Читать книгу Mountains and Moorlands - W. Pearsall H. - Страница 15

In general, of course, leaching will preponderate in upland regions and enriched soils will be commoner in lowland regions. Nevertheless enriched soils are always to be found occupying characteristic localities in mountain areas. Thus there is a flushed area around every springhead and around every rivulet. However, the water need not emerge as a separate spring but may perfuse the surface soil—a type of flush that can be recognised by a zone of greener vegetation. The various types of “damp flush” may be associated with a soil of almost any physical category. Enrichment by water from a higher level is greatest when such water has penetrated into the rock by means of structural fissures, and permeated the rock strata on its way down; a mere receiving area for surface run-off from acid upland soils is often as severely leached and as acid as the upland soil itself.

Parallel with enrichment by water (“damp flushes”), there is enrichment by presence of freshly-weathered rock particles, and areas of this type might be called “dry flushes.” The lower part of any steep slope is constantly enriched by such particles washed down from above. Screes and gravel-slides, in which the breakdown of new rock by weathering continually yields a supply of bases, could thus be considered among the enriched or flushed habitats. In this category also comes any unstable surface, crag, gullies and the like, where new rock surfaces are being exposed by erosion.

It will be observed that the flushed habitats are determined by a diversity of factors producing enrichment and have thus few physical characteristics in common. They tend to fall technically into four categories:

1 Bare rock or oversteepened slopes with soil particles washed away or present only in narrow fissures. Enrichment by continual weathering of freshly exposed rock surfaces.

2 Block scree in which leaching tends to preponderate over weathering, although the latter nevertheless does continually refurnish some of the bases lost, especially in the case of more rapidly weathering and base-rich rocks.

3 Unstable scree-slopes and solifluction areas with movement and accumulation of weathered soil particles, often below the surface layer of coarse detritus; enrichment both by weathering and particle accumulation.

4 Accumulation areas—nearly always showing fine and deep soils, with enrichment mainly by accumulation from above.

Upwelling of base-rich waters may occur in conjunction with any of the four categories above, although it is rarest in a and commonest in c and d. In both c and d the total “flush” effects normally counteract losses by leaching unless the soils are deriving from base-poor rocks.