Geoscience Reference
In-Depth Information
tell, Bodmin Moor and Dartmoor in succession to the east. These distinct granite areas
at the surface can be visualised as the tops of fingers extending upwards from a single
continuous granite body detectable by gravity surveys at greater depth under the spine
of Southwest England (Fig. 44). The deep body extends for some 200 km along the
length of the mountain belt.
FIG
43.
Polished slab cut in the Dartmoor granite showing typical granite texture. Crys-
tals of quartz (light grey), feldspar (white) and biotite (black) have interlocked as the
magma (molten rock) solidified on cooling. (Copyright Landform Slides - Ken Gardner)
FIG
44.
Diagram showing the large granite body below the bedrock of Cornwall and
Devon, and the way the granite bosses now visible at the surface are upward extensions
of this larger body.
Although there was probably some time range in the arrival of different granite
bodies in the upper crust, the main episodes took place at the very end of the Carbon-
iferous and during the earliest Permian, roughly 300 million years ago.
The arrival of the granites from below was only one part of the invasion of the up-
per levels of the bedrock that took place at this time. Widespread mineralisation around
the granites has probably been even more important than the arrival of the granites
themselves, in terms of human history. The term mineralisation is used to cover the
alteration of the solid granite and the surrounding (older) bedrock that has, in some
areas, been caused by the movement of very hot and chemically rich water, using the
