Geoscience Reference
In-Depth Information
Drainage
Many conspicuous drainage features owe their existence to the
underlying bedrock or superfi cial geology (e.g. Figure 10.15).
Table 10.7 summarizes some common examples. Care is
needed in interpretation, as both bedrock and superfi cial
deposits may infl uence drainage. For example, waterlogged
ground could be due to a thin layer of impermeable glacial till
that is itself underlain by permeable sandstone. Some drainage
patterns may be antecedent: i.e. they pre-date the formation of
a later landscape (e.g. a young mountain range) and hence bear
little or no relation to the underlying geology; this is true of
10
(a)
(b)
Figure 10.15
Examples of drainage and vegetation contrasts.
(a) Marshy ground with rushes (foreground) marks the outcrop of
impermeable Silurian mudstones, while most of the sheep are grazing on
the well-drained pasture underlain by permeable Carboniferous Limestone
(exposed in the crags behind). The contact between the two rock types is
marked by a distinct break in slope, conspicuous against the woodland
near the left-hand side of the photograph. Witherslack, Cumbria, UK.
(b) The green verdant vegetation and fl atter foreground is underlain by
interbedded limestones and mudstones. The steeper slopes with vegetation
typical of acidic soils (heather, bracken) are underlain by sandstone. The
contact is marked by the vegetation change, running from just left of the
group of trees on the horizon obliquely left behind the plantation on the
left-hand side. This locality is near Alnwick, Northumberland, UK. (a: Tom
W. Argles, The Open University, UK; b: Angela L. Coe, The Open
University, UK.)
