Sir,
In a recent letter regarding the origin of ice-cored rock glaciers, Reference CarraraCarrara (1973) suggested that the debris at their surfaces may represent a succession of shear moraines formed during glacier retreat. Referring to a photograph of the ice core of the Arapaho rock glacier, Colorado Front Range (Reference Outcalt and BenedictOutcalt and Benedict, 1965), he concluded that debris bands in the ice “may well be shear planes”, and that shearing “would be one mechanism for obtaining a surficial mantle on the ice body”.
Field studies of the Arapaho rock glacier support part, but by no means all, of Carrara’s hypothesis. Figure 1 is a photograph of the surface of a debris layer (ablation surface) from the ice exposure in question. Mineral and organic inclusions are arranged in parallel streaks, orientated in a down-glacier direction, and indicating that Carrara is probably correct when he suggests that differential movement has occurred.
Fig. 1. Plan view of a debris layer in the ice core of the Arapaho rock glacier, Colorado Front Range.
There is no evidence, however, that shearing has contributed a significant amount of coarse debris to the surface of the rock glacier. During the summer of 1966, erosion by a melt-water stream exposed a discontinuous 220 m long vertical section of buried ice, extending along the axis of the rock glacier from the shallow depression at its rear to a position about 400 m behind its front. The thickness of the exposed ice varied from 1.0 to 9.8 m. Examination of the ice core revealed only a few stones that were larger than 2 cm, and none that was larger than 6 cm. The ice is much too clean and contains stones that are at least an order of magnitude too small to be the source of the thick accumulation of boulders on the surface of the rock glacier. The latter have an average diameter of approximately 1 m (Reference WhiteWhite, 1971) with occasional boulders 15–20 m in maximum dimension.
Along the walls of the melt-water channel, the thickness of the debris mantle ranged from 0.2 to 2.4 m, increasing down-valley. The debris was composed of two units: (1) a poorly sorted basal sand layer containing gravel and a few cobbles; and (2) a surface layer of large open-work boulders. Each layer appears to have originated by a different mechanism.
Boulders in the surface layer are rough and angular. Unlike the smooth, predominantly subrounded boulders found on historic moraines a few meters to the north, they show no evidence of modification by glacial transport. I attribute the upper layer of coarse angular debris to rockfall on to the glacier surface, but I am uncertain whether the boulders accumulated gradually through a succession of small rockfall events, or abruptly, as the result of a single catastrophic rockfall avalanche (Reference MudgeMudge,1965).
The origin of the basal sand layer is undoubtedly complex. Some of the material may have sifted and washed downward from overlying rockfall debris and some, as suggested by Carrara, may have been brought to the surface of the ice by shearing. Most, however, appears to be ablation till, added to the base of the debris mantle during melting of the underlying ice.
Additional information concerning the ice core of the Arapaho rock glacier may be of interest:
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i. Ablation surfaces dipped up-valley at angles of 32–60° and showed no systematic variation in dip along the length of the exposure. A strong air-bubble lineation, less steeply inclined, locally intersected ablation surfaces at angles of 20–30°. Small healed crevasses were not offset where they crossed debris bands in the ice, suggesting that there has been no recent differential movement along the ablation surfaces.
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ii. Thin-section studies by Charles A. Knight (National Center for Atmospheric Research) indicate that the ice is highly modified, and they are consistent with the hypothesis of its glacial origin. Samples from both the rock glacier and the nearby Arapaho Glacier showed a broad range of crystal sizes and shapes, with individual grains as large as 30 mm in diameter. Interlocking grain boundaries were common and air bubbles were distributed throughout the samples with little regard for crystal boundaries. In contrast, samples of recent snow-bank ice from the lateral trough at the south edge of the rock glacier were characterized by smaller equidimensional crystals, a narrower range of grain diameters (0.5–3.0 mm), an absence of interlocking grain boundaries and restriction of air bubbles to the contacts between adjacent crystals.
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iii. Radiocarbon ages of 1000±90 b.p. (I-2562) and 955±95 b.p. (I-3858) were obtained for pollen, plant fragments and insect remains collected from ablation surfaces 310 and 300 m, respectively, from the cirque headwall. The oldest sample was overlain by 90–120 cm of gravelly sand, capped by an additional 120 cm of boulders; the youngest was insulated by 20 cm of sand, beneath a 65–75 cm thick layer of boulders.
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iv. Peter J. Mehringer, Jr (Washington State University) found a predominance of Artemisia (45.5%) and Pinus (17.5%) pollen in a sample collected from the 1000 year old ablation surface. Comparison of the Picea/Pinus ratio (0.03) with modern ratios determined by Reference MaherMaher (1972) along an altitudinal transect in the Front Range suggests that vegetation zones were depressed only slightly, if at all, at the time of pollen deposition.
In the Colorado Front Range, as in many other mountain areas, the distinction between “rock glaciers” and debris-laden ice glaciers or snow patches is largely artificial. Where modern glaciers are clean, there is a tendency to emphasize the uniqueness of glaciers that are buried beneath thick layers of insulating debris. The significance of the Colorado Front Range rock glaciers is not that they indicate a climate “not quite severe enough to produce or sustain ice glaciers” (Reference MadoleMadole, 1972) but rather that the environment at the time of their formation favored both (1) glacierization and (2) extensive rockfall from cirque headwalls.
28 April 1973
