Geology Reference
In-Depth Information
Figure 5.19.
Blocky corestone morainic accumulation on the margin of Touça glacier valley (Serra do Gerês,
northern Portugal).
of the weathered rock is largely the work of wash, rills and rivers. A spectacular example of the
concentration of residual boulders by running water occurs in the high valley of the Touça River,
in northern Portugal, where glacial meltwaters have evacuated grus leaving a lateral moraine
formed by a train of large boulders on the side of the river channel over a length of some 10 km
(Fig. 5.19).
But wind-driven waves have achieved exposure of corestones by the preferential stripping of
grus, so that entire beaches are composed of corestones, as for example on the Costa da Morte, in
western Galicia, between Cape Vilano and Camelle, Spain and in Cape Willoughby, Kangaroo
Island, South Australia, where the sea has stripped the regolith to expose an etch surface in gran-
ite, and the released corestones have been washed into a bouldery shingle beach or coido (see also
Chapter 12). Wind may play some small part in the exposure of corestones in arid and semi-arid
regions, and solifluction is significant in nival areas, but it is running water that is primarily
responsible for the exposure of corestones as boulders.
5.8
BOULDERS OF EPIGENE ORIGIN
Not all boulders are of subterranean origin. Some are glacial erratics, some result from frost
action, and yet others result from the disintegration of massive arcuate sheets or slabs which are
typical of bornhardts or domical hills (Barbeau and Gèze, 1957 and see Chapters 2 and 6). These
arcuate masses of rock are split by steeply inclined radial fracture planes (cross or fan joints). They
effectively subdivide the thick slabs into blocks, which can be seen
in situ
in orderly arrangement
on the flanks of many bornhardts and nubbins (see Chapter 7).
Some sheets disintegrate while still beneath the land surface and the blocks are converted to
corestones set in grus
(Fig. 5.20)
.
Others, however, have been rounded and yet others completely
broken down by epigene or subaerial agencies, i.e. by processes active at the land surface. There
are many variations in the pattern of weathering, but in some places, for instance in the Monte
Pindo of Galicia, Spain and in Corsica, France, the sheet remnants preserved on upper slopes
consist of clusters of angular blocks, many with tafoni well-developed, whereas those resting on
