Geoscience Reference
In-Depth Information
and other sedimentary rocks, which have not been subject
to the same formation mechanisms and must therefore
be formed by different processes. The formation of Ayers
Rock in Australia is a resistant sandstone that has survived
weathering processes that have lowered the surrounding
landscape to leave the domes clearly elevated from the
plains. Thus, the key characteristics of such dome-like
formations in arid zones are perhaps that they are all,
regardless of the processes that formed them, harder rocks
than those found in the local landscape and have thus been
able to resist weathering since their formation.
10.3.1.2
Substrate ramps
Substrate ramps are formed in erodible strata at the base of
caprocks and are often characterised by a distinct change
in gradient from the near-vertical caprock to the 30-40
◦
ramp, where the caprock is still present (Figure 10.13).
Upslope or headward extension of these features is ei-
ther due to fluvial processes of basal sapping or erosion,
which undermine caprock and lead to failure and collapse
or the effect of gravity on the vertical caprock (perhaps in
combination with freeze and thaw action), which will also
promote collapse. Such collapse may result in the exten-
sion of a debris-covered substrate ramp, covered by large
amounts of talus or talus ledges (see below). Substrate
ramps may be formed in highly erodible material, such
as limestones or sandstones, which are prone to erosion
by sheet wash and dissolution and may therefore become
gullied or highly dissected by overland flow. Figure 10.14
illustrates the various forms of rock slopes and shows
how substrate ramps often intersect with the more durable
caprocks well below the margin between the two strata.
Figure 10.12
Granite inselbergs.
skin of an onion) peel away from the bedrock slope to re-
veal fresh surfaces (see Figure 10.12). This process, often
termed 'sheeting', leads to domed formations, with con-
vex slope forms and very abrupt changes in gradient with
the surrounding landscape. Numerous hypotheses exist to
describe the process of sheeting and how it leads to the
formation of inselbergs. These hypotheses are based on
either (1) rock-formation processes - planes are formed as
the batholiths intrude into overlying material or crust ma-
terial is compressed to form concentric and parallel faults
within the granite - or (2) post-formation processes -
stress releases after formation and secondary shearing or
faulting within the massive bedrock form concentric lay-
ers of weakness. It is likely that these processes are not
mutually exclusive and can operate alongside each other
depending on the characteristics of the bedrock. For exam-
ple, granite batholiths that exhibit stress-release sheeting
(from internal stresses within the bedrock) may also be
recovering from the removal of overlying material that
has been eroded, thus unloading the slopes and allowing
them to expand due to lower compression from the (now
absent) overlying material.
The bornhardts of East Africa (named after the German
geologist Willhelm Bornhardt) show classic dome-shaped
forms, within massively bedded igneous rocks. However,
10.3.1.3
Rock-slope erosion features
The process of sheeting, which exfoliates layers of rock
to form dome-like rock slopes, is described above. In par-
allel to sheeting, it is also common for bare rock slopes
to be weathered via fluvial processes, which sculpt slopes
via dissolution of the bedrock, a process that can be ac-
celerated with slightly acidic precipitation in contact with
calcareous, sedimentary bedrock.
In addition, wind action leads to the erosion of bare rock
slopes, in areas such as the sandstone Colorado plateau,
southwestern USA. The work of Loope
et al
. (2008) dis-
cusses the importance of the wind-sculpting of bare rock
slopes, leading to the formation of a variety of features,
which hitherto may have been interpreted as fluvial land-
