Environmental Engineering Reference
In-Depth Information
Figure 2.23.
Distribution of fresh and variably weathered rock, controlled largely by variation in rock
substance type, at Split Rock Dam, New South Wales.
Figure 2.24.
Shape of upper surface of fresh rock in a situation where the rate of valley erosion has been
exceeding the rate of lowering of the chemically weathered zone.
the development of the profile. In tectonically stable areas where the effects of erosion have
been relatively small, great depths of weathered rock have developed. For example, large
areas of Western Australia which have been relatively stable and mainly exposed since
Precambrian time, are now almost flat “peneplains”, underlain by extremely weathered rock
to depths of 30 m to 50 m. In tectonically active areas uplift and subsequent erosion occur,
and result in much shallower weathered profiles in comparable rocks. In the areas of North
America and Scandinavia eroded by ice-sheets during the Pleistocene glaciation, there is
generally no chemically weathered profile.
Figure 2.24 shows a situation which is frequently seen: weathered profiles which are deep-
est beneath hilltops, ridges and plateaus, and shallow or absent at or close to the floors of val-
leys. This situation may have arisen simply because the rate of valley erosion (i.e. removal of
weathered materials) has exceeded the rate of deepening of the weathered profile. However,
in some places situations similar to that in Figure 2.24 can have a different origin as indicated
in
Figure 2.25
.
This shows the situation in parts of southern and eastern Australia, where
mountain ranges were formed during late Tertiary time, by the faulting and uplifting of
deeply weathered rock. Relatively rapid erosion of river valleys since late Tertiary time has
resulted in composite weathered profiles as shown on Figure 2.25. The rock near the valley
floors is mainly fresh but is usually somewhat mechanically loosened due to destressing.
Ancient weathered profiles can be buried and preserved beneath younger sediments or
even sedimentary rocks.
Figure 2.26
is a diagrammatic cross section showing the geological
