Geoscience Reference
In-Depth Information
Cornish killas in Areas 1 and 2. Mudstones and sandstones are the commonest types
of sedimentary bedrocks in the Exmoor landscape, originally formed in the Devonian
basinal setting illustrated in Figure 38.
Exmoor was never glaciated, and the upland plateau surface may be as much as
200 million years old, formed in the Early Jurassic. Gently rounded hillsides and gentle
valleys are typical of most of the higher ground, suggesting that the surface was deve-
loped over a long period of stream action. A more recent valley incision is also appar-
ent and probably relates to steady upward movement of the ground relative to the sea
during Tertiary times.
FIG
83.
Heddon's Mouth, Exmoor, 5 km west of Lynton. (Copyright National Monuments
Record, English Heritage)
As the slope map (Fig. 84) shows, local slope patterns in both Exmoor and the
Culm area of Landscape
B
have a tendency to run broadly west to east. This is the
result of the direction of the folding that formed during the Variscan convergence.
Local erosion of tilted sedimentary layers and their fold axes has produced ridges and
hillcrests with this orientation. The most obvious east-west slope is the 'Exmoor line',
running eastwards from Barnstaple and South Molton (Fig. 80), which marks the join
of the more easily eroded Carboniferous sediments (Landscape
B
) and the underlying
Devonian sediments.
The coast of Exmoor has some of the highest cliffs in England, formed where the
old hilly plateau of Exmoor, with tops locally over 300 m in elevation, has been cut in-
to and removed by young coastal erosion. There is so much interest along this coastline
that I will comment on it locality by locality, from east to west (Fig. 81).
Westwards from Minehead, the Bluff is the northernmost point of an isolated hill
range that reaches elevations of more than 300 m and runs for some 7 km along the
coast. There is a marked change of slope at 200 m, above which the topography is
gently hilly, whereas below it descends steeply in numerous landslides towards small
