Geoscience Reference
In-Depth Information
Table 10.7
Drainage characteristics of the underlying geology.
Drainage feature
Possible cause(s)
Wet, boggy ground, marsh
Impermeable substrate (e.g. mudstone, granite, gneiss, glacial till)
Dry, well-drained ground
Permeable substrate (e.g. limestone, chalk, some sandstones, gravel, blown sand)
Ephemeral streams
Permeable bedrock (e.g. limestone, chalk), or low rainfall (arid areas)
10
Spring lines, seeps
Boundary between permeable (up-slope) and impermeable (down-slope) substrates
(e.g. fault, unconformity, or stratigraphic boundary)
Sinkholes
Boundary between impermeable (up-slope) and permeable (down-slope) substrates
(e.g. fault, unconformity, igneous or stratigraphic boundary); typical of limestone
areas
Potholes, caves, karst
features
Typical of limestone areas, rare in regions underlain by other rocks
Dendritic drainage
Area underlain by similar substrates (or even a single rock type)
Rectangular drainage
Exploiting regular lines of weakness in bedrock, such as joints in granite
Radial drainage
Water drains outward from a raised feature (e.g. structural dome, granite
intrusion, volcanic centre)
Linear drainage
Lines of relative weakness in bedrock (e.g. faults, fold axes, soft strata, dykes)
several major Himalayan rivers that cut deep gorges through
the range. However, some watercourses may be defl ected by
strike-slip faults, preserving offsets that betray the presence and
kinematic sense of the fault (Figure 8.1d, p. 166).
Soils and vegetation
Other clues may be drawn from soils and vegetation. Fragments
of the underlying rock (fl oat) may be found in the soil (Figure
10.16a, p. 232), although these may be misleading in formerly
glaciated areas with extensive till or river gravel deposits that
contain pebbles from several (perhaps distant) sources.
Otherwise, the mineral content may give some hint of the
bedrock: sandy soils could refl ect sandstone or granite; red,
iron-rich soils occur commonly on mafi c rocks (e.g. basalt). An
effi cient way of investigating the soil type down to a few
decimetres is by using a hand auger to take a sample of the soil
and particularly the subsoil (Figure 10.16b). As well as
information on the soil type, the auger may yield fragments of
weathered bedrock (regolith) that is not exposed nearby, and it is
a common reconnaissance tool. The vegetation is strongly
infl uenced by both soil type and drainage, so certain rock types
support distinctive plant associations (e.g. calcareous grasslands
on chalk; acidic soils hosting gorse bushes and heather on
sandstones (Figure 10.15b) and peat bogs on granite).
