Geology Reference
In-Depth Information
into question by the late Holocene development of similar forms on prehistoric human monuments
in Brittany (Fig. 8.23).
Nubbins are prominent in humid tropical regions, such as monsoonal northern Australia and
Hong Kong, and this can be explained in terms of the rapid disintegration of the outer shells or
sheet structures of bornhardts in the high temperatures and seasonally abundant groundwaters typ-
ical of such regions. Sheet structure is weathered and broken into blocks beneath ground level.
Nubbins are however known from other climatic settings such as Namaqualand, in Western Cape
Province of South Africa, and the Alice Springs area of the Northern Territory, Australia. In each
of these examples, the nubbins occur in topographic basins to which surface and groundwaters
gravitate so that they may develop in local moist sites. On the other hand, nubbins in present mid-
latitude cool areas, such as the Bohemian Massif (Demek, 1964), and desert settings, as in the
Mojave Desert of the southwestern United States (Oberlander, 1972), have been attributed to
development in warm humid climates followed by climatic change.
Castle koppies have a peculiar bimodal distribution, or apparently so. On the one hand, they are
characteristic of cold lands, and are the well-known tors of the granite uplands of southwestern
England (Linton, 1964). They are also well represented in the Pyrenees, the Bohemian Massif and
the Massif Central of France. They appear to be due to intense frost action and to ground-ice active
in the regolith at the margins of the residuals. On the other hand, the classical koppie country is the
high veld of southern Africa (Lageat, 1989) and especially Zimbabwe. They are associated with
palaeosurfaces of low relief. Here the koppies appear to be the result of long-continued subsurface
marginal weathering or residual masses the crests of which are just exposed. As with nubbins, kop-
pies are also developed in local wet sites, as at the Devil's Marbles, Northern Territory (Fig. 7.9).
Thus, the basic mechanism for koppie development may be the same though the processes
involved are very different. In these terms koppies are convergent landforms.
Frost action is involved in the formation of another distinctive residual in granite. The freeze-
thaw mechanism is evidently effective in exploiting well-developed fractures, and for this reason
towers and frost-shattered bedrock are well represented in cool or seasonally cool climates. Good
examples are found in the Organ Mountains of southern New Mexico, in Northern Canada, in
the Mt Whitney area of the eastern Sierra Nevada, and in Cathedral Rocks, in the Yosemite,
both in California (Figs 7.12b and 7.13). The Needles, in South Dakota, provides another
well-known example (
Fig. 12.13)
.
The twin peaks of Fitz Roy and Cerro Torre in the Province of
Santa Cruz, in Argentine Patagonia, and, although smaller, several hills in the Sierra de Gredos
of the Spanish Central Massif and in the Pitões das Junhas of northern Portugal, provide other
examples.
Though sheet structure is widely developed, pseudobedding (Fig. 2.2) restricted to near
surface zones may be due to frost action, to the freezing and expansion of meteoric waters that
percolated into the country rock, causing the separation of one layer after another. Such features
may well, however, reflect exploitation by weathering agencies of latent stress fractures or lin-
eations in the rock.
12.4
THE COASTAL CONTEXT
Many typical granite forms are found in coastal zones in various parts of the world. Some are
structural in origin. Thus, the sheet structures exposed in the littoral of the Pearson Islands (Fig. 1.8),
in the eastern Great Australian Bight, are most likely (see Chapter 2) a manifestation of tectonic
stress, and are identical with those exposed, for example, in the sidewalls of deeply incised valleys
(Fig. 2.6). Again, geos due to the exploitation of fracture zones, or, more commonly, to the preferen-
tial weathering and erosion of weaker veins intrusive into the granite host mass, are fairly common,
and sea caves, characteristically pear-shaped in cross section (Sjoberg, 1986, 1987 and see
Fig.
12.15)
,
due to the exploitation by waves of steeply inclined fracture zones have been reported from
several sites on the Swedish coast, and inland, where they are used as indicators of isostatic move-
ments (Sjoberg, 1986, 1987).
