Humlum, Ole: Natural cairns on rock glaciers as an indication of
a solid ice core. Geografisk Tidsskrift 88:78- Copenhagen,
1988.

A special surface feature taking the form of natural cairns is observed on rock glaciers in Disko, central West Greenland, known to possess a core of solid ice. These block accumulations are perceived as developed by the gradually outcropping of large englacial blocks from the ice core due to surface ablation, thereby lifting surface debris lying above. Below a line on the rock glacier where the surface debris layer equals or exceeds the depth of summer thaw, no more cairns are developed. Natural cairns as these on rock glaciers are suggested as a criterion for the existence of a solid ice core within the rock glacier, and the designation rock glacier cairn is proposed for them.

Keywords: Rock glacier, Glacier-derived rock glacier, West
Greenland, Disko.

Ole Humlum, ass. prof., Institute of Geography, University of
Copenhagen, Øster Voldgade 10, DK-1350 Copenhagen K,
Denmark.

Since the pioneering papers by Spencer (1900), Rohn (1900), Cross and Howe (1905), Howe (1909) and Capps (1910), rock glaciers have been described from many alpine areas throughout the world. The most comprehensive descriptions to date are probably those by Wahrhaftig and Cox (1959), Whalley (1974a), Höllermann (1983) and Haeberli (1985).

Many papers on rock glaciers have dealt with terminology,
typology and genesis (see recent review in Martin and
Whalley, 1987). There are, however, no complete agreement
among investigators on the basic rock glacier classification.
Present opinion is that one or more of the following
three main types of rock glaciers exist:
A. Glacier-derived rock glaciers (Capps 1910, Pillewizer

1957, Grötzbach 1965, Barsch 1971, Potter 1972,
Whalley 1974a, 1983).

B. Talus-derived rock glaciers (Crawford 1913, Outcalt
and Benedict 1965, Wahrhaftig and Cox 1959, White
1981,Shakesbyet.al. 1987).

C. Landslide-derived rock glaciers (Howe 1909, Shalley

1974b, Johnson 1984a, 1984b).

Glacier-derived rock glaciers are expected to contain a
core of massive ice, whereas many talus-derived rock
glaciers are envisaged as being ice-cemented by interstitiel
ice and some are expected to contain larger bodies of solid
ice derived from avalanche snow or refreezing water.

Only relatively little is known about the interior structure and ice content of rock glaciers. The most comprehensive description on this is probably the paper by Haeberli (1985). The general lack of detailed knowledge on this topic is caused by several factors. First of all, intensive rock glacier research has no long tradition, secondly, research on rock glaciers is hampered by the environment in which they occur, and thirdly, the very nature of the rock glacier itself represents a very perceptible difficulty. Most rock glaciers have a surface layer consisting of blocks up to several meters in size. Many rock glacier studies have therefore relied heavily on indirect means, first of all the study of aerial photos, but also implications of the morphology, topographical location and surface texture of the rock glacier. Only in a few cases shallow excavations and geophysical soundings have been carried out (see Barsch 1973 and Haeberli 1985).

Also in the future logistic problems will probably force investigators to apply different indirect means in the study of rock glaciers. It is the purpose of the present paper to draw attention to a small surface feature, which apparently only exists on rock glaciers with a solid ice core. Concurrent with different criteria suggested for use in aerial photo interpretation of rock glaciers (Outcalt and Benedict 1965, Vernon and Hughes 1966, Humlum in press), the field observation of this small-scale surface feature may hopefully assist in discriminating between different types of rock glaciers as to the character of ice content.

ROCK GLACIER CAIRNS

The present author is engaged in a study on glaciers and rock glaciers in Disko Island, central West Greenland (70°N,fig. 1, Humlum 1983,1984,1985, 1986,1987, and in press). About 1700 individual rock glaciers exist in Disko, both glacier-derived and talus-derived features. No landslide- derived rock glaciers have been found. At

nine rock glaciers natural sections near their terminus have demonstrated the existence of a solid ice core, apparently consisting of glacier ice (fig. 2). In advance to the field investigations, all these rock glaciers were classified as glacier-derived and probably containing a solid ice core on basis of observations from aerial photos. Fig. 3 displays playsone of above rock glaciers. The individual ice crystals in samples taken from the ice cores were usually 0.5 to 3 cm in size. The solid ice contained both debris and air bubbles, organized in a layered structure, just as normal foliated glacier ice is (see fig. 2).

At all of the above rock glaciers a somewhat peculiar

surface feature was observed (fig. 4, 5, and 6). Natural cairns 1-2 m high occurred several places on the rock glacier surface. In most instances their frequency was rather great near the rock glacier head, where up to several scores could be recognized within visual distance, and declining towards the rock glacier terminus. Some have, however, been observed quite close (150 m) to the terminus.The lower part of these cairns always appears to consist of one large boulder, often 2-4 m in size. In several occasions this basal boulder showed distinct vertical striae (fig. 6).

These natural cairns are perceived as an indication of

the presence of a solid ice core beneath the rock glacier surface. The line of reasoning is outlined in fig. 7. Rock fragments derived from the headwall above the glacier falls upon the accumulation area below, where they are buried by snow and incorporated in the glacier. Subsequently the rock fragments are transported towards the ablation area in englacial position as glacial debris. Below the equilibrium line (ELAI in fig. 7) the fragments crop out on the ice surface concurrent with the loss of ice. During this process, very large boulders, still with their lower part firmly embedded within glacier ice, may slowly lift fragments lying above, relatively to adjoining debris lying on the ablating glacier ice surface. As the large boulders penetrate surface debris, their flanks may become vertically striated due to the contact with adjoining blocks lying on the ice surface. This mechanism would not work in the absence of a solid ice body below the surface debris, and I would, therefore, suggest this phenomenon to represent a useful criterion for the presence of an ice core within rock glaciers where it is observed. This ice core may or may not consist of glacier ice. The emerging boulders lifting surface debris take the appearance of natural cairns, and I would suggest that they should be referred to as rock glacier cairns.

The rock glacier cairns are unstable features, and their duration of life may well be rather short, say, a couple of years or more. As is observed, they must be expected to form most frequently not far below the rock glacier head, where the surface layer of debris still is thin. Farther below, the layer of surface debris may attain or exceed the thickness of the active layer, and no further ablation from the ice surface will take place. The Balch ventilation effect (Thompson 1962) may even lead to accretion of superimposed ice at the base of the surface debris layer (observed by the author in Disko), and in this case a second equilibrium line (ELA2 in fig. 7) may be present on the rock glacier itself, separating an upper ablation area from a lower accumulation area extending almost to the front at the terminus. When this is the case, no new rock glacier cairns will develop below the ELA2. The decreasing frequency of cairns towards the rock glacier terminus is, therefore, concurrent with the hypothesis outlined above. The other way round, the presence of fresh rock glacier cairns should be taken as an indication of the thickness of the surface layer being less than the local depth of summer thawing.

ACKNOWLEDGMENTS

This paper is based on observations obtained while serving
as scientific director of the Arctic Station (University

of Copenhagen) in Qeqertarsuaq/Godhavn, Disko 1983-86. observations were collected during the summer of 1987. The Danish Natural Science Research Council made funds available for this field work. Research student Poul Frich (Univ. of Copenhagen) skillfully participated in the 1987 field work. My sincere thanks further goes to the staff at the Arctic Station, especially to Jakob Broberg, skipper of the research cutter "Porsild".

References

Barsch, D. (1971): Rock glaciers and ice-cored moraines. Geograf.
Annal. 53 (3-4):203-206.

Barsch, D. (1973): Refraktionsseismische Bestimmung der Obergrenze des gefrorenen Schuttkorpers in verschiedenen Blokgletschern Graubiindens, Schweizer Alpen. Zeitschr.f. Gletscherk. Glazialgeol. 9:143-167.

Jr., S. R. (1910): Rock glaciers in Alaska. Journ. Geol.
18:359-375.

Crawford, R. D. (1913): Geology and ore deposits of Monarch
and Tomichi Districts, Colorado. Col. Geol. Surv. Bull. 4, 317
pp.

Cross, W. and Howe, E. (1905): Geography and general geology
of the quadrangle. US Geol. Surv. Atlas of the U.S., Silverton
Folio, Colorado, No. 120:1-

Grötzbach, E. (1965): Beobachtungen an blockströmen im afghanischen
Hindukusch und in den Ostalpen. Mitt. Geogr. Gesellsch.
in München, 50:175-201.

Haeberli, W. (1985): Creep of mountain permafrost: internal structure and flow of alpine rock glaciers. Mitt. d. Versuchsanstalt f. Wasserbau, Hydrologie und Glaziologie, 77, Zürich, 142 pp.

Höllermann, P. (1983): Blockgletscher als Mesoformen der Periglazialstufe
- Studien aus europäischen und nordamerikanischen
Hochgebirgen. Bonner Geogr. Abh. 67, 83 pp.

Howe, E. (1909): Landslides in the San Juan Mountains, Colorado:
including a consideration of their cause and their classification.
U.S. Geol. Surv., Prof. Pap. 67.

Humlum, O. (1983): Rock glacier types on Disko, central West
Greenland. Geogr. Tidsskr. 82:59-66.

Humlum, O. (1984): Altitudinal trends of talus-derived lobate rock glaciers on Disko, central West Greenland. Geogr. Tidsskr. 84, Occasional papers Denmark, vol 7, XXV Int. Geogr. Congress Paris Alpes 1984:35-39.

Humlum, O. (1985): The glaciation level in West Greenland.
Arctic and Alpine Research 17(3):311-

Humlum, O. (1986): Mapping of glaciation levels: comments on
the effect of sampling area size. Arctic and Alpine Research
18(4):407-

Humlum, O. (1987): Glacier behaviour and the influence of
upper-air conditions during the Little Ice Age in Disko, central
West Greenland. Geogr. Tidsskr. 87:1-

Humlum, O. (in press): Rock glacier appearance level and rock
glacier initiation line altitude: a methodological approach to
the study of rock glaciers. Arctic and Alpine Research 20(2).

Johnson, P. G. (1984a): Paraglacial conditions of instability and
mass movement. A discussion. Zeitschr. f. Geomorph. N.F.
28(2):235-250.

Johnson, P. G. (1984b): Rock glacier formation by high-magnitude
low-frequency slope processes in the southwest Yukon.
Annal. Ass. Amer. Geograph. 74(3):408-

Martin, H. E. and Whalley, W. B. (1987): Rock glaciers. Part 1.
Rock glacier morphology: classification and distribution.
Progress in Phys. Geograph. 11(2):260-

Outcalt, S. I. and Benedict, J. B. (1965): Photo- interpretation of
two types of rock glaciers in the Colorado Front Range, USA.
Journ. Glaciol. 5(42):849-856.

Pillewizer, W. (1969): Untersuchungen an Blockströmen der Otzaler
Alpen. Abh. d. Geograph. Inst. d. Freie Univ. Berlins
5:37-50.

Potter, N. (1972): Ice-cored rock glacier, Galena Creek, Northern
Absaroka Mountains, Wyoming. Bull. Geol. Soc. Amer.
83(10):3025-

Rohn, O. (1900): A reconnaissance of the Chitna River and
Skolai Mountains, Alaska. In "U.S. Geological Survey" 21st
Ann. Rep. Pt. 2:399-440.

Shakesby, R. A., Dawson, A. G. and Matthews, J. A. (1987): Rock glaciers, protalus ramparts and related phenomena, Rondane, Norway: a continuum of large-scale talus derived landforms. Boreas 16(3):305-

Spencer, A. C. (1900): A peculiar form of talus (abstract). Science
11:188.

Thompson, W. F. (1962): Preliminar notes on the nature and distribution of rock glaciers relative to true glaciers and other effects of the climate on the ground in North America. Internat. Associat. Scient. Hydrol. (lASH), Comm. Snow Ice, Symposium Obergurgl, Publ. 58:212-219.

Vernon, P. and Hughes, O. L. (1966): Surficial geology, Dawson,
Larsen Creek map-areas, Yukon Territory. Geol. Surv. Canada
Bull. 136,25pp.

Wahrhaftig, C. and Cox, A. (1959): Rock glaciers in the Alaska
range. Geol. Surv. Amer. Bull. 70:383-436.

Whalley, B. (1974a): Rock glaciers and their formation. Reading Geographical Papers, eds. D. Bonnar, C. Chartres, M. Ebery, and P. Williams, University of Reading, Reading, England, No 24, 60 pp.

Whalley, B. (1947b): Origin of rock glaciers (correspondence).
Journ. Glaciol. 13(68):323-324.

Whalley, B. (1983): Rock glaciers - permafrost features or glacial
relics? Proc. Fourth Int. Conf. Permafrost, National Academy
Press, Washington, D.C. 1:1396-1401.

White, S. E. (1981): Alpine Mass Movement Forms (Noncatastrophic):
Classification, Description and Significance. Arctic
and Alpine Research 13(2): 127-137.