The Editor,

Journal of Glaciology

Sir,

The accumulation of data about Arapaho rock glacier in Colorado Front Range is growing (Reference CarraraCarrara, 1973; Reference BenedictBenedict, 1973[b]; Reference WhalleyWhalley, 1974), much to everyone’s benefit. I have been observing this tongue-shaped rock glacier for more than 14 years (Reference Waldrop and WhiteWaldrop and White, 1965; Reference WhiteWhite, 1971[a], [b]), and wish to add information which may help and leave a question to be answered.

In 1961, I descended into the rock glacier to a depth of 4 m and discovered water flowing across a solid ice core, but did not then realize its significance. It was Benedict in late August 1963 who first recognized the ice was that of an old buried glacier, and photographed it (Reference Outcalt and BenedictOutcalt and Benedict, 1965, fig. 6). On 22 August 1966, several of us climbed into the central longitudinal furrow and down a narrow channel over 12 m deep in the ice. Sun-warmed water from the saucer-shaped Arapaho South Glacier (Fig. 1) between the cirque headwall and the head of the rock glacier had melted this channel into the old glacier (now an ice core) beneath 2 to 3 m of rocky rubble. Benedict later collected ice there with pollen, plant and insect remains in it dated radiometrically (Reference BenedictBenedict, 1973[a], [b]) at ages close to the end of Audubon stade advance of Neoglacialion. Audubon is a new name for Colorado Front Range mid-Neoglacial deposits (Reference MahaneyMahaney, 1972).

Following the terminology of Reference PotterPotter (1972), Arapaho rock glacier is an ice-cored rock glacier, with its down-valley half containing interstitial ice. Formerly, it was a short debris-covered glacier, but now is not much different from any of the other eleven tongue-shaped ice-cored rock glaciers east of the Continental Divide or cast of the highest ridges in this part of Colorado Front Range (Reference WhiteWhite, in press). Six tongue-shaped rock glaciers west of the Divide contain interstitial ice only, and are termed ice-cemented rock glaciers (Reference PotterPotter, 1972).

No doubt exists as to the rock-fall origin of the huge blocks along the southern up-valley half of Arapaho rock glacier. More than 50% of them are > 25 m³ (Reference WhiteWhite, 1971[b]), and may be matched with scars high on the rock face from which they fell. They did not arrive in one rock fall, inasmuch as they are spread over a down-valley distance of 280 m. It took the farthest blocks perhaps > 1 800 years to be progressively dispersed this far. In addition, lichen growing on rocks on the mid-part of the rock glacier head show the rock rubble can not be younger than late Audubon stade. Diameter of the largest thallus of Rhizocarpon geographicum growing on the rock glacier surface above the ice dated by Benedict (Reference Benedict1973[b]) is 49 mm. Using his lichen-growth curve (Reference BenedictBenedict, 1967), those rocks became stable about 1 170 years ago. And near the actual head of the rock glacier, the largest Rh. geographicum thalli are 20 mm. These, however, are in a shallow basin where snow remains throughout the year except for 2 months in mid-summer. Per cent of lichen cover on the rocks even so is 20%, an amount falling within the 10–40% range for Audubon stade Reference Benedict(Benedict, 1967,1968). Lecanora thomsonii is the dominant lichen, characteristic of a lichen cover from the Audubon stade.

Fig. 1.

Arapaho rock glacier, Arapaho Soulh Glacier, and Arapaho Glacier, in cirque al head of Arapaho Valley, Colorado Front Range. Arapaho Peak stade moraines shown by triangles. Central longitudinal furrow on Arapaho rock glacier is sinuous line. Continental Divide is dashed line. Map adapted from U.S. Geological Survey Monarch Lake quadrangle, 7.5 min. series, 1958.

The lack of moraines of Arapaho Peak slade (new name for the most recent Neoglaciation: Reference BenedictBenedict, 1973[a]; formerly referred to in this mountain range as Gannett Peak stade) and of rock debris supplied from the cliff is a problem still bothering me (previously discussed, While, 1971[b]). Yet in the same cirque, 100 m north of Arapaho rock glacier, great moraines of Arapaho Peak stade (Fig. 1) are directly in front of the fast-shrinking Arapaho Glacier. And in front of these moraines is a moraine and small rock glacier of Audubon stade (cf. Reference Mahaney and MahaneyMahaney, 1974). Blocks pushed from the south headwall to test the distance they can travel do not reach the head of Arapaho rock glacier across Arapaho South Glacier, nor, based on my photographs, have any blocks done so since 1939. The Audubon stade glacier formerly existing there was not sufficiently nourished during Audubon–Arapaho Peak interstade time and the deep saucer-shaped depression in the ice may have been produced then as it exists today. Thus, no moraines were built behind Arapaho rock glacier in the Arapaho Peak stade.

But this leaves the question of why Arapaho Peak slade glaciers did not fill both cirque heads. Could the configuration of the windward slope which formerly allowed snow to be fed to Arapaho South Glacier have become so changed by erosion during the 600 year interval between the close of Audubon stade and the beginning of Arapaho Peak stade that growth of a glacier comparable to Arapaho Glacier where Arapaho South Glacier now is, no longer was possible in Arapaho Peak stade time?

27 February 1975