Geology Reference
In-Depth Information
13
Retrospect and prospect
“
Ages are spent collecting materials, ages more in separating and combining them. Even when a
system has been formed, there is still something to add, to alter or to reject. Every generation
enjoys the use of a vast hoard bequeathed to it by antiquity, and transmits that hoard, augmented
by fresh acquisitions, to future ages. In these pursuits, therefore, the first speculators be under
great disadvantages, and when they fail, be entitled to praise
.” (Lord Macaulay, Essay on Milton).
No landform is restricted to granite. Many are better or more commonly developed in granite
than in any other rock type but none is unique to that material. All have their congeners in other
lithological environments. The corollary of this is that the factors, processes and mechanisms
invoked in explanation of granite landforms have a wider application and indeed are germane
to the understanding of landforms and landscapes in general. Thus, many granitic forms are of
two-stage origin, and originated at the weathering front. There, shallow subsurface groundwaters
exploited structural weaknesses, major and minor. There can be no doubt that many familiar land-
forms have their origins in the subsurface though it would be imprudent to assume that all repre-
sentatives of a particular form are of such an origin. There can be no doubt, for example, that some
runnels or flutings, and some rock basins, have formed after the exposure of the host surface,
though equally, many demonstrably have formed at the weathering front. Similarly, though many
bornhardts are of two-stage origin, others, due to upfaulting, for example, may well be epigene
features. The local evidence must be sought. If the problem is appreciated, however, the greater the
chance of finding relevant evidence.
At a quite different scale, pitting, notches and flared slopes are good indicators of recent soil
erosion and of the thickness of soil evacuated (e.g. Fig. 13.1a; also Fig. 3.5e). The flared, and
notched, basal slopes shown in Fig. 13.1b, suggest that the thickness of detritus stored in these
basins and valleys on the flanks of Domboshawa, a bornhardt near Harare in central Zimbabwe,
was formerly much greater.
The implications of such a two-stage origin are several, but in particular, besides emphasising
the great antiquity of the geological factors in part responsible for many contemporary forms,
there is a strong possibility that as groundwaters are ubiquitous, so are the forms they generate.
Many granitic forms, and their congeners in other rock types, appear to be azonal. This is a con-
clusion of general significance.
The Earth's crust is not quiescent but continues to be active - more at and near plate margins
than elsewhere but everywhere active. Differential movements cause stress and strain, as witness
the many modern or neotectonic landforms known from both granitic and other terrains, and man-
ifested also in contrasted stress regimes and in varied fracture densities. These crustal characteris-
tics find particularly clear expression in granitic terrains.
At and near the Earth's surface granite is brittle and neotectonic forms, involving warps as well
as fractures, are well-developed and preserved. Minor scarps, A-tents and wedges are especially
well represented and preserved, but antiforms and synforms also find topographic expression.
Tectonic stress is significant in the formation of such features as bornhardts and sheet structure but
