Geoscience Reference
In-Depth Information
Between the broad climatic zones of hill and lowland there is an intermediate
belt, fringing the moorlands. This is narrow in some areas, and geographically more
extensive in others, as in Wales. The freely drained soils of this 'upland margin' zone
combine something of the character of the principal soils of its two neighbouring cli-
matic zones, as described later in this chapter.
In the extensive lowland zone that dominates southern Britain, and also extends
up the eastern side of Scotland, regional climatic differences have little direct effect
on soils, though they do affect the possible crop options and their yields. Rainfall and
temperature variations from year to year at one place can be greater than differences
in long-term averages between places. The contrasts between soil classes in the low-
lands, which are very great, are due mainly to the influence of parent materials, and,
within a single parent material, to landform.
Contrasts in parent material range between almost pure calcium carbonate rocks
such as the chalk, which in most locations carry shallow calcareous soils, and virtu-
ally pure silica sands and sandstones, on which highly acid soils develop. Although
many soils form directly from parent bedrock at shallow depth, others are developed
on transported geological deposits.
It may be thought that a geological map would show what parent material is
present at any particular site. However, such maps display either the strati-graphic age
of the solid rocks, or the mode of origin of the overlying alluvial or drift deposits,
rather than their composition, which is what influences soil properties. Limestone
bedrock shown on the geological map may, for instance, be overlain by shallow silty
glacial drift derived from non-calcareous shales outcropping elsewhere. Additionally,
within one rock age class or map unit there may well be considerable variation in rock
chemistry and other relevant properties. Bedded sandstones, shales, and limestones
may all occur in rapid succession at one place in a single rock unit, while rock of the
same age-based unit can be limestone in one area and sandstone in another. Thus, al-
though geological maps give a general picture of the potential source materials for
soil formation, only field examination of soils themselves, and possibly laboratory in-
vestigation also, can confirm their parent material origins.
Among landform influences, steepness of slope and position on the slope have
the most important effects on soils. Together with soil texture characteristics de-
scribed in the previous chapter, they control drainage as illustrated in Figure 11 . On
clay-rich, poorly permeable materials, waterlogging persists near the surface on both
level and gently sloping ground; poorly drained soils commonly occur on crests and
lower parts of slopes while freely drained soils can be restricted to quite steep slopes.
On lighter-textured (sandy) parent materials, however, freely drained soils predomin-
ate. The water that has moved through the soil flows downslope in the underlying par-
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