Geoscience Reference
In-Depth Information
B
A
C
10
(a)
(b)
Figure 10.14
Some landscape clues to bedrock geology. (a) Two breaks in slope (arrowed) mark the
boundaries between massive gabbro (crags on left) and cleaved mudstones (underlying fi elds on right) at Carn
Llidi, southwest Wales, UK. The intermediate slopes are underlain by contact metamorphosed mudstones and
gabbro debris. (b) Steep scarp slopes mark resistant strata, near Haltwhistle, UK: a sandstone at A and the
Whin Sill (dolerite sill) at B and C. The horizon dipping to the right (south) follows the dip of the sill. Depressions
between the scarps are underlain by softer siltstones and limestones. (a and b: Tom W. Argles, The Open
University, UK.)
Table 10.6
Topographic clues to bedrock geology.
Topographic feature
Possible cause(s)
Break in slope
Stratigraphic boundary, fault, unconformity, limit of metamorphic aureole
Depression, ravine
Fault, shear zone, layer of softer material, synformal fold
Ridge
Dipping resistant rock layer, dyke, mineralized fault zone, antiformal fold
Escarpment
Edge of gently dipping strata, fault
lithological boundary (Figure 10.13a). Gently dipping strata can
be mapped out by the succession of scarp and dip slopes
(Figure 10.14b), even where rock is not exposed. Systematic
offsets of scarp features may indicate faults, especially where
the offsets coincide with linear depressions. Curved ridges may
indicate folding that can be traced out, either on the ground or
by reference to aerial photographs. Where scarps or ridges die
out gradually, you might suspect that the resistant bed forming
the feature has pinched out. Table 10.6 lists how some
common topographic features might refl ect the bedrock
geology, with some examples shown in Figure 10.14 (see also
Figure 8.2, p. 167). Conventional symbols for these features on
a fi eld map are included in Appendix A10, Figure A10.3.
