This section is from the book "British Wild Flowers - In Their Natural Haunts Vol5-6", by A. R. Horwood. Also available from Amazon: A British Wild Flowers In Their Natural Haunts.
The accumulation of talus slopes on sloping hillsides forms a transition between bare rock and rock with a thick layer of soil, the limestone pavement, and sandy and gravelly soils, being other transitions between them.
In the case of arctic-alpine vegetation, disintegration owing to alternation of heat and cold, landslips, erosion by water or by sand, etc.. play a considerable part in the accumulation of talus, as does rain-wash against vertical cliffs, whilst in the past glacial action has produced a considerable scree or detritus at higher elevations.
Rock debris is formed and is colonized by lichens and mosses, which provide a layer of humus and pulverized grit or sand, etc, for higher plants. Indeed, a large part of it is colonized by Rhacomitrium, forming an open association with bare patches of rock between, which lower down becomes closed and includes higher plants, and still lower is succeeded by heath.
It is exposed to wind, to intense heat, and drought, owing to the thin soil and steepness of the slopes, and is subjected, also, to insolation and the prolonged influence of a snow covering. The intensity of the struggle for existence under such conditions is shown by the paucity of species.