Geology Reference
In-Depth Information
Figure 5.13.
Shear planes with conjugate system of discontinuities and corestones exposed in road cutting
at A Coruña, Galicia, NW Spain.
On the other hand, it has been suggested that sheet fractures and structures are due to shorten-
ing/compression (Chapter 2), implicitly acting on brittle stage at some depth in the crust. The
resultant zones of strain were planes of weakness, for some were intruded by veins and sills, but
they were not manifested as fractures until erosional unloading had diminished the lithostatic load.
The orthogonal fracture systems that are the basis of many bornhardts (Chapter 6), as well as boul-
ders, are similarly initiated by stress at depth, but do not find expression as partings until lowering
of the land bring them close to the surface. Thus, sheet and orthogonal fractures can be regarded
as released tectonic forms. There is an epigene or external component involved in the development
of the fractures and associated forms, but it is indirect and referred.
The weathering and erosion of passive structures have produced a wide range of forms, major
and minor, in granitic and in other lithological environments. Many of the weaknesses exploited,
however, are due to crustal events dating from the distant past, and ranging from hydrothermal
invasions, to the intrusion of stocks and the development of anatectic folds, to the development of
foliation or cleavage. It is misleadingly oversimple to refer to associated forms as structural with-
out reference to the crustal activities in which they are based. They can be regarded as exploited
crustal features, magmatic, thermal or tectonic (see
Figs 5.10
and 5.13).
Corestone boulders are examples of exploited tectonic features. In most instances their shape
and size vary with fracture patterns though in some magmatic processes resulting in mineral band-
ing (see below), for example, have played a part. These bedrock properties did not, however, find
expression until erosion brought the country rock near the surface and into the zone of shallow
groundwaters.
5.5
TYPES OF PERIPHERAL OR MARGINAL WEATHERING
Several variations in the type of weathering developed marginal to corestones have been noted in
the field, though whatever the details, the end product is still referred to as spheroidal weathering.
