km glacier terminus of Höfdabrekkujökull, i. e. just north of thearea studied by HEIM. On this basis, I enjoyed reading the paper by HEIM (1984), but I disagree as tothe supposed origin of the so-called "glacier-foot". This will shortly be outlined below. In his paper,HEIM (1984) mentions my 1983-paper, and points out correctly that my interpretation differs from thatproposed by hirn. However, he did not mention the nature of the disagreement.OBSERVATIONS ON THE "GLACIER-FOOT"As described by HUMLUM (1983, 1985) and HEIM (1984), Höfdabrekkujökull/Kötlujökull is at presentadvancing at a rate of 10-25 m/yr. Because of this, a 2-10 m high push moraine is being formed alongthe glacier front wh ich is especially pronounced where the glacier overrides glaciofluvial sediments. Themoraine, which may have an imbricated inner structure, contains bodies of solid glacier ice at some loca-tions (HEIM, 1985), whilst at other locations no ice is found within it (HUMLUM, 1983, 1985).In the summer of 1977, when I had the opporunity to study theglacier margin of Höfdabrekkujökull forthe first time, the advance noted above had just been initiated following several years of backwasting(since ab out 1955; see KRÜGER & HUMLUM, 1981). At this time theglacier front was generally not assteep as is observed today (10-40 deg. versus 50-90 deg.). The lower part of the glacier front was thencovered by a considerable amount of debris.In 1979 the overall appearance of theglacier terminus had changed considerably. At severallocations theglacier was now clearly advancing, and the general surface gradient at the terminus had increased to30-70 deg. Along sections of the margin, the debris covered remnants of the old glacier terminus werebeing pushed over ice-free terrain by reactivated ice further upglacier (Fig. 1). The visual appearance ofthe glacier torminus were thus identical to the "giacier-foot situation" described by HEIM (1984).*' Dr. OIe Humlum, Scientific Director , Arctic Station, University of Copenhagen, DK-3953 Godhavn, Greenland127Fig. 1: The terrninus of Höfdabrekkujökull in July 1979. A frontal apron consisting of glacier ice covered by supraglacial debris is promi-nent. Tho the right the acitve, exposed part of the glacier terminus is seen.Abb. 1: Die Gletscherstirn des Höfdabrekkujökull im Juli 1979. Davor ein Schurz von Gletschereis, welches mit supraglaziärem Schutt be-deckt ist. Rechts ist der aktive, exponierte Teil des Gletscherabfalles sichtbar.GENESIS OF THE "GLACIER-FOOT"Three years later, during the summer of 1982, a situation as shown in Fig. 1 was still apparant at severa1localities along the northeastern margin of Höfdabrekkujökull, In all cases, my clear impression - part-ly justified by comparison with photographs obtained in 1977 and 1979 - was that the bodies of glacierice found along the lower part of the steep, active front were remnants of the distal part of an older icesurface. In no cases did they appear to be derived by extrusion from the lowermost part of the glacierfront. Along the distal rim of these remnants, a push moraine was being forrned as these ice bodies werebeing pushed by the active ice upglacier, just as a push moraine was being formed along the active icemargin at piaces where no ice remnants were present.The extreme northeastern part of the glacier was not included in the general advance until much later,and along this part of the glacier terminus it was.still possible in 1982to find examples of almost detachedremnants of the older glacier front, which laterally gave way to other remnants in contact with the activeice. Those remnants were being pushed over the foreland beyond, thereby producing a push morainealong their distal rim (Fig. 2) - much the same as the situation described by HEIM (1984).I would therefore suggest, that also the "glacier-foot"-features described by HEIM (1984), at least themajority, are also reactivated remnants of an earlierglacier surface. These remnants have survived eitherbecause of a substantial cover of supraglacial debris, or because of burial by glaciofluvial sediments.Very high positive net sedimentation rates (up to 3 cm/day) are common in many fluvial sedimentationbasins close to the present ice margin. I am therefore sceptical about the concept of a "glacier-foot"128Fig. 2: Folding of glaciofluvial sediments along debris-covered remnant ice bodies, which are being pushed over ice-free terrain by activeice farther to the right. July 1982Abb. 2: Faltung von glazifluviatilen Sedimenten an schuttbedeckten Toteiskörpern, welche von aktivem Eis (weiter rechts) tiber eisfreienUntergrund geschoben werden. July 1982.produced by extrusion flow from the lowermost part of a steep glacier front. Furthermore, I do not findit quite appropiate to involve the theoretical developments by GLEN (1955) and NYE (1957) on ice defor-mation in the present context (see HEIM 1984: 34), as these theoretiea1 deseriptions may be invalidatedby large surface gradients such as those experienced elose to a glacier margin.Having presented this alternative point of view, I want to emphasize that professor HEIM has presenteda very valuable and thorough description of arecent push moraine. This is clearly a field of glacial geo-morphology where the general glaciological situation eharaeterizing many glaciated areas in our centurybefore 1960-1970 (backwasting glaciers) has made investigations on this subjeet at recent glaciers diffi-cult, if not to say impossible. More work on the formation of moraines at recent glaciers is thereforemuch needed. Finally, I would like to draw attention to a coming papier by KRÜGER (in press), in whichimportant supplementary results from the continuing work by the research group on the push moraine atHöfdabrekkujökull will be presented.This paper was prepared while serving as scientific director of the Arctic Station in Greenland, and is pa-per nr. 39 from the Aretic Station.ReferencesOie n, J. W. (1955): The creep of polycrystalline ice. - Proc, R., Soc., Ser. A., 228: 519-538.He im, D. (1984): Stauchmoränengenese durch die Entwicklung eines "Oletscherfußes" am Kötlujökull, Stidisland. - Polarforschung54 (I): 21-36. .H u m I u n, D. (1983): Dannelsen af en disloceret randmorrene ved en avancerende isrand, Höfdabrekkujökull, Island. - Dansk geol.Forening, Arsskrift 1982: 11-26.H lJ m I um, O. (1985): Genesis is of an imbricate push moraine, Höfdabrekkujökull, Iceland. - J. Oeol. 93: 185-195.129Kr ü g e r , J. & O. H u m 1 um (1981): The proglacial area of Myrdalsjökull: general report on the Danish Geornorphological Expedi-tion to leeland 1977. - Folia Geographiea Daniea XV (I): 1-58.K r ü g e r , J. (in press): Formation of a push moraine at the margin in of Höfdabrekkujökull, South Iceland. - Geogr , Annaler ,Nye, J. F. (1957): The distribution of stress and velocity in glaciers and iee shects. - Proc. R. Soc., SeL A 239: 113-133.Genesis of the Push Moraine at KötIujökull, Ieeland:A ReplyBy Dieter Heim *When my co-workers first talked about iee-slabs extruding from the base of Kötlujökull, I beeame angryand told them, that those slabs eould eorrespond only with old remnant iee previously buried by fIuvio-glacial debris and then pushed upwards by the progressing glaeier. However now - surely to learn a les-son - again and again I have to defend my glaeier-foot against old iee-reliets.In fact, the Fig. 2 of Dr. Humlums "Commentary" appears to represent a similar situation at the north-eastern margin of Kötlujökull 1982 as I found at the southeastern and eastern margin in July and August1983, ifthe debris ridge in Dr. Humlums figure ends to the right at a steep glaeier slope. And perhaps thisridge hides old reaetivated iee remnants (though I suppose, that one has to exeavate that ridge to make su-re). But at the southeastern and eastern margin of Kötlujökull exeavations of 9 iee-eores distributed over5 km glacier margin as well as some less extensive diggings clearly showed, that here iee grew out of theglacier slope without joints in 1983. When the glacier iee showed foliation, the closest part of the extru-ding ice-slab showed the same foliation. Often one eould observe the top of a ridge shaped iee-eore ben-ding out of a small meltwater generated ridge of the glaeier slope (e. g. the left ridge in Fig. 1) and endingin the third push moraine ridge in front of the glacier-foot.Dr. Humlum's eomments with regard to the evolution of the margin of Kötlujökull sinee 1977 are gene-rally in good agreement with my own results (HEIM, 1983: 27; 1984: 23-26). However, aeeording to air-photo surveys the progression of the glacier already began before August 1975. Itwas clearly reeogniza-ble at the southern margin in July 1978 to me, and during the following years it sueeessively seized thesoutheastern and eastern margin. By 1983 this part of the glacier was eertainly in a more advaneed stageof evolution than the northeastern part in 1982, and I suppose that this eaused some different observa-tions in our research areas.In the first stage of evolution the progressing glacier eonfIieted with an apron of old iee remnants, in our* Prof. Dr. Dieter Heim, Institut für Geowissenschaften der Universität. Saarsrr. 21, D-6500 Mainz.130

Genesis of the Push Moraine at Kötlujökull, Iceland: A Commentary

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