Geology Reference
In-Depth Information
Figure 7.17.
Koppie in the Margeride, central France.
fractures (Figs 2.2a and c, and 7.16b). Rock basins are well-developed (Fig. 1.2f), as are frost shat-
tered plates, known locally as clitter. Details vary but most workers consider that the tors were
initiated by differential, structurally-controlled subsurface weathering during the Early Tertiary,
in warm humid conditions, and that the weathered mantle was later stripped, in part at least by
solifluxion during Pleistocene glacial phases. The Massif Central, and the Margeride, also in cen-
tral France, are morphologically similar (Fig. 7.17).
The granitic (monzonite) areas of the Sierra Nevada of California (Bateman and Wahrhaftig,
1966) are characterised by deeply dissected (relief amplitude circa 600-650 m) glaciated river val-
leys such as that of the Merced River. The intervening high plain or plains, such as the Dana
Plateau, are remnants of ancient, largely unglaciated planation surfaces which have been stripped of
their regolith and which are thus of etch type (Fig. 1.1g). Superbly developed and exposed domes
or bornhardts are revealed in and adjacent to the dissected areas. Sheet structure is well-developed,
including banks of thin sheets on cirque headwalls and other recently sculpted surfaces (Fig. 2.7c).
Rock basins and slots are also prominent.
In the humid tropics, with high temperatures, abundant moisture and high inputs of organic
chemicals and activities, weathering is rapid, with the alteration of micas measured in decades and
of feldspars in centuries (Caillère and Henin, 1950). Silica, especially in an amorphous state, is
also dissolved. As a result, the landscape is typically blanketed by a thick regolith, and bedrock is
largely obscured. Only on the coast, in river channels, on steep slopes, and where there has been
vegetation clearance for agriculture or as a result of construction works (road cuttings, quarries,
tunnels and adits), the fresh bedrock is exposed. The overall topography is one of all slopes
(Fig. 7.18a)
,
though frequently with numerous blocks and boulders (nubbins) and with a tendency
to convex slopes and hence domical morphology. River patterns are frequently angular and are in
many instances demonstrably related to fracture patterns in the local bedrock.
Though limited, such exposures as suggested in other environments, weathering is characteris-
tically incomplete, with corestones or core-boulders, to use Scrivenor's (1931) apt term, widely
preserved (Fig. 5.4). Mass movements of debris are common and as a result of these, plus stream
erosion, many such corestones are exposed as boulders. The evacuation of detritus has led to boulders
and related forms, such as miniature towers, being exposed on peaks and ridge crests. The steep,
bare and frequently slightly convex-outward rock faces found on some steep hillsides are con-
strued as sheeting planes.
