Geoscience Reference
In-Depth Information
There are a number of gorges resulting from the deep incision of rivers and
streams into the granites and their surrounding materials. Around the Dartmoor granite,
the valleys of the River Dart to the east and the Lydford Gorge to the west (
a3
) are ex-
amples of these. South of the Bodmin Moor granite, the River Fowey also has a spec-
tacular and well-known gorge at the Golitha Falls (
a4
).
Tors are remarkable features of both the Dartmoor and Bodmin Moor granite areas
(Fig. 68). They provide a focus for visitors in granite scenery that is often otherwise
rather featureless and empty, and there are well over a hundred tors on Dartmoor alone.
Tors tend to look like heaps of granite blocks, but a closer inspection shows that they
are not jumbled but rather blocks that 'belong' next to their neighbours. These linked
blocks are relict volumes of a much larger volume of granite, most of which has dis-
integrated and been removed by weathering. Tors are very much features of granite
weathering, suggesting that the coarse interlocking crystal texture and general lack of
layering have caused these remarkable landforms to appear.
Many tors occur on the most elevated parts of the scenery, looking like man-made
cairns. Others occur on the slopes of valleys, but it is clear that tors will only form
where down-slope processes, driven by gravity, can remove the weathering debris from
around them. Cracks in the granite (technically called
joints
) give tors much of their
distinctive appearance: near-vertical joints produce towers and pillars, while roughly
horizontal joints give the rocks a layered, blocky appearance (Fig. 69). Most of the
joints seem to have formed during the arrival of the granite material from below (intru-
sion), either due to contraction from cooling of the newly solid material, or due to other
stresses acting shortly after solidification. The flat-lying joints (horizontal on hill tops,
and parallel to slopes elsewhere) may also be due to the erosion of the present scenery,
allowing the granite to expand and fracture as the weight of the overlying material is
removed.
