Читать книгу Mountains and Moorlands - W. Pearsall H. - Страница 14

The initial stages of soil development in which rock fragments predominate are what we can only call skeletal soils. They are found principally where the surface is unstable or where further development is retarded by hard rocks or low temperatures. Of these factors, the low temperatures have also distinct qualitative effects, because while they greatly retard chemical modifications, the associated physical disintegrations caused by frost and solifluction are especially vigorous. There may thus be much physical commination of rock fragments with little chemical change. Thus the soils, even if finely divided, are immature in the developmental sense because of the deficiencies in their chemical and biological equipment. Soils of this general type occur on mountain-tops, where they are found under the mountain-top detritus except, perhaps, where it is especially coarse and deep. Generally, however, the detritus seems to be a superficial layer of stones extruded from below during the frost-caused or solifluction movements of the materials. Beneath the stones there is commonly a sandy loam, generally brown in colour and little leached. When vegetation is present this merges at the top into the almost black humus that collects among the surface detritus. The depth of the soil varies with the nature of the rock and the degree of erosion, but it is usually between one and three feet, and then comes disintegrating rock.

Parallel to these summit soils in general quality may be the scree-slopes of finely-divided material which occur lower down a mountain, often approaching stability but still subject to soil-wash and soil-creep, and so often distinguished as creep-soils. These show great variability in detail, but, like the mountain-top detritus, they often show coarse material at the surface and finer below. As they approach stability, they merge into the woodland soils described below, but in the earlier “gravel-slide” stages, a vertical section usually reveals a sequence of more or less alternating sandy or stony layers parallel to the slope (see Fig. 8). All soils of these types are alike in possessing a high base-status because they consist mainly of rock particles as yet not greatly modified by chemical change.

In all upland habitats there are in addition the overall trends caused by continual washing by rain, and as a result every exposed and porous surface will be more or less leached. Wherever leaching has taken place there must be corresponding areas that receive the products of leaching. The water that carries away lime or other bases from the higher upland surfaces must produce elsewhere lime-rich or base-rich habitats. Areas of this latter type may be distinguished as flushed or enriched habitats to distinguish them from the leached or impoverished ones.