27

There appears to be no doubt that at least some of the rock glaciers
are advancing. Four of the rock glaciers studied are pushing their way
into forest or across meadowland. All have the same characteristics: a
smooth, lobate ground plan; a very steep, unstable snout; and, perhaps
most significant, an almost total lack of ‘runaway’ boulders ahead of the
snout. Trees and bushes have been pushed down and overridden in such
a way as to indicate that the rock glacier is advancing along the present
ground surface. Some of the rock glaciers are flowing down relatively
gentle gradients, and have become detached from the talus behind them.
The upper end of one such ‘detached’ rock glacier is marked by a grassy
floored depression 40 feet deep (See Figure 1). These features would
seem to indicate that the rock glaciers studied cannot have reached their
present positions by landslides, piecemeal rock-fall, or by avalanching,
but must have flowed slowly, as a more or less plastic mass. The fragments
of rock glaciers carry somewhat less lichen than those of the felsenmeer;
the lichen coating is commonly best developed on the upstream faces of the
blocks, and may be an indication that the surface blocks of a rock glacier
rotate backward during its flow. Further evidence of the activity of rock
glaciers may lie in the observation that marmots have made their home in
talus and felsenmeer in all parts of the map-area, but none was found
living in rock glaciers.

Just as there seems to be no evidence of a gradation between debris-
loaded ice glaciers and most of the rock glaciers in Aiken Lake map-area,
there is in most places an apparent lack of gradation between normal talus
or felsenmeer and rock glaciers. One of the most remarkable features of
the rock glaciers observed in the area is their conspicuous appearance; a
given pile of rock fragments appears to be a rock glacier, or it does not;
the distinction between talus, felsenmeer, and moraine is often difficult and
in part arbitrary, but the rock glaciers are mostly unique and easily
recogn?zed.

The only consistent difference observed between the rock glaciers and
the adjacent talus is a coarser size and a much better sorting of fragments
on the surface of the rock glacier than in the talus (See Figure 2). Careful
examination of the bedrock above both talus and rock glaciers in three
different parts of the map-area, however, has shown no recognizable
difference in the joint pattern or intensity of shattering of the rock that
has supplied both deposits. It may be that rock glaciers are sorted during
their flow, with coarse fragments jostled to their upper surfaces, rather
than that they have developed from more coarsely jointed bedrock.

It seems obvious that the rock glaciers in Aiken Lake map-area are
in part normal talus and felsenmeer that has been rendered mobile. The
suggestion that the mobility is controlled mainly by interstitial ice is
reasonable; the difficulty in explaining rock glaciers lies in accounting for
the localization of that control. Rock glaciers, talus, and felsenmeer must
all contain interstitial ice, at least in their surface layers, during winter
and spring, and a considerable amount probably remains from year to year.
The Granite Basin adit showed ice in the talus, 5 to 15 feet from the surface,
in July. If it is postulated that rock glaciers contain more interstitial ice
than do talus, or felsenmeer, rock glaciers should be best developed in local
drainage channels, or should head in snow-catchment basins; there should