Geoscience Reference
In-Depth Information
FIG
59.
Sketch map showing the orientation of some of the main faults in West Cornwall.
Another similarity to Land's End is the widespread topographic platform at about
140 m above sea level. This platform is particularly clear north of the Carnmenellis
granite but is also obvious in the Godolphin granite (Fig. 57). In the Porthmeor and
Camborne areas the platform is particularly distinctive, and the continuity of its land-
ward slope is clear on the slope map. It has generally been assumed that these platforms
were cut by storm waves when the sea stood at this level about 3 million years ago. At
this time, West Cornwall would have consisted of granite islands, like the present Isles
of Scilly, while the surrounding Devonian bedrock (killas) was submerged.
The western part of the St Austell granite (
A5
) lies within Area 1, and again its
resistance to landscape erosion is shown by the high ground that it occupies. The re-
markable feature of this granite is the way it has been altered by the circulation of hot
fluids. Much of the feldspar in this granite has been altered to the mineral kaolinite,
which is a member of the clay mineral group that is the key component of china clay.
The result of this is that the St Austell granite has been quarried, particularly in its
western part within Area 1. The kaolinite has been extracted from the rotted granite by
high-pressure water jets, which leave large volumes of quartz and feldspar grains that
are heaped up in enormous and obvious spoil heaps.
Most of the original heathland and moorland on the St Austell granite has been
destroyed by the mining industry. More recently, the Eden project redevelopment of
one quarry complex (in Area 2) has brought many visitors to the area.
