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may act as a binding, rather than a lubricating, medium. It might be that
by the time that the ice has dissipated to the extent that the mass is
potentially mobile, most of the remnant glaciers and their moraines are
marooned on a cirque floor so flat that a rock glacier cannot develop.

No fully satisfactory explanation of the origin or maintenance of the
rock glaciers in the Aiken Lake map-area is known to the writer. They
are definitely not merely an end phase of present glaciation, although some
may develop from dying glaciers. Oncé started, they appear to be more or
less self-perpetuating under present climatic conditions, and develop into
unique, flowing bodies, distinct from the surrounding talus and felsenmeer.
Some of them are actively advancing at levels far below the present snow-
line, and they continue to flow across relatively gentle gradients even when
detached from their steep parent slopes. They maintain their identity and
peculiar behaviour, which must be due in part to freeze-and-thaw processes
in their interstitial ice, over a wide range of temperature, moisture supply,
and drainage conditions; for a single rock glacier may have a vertical range
of more than 1,000 feet, and may head on a dry ridge, with its lower part
occupying the floor of a valley carrying a stream of considerable volume,
which is forced to percolate through the rock glacier. Most inexplicable
of all, adjacent deposits of talus, composed of the same type of material,
apparently shattered to the same extent, and exposed to the same conditions,
may not develop rock glaciers; and gradational phases appear to be rare.

SOLIFLUCTION

The mountains and valleys above timber-line in the less rugged parts
of the map-area support tundra vegetation, which ranges from sparse
lichens and mosses to a thick mat of grasses and flowering plants growing
on many feet of peaty material. Much of the ground is frozen at depth
throughout the year, but alternate freezing and thawing of the surface
layer has facilitated a slow downslope flow, or solifluction, analogous to
the felsenmeer creep of more alpine areas. Where typically developed in
the northern and central parts of the map-area, solifluction has resulted in
low, bulbous terraces, turf-covered ridges, parallel strips of muskeg ground,
and alplands where the surface material has been sorted into polygonal
areas or long lanes of coarse boulders, separated by fine material.