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istance to erosion of this material compared with the Carboniferous material that forms
the bedrock further south. Another possible factor is suggested by the remarkable way
that many of the river systems of the southwest drain to the south coast, despite their
sources being remarkably close to the north coast (Fig. 48). This is the case for the Exe,
flowing from Exmoor southwards via Exeter to Exmouth, and, further west, the Tamar,
which begins northeast of Bude and flows southwards past Launceston and Tavistock
before discharging into Plymouth Sound. It looks as if this part of the Southwest Re-
gion has been tilted southwards as these river systems developed on either side of the
high ground of Dartmoor, where the granite resisted erosion. A southerly tilt would
also be consistent with a preferential uplift of the Exmoor Hills to the north.
FIG
48.
River pathways, mean flow rates (m
3
/s) at some river stations, main drainage di-
vide (red line) and main granites of the Southwest Region.
The southern areas of hills correspond so clearly with the areas of granite outcrops
that there can be little doubt that the greater resistance to erosion of the granite explains
their higher elevations. But how long has this erosion been taking place? Emplacement
of the granites was over by the end of Carboniferous times (about 300 million years
ago) and there is evidence of pebbles in the New Red Sandstone from the Dartmoor
granite and from the altered bedrock close by. Although the precise age of the earli-
est New Red Sandstone is uncertain, it does not appear to be much younger than the
age of granite emplacement. However, it appears that the granites were not being signi-
ficantly eroded in quantity much before Cretaceous times, 200 million years later and
about 100 million years ago. Since then, the granites have been eroded into the present
patterns of local hills and valleys, but at very variable rates as climate, coverage by the
sea and rates of river erosion changed.
Each of the main granite bodies corresponds closely to an area of high ground,
and their maximum heights tend to be greater towards the east (44 m for the Isles of
Scilly, 247 m for Land's End, 252 m for Carnmenellis, 312 m for St Austell, 420 m for
