Geology Reference
In-Depth Information
and boulders, and as already described (Chapter 5), they may be bordered by laminated rock that
is easly weathered below the soil. The elongated masses of fresh rock may be several tens, or even
a few hundreds, of metres, in diameter ( Fig. 6.11a and b), and give rise to small scale bornhardts.
The mechanism could also e xplain the asymmetr y of some domical hills such as the elephant
rocks described from some parts of the world.
In this connection, it is of interest that many years ago Brajnikov (1953) suggested that some of
the bornhardts (morros) of southeaster n Brazil dif fer from other bor nhardts (which are widel y
accepted as being huge projections o f bedrock) in that the y are gigantic “floaters”, or detached
masses of fresh rock set in a matrix of weathered material. Such detached masses of fresh bedrock
could be associated with major shear zones.
On the other hand, many bornhardts appear to reflect bedrock str ucture and to be due to the
exploitation of weaknesses in the crust by various external agents of weathering and erosion. In
the Guitiriz region of nor thwestern Galicia, the rounded g ranite hills display low domical out-
crops, as well as large residual boulders and small koppies. Exposures in shallow quarries suggest
a direct relationship between surface form and subsurface structure, for in the g ranite there are
developed man y features due to horizontal shor tening, including o verthrusting and v ertical
wedges, as well as small, mainly large-radius, domes developed in granitic rocks (Figs 2.9a-c).
Variations in composition and texture have been neglected as causes of differential weathering
and erosion within plutonic bodies. Most plutons are comple x composite features within w hich
there are several bodies of varied petrological, and hence weathering and erosional characteristics.
Some of these internal variations are pronounced, others subtle. In some cases, as for instance in
the exposures of Hiltaba Granite Suite of northwestern Eyre Peninsula, South Australia, there is no
correlation between petrology and topography. Stone Mountain, Georgia, is a granitic dome, and
some of the adjacent plains are eroded in biotite gneiss, though on the northern and eastern sides
of the residual g ranite also underlies the plains ( Fig. 6.12) , suggesting that f actors additional to
composition are involved. But in French Guyana leucogranites form the residuals that stand above
plains eroded in biotite gneiss, g ranitic residuals stand abo ve plains eroded in schist in the Air
Mountains of the souther n Sahara, and in par ts of Namibia and in souther n Africa potash-rich
masses tend to be upstanding. Aplogranite and f ine grained granite are cited as gi ving rise to
Figure 6.12. Map of Stone Mountain, Geor gia, eastern USA, sho wing relation betw een topography and
geology (Herrmann, 1957).
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