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On the scale of the whole Southwest Region, the main upland areas are Exmoor in the north and the zone of distinct granite domes in the south, extending from Dartmoor to Land’s End.

The highest point of Exmoor is Dunkery Beacon (519 m). Exmoor has been eroded from Devonian bedrock, and may owe some of its elevation to the greater resistance 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 Region 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 divide (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 earliest 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 significantly 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 Bodmin and 621 m for Dartmoor). This gradient is overall only about 3 m per km. The geophysical data on the large, deep granite body (Fig. 44) recognised below the surface granite bodies do not provide independent evidence for a slope of this sort deep down. Some tilting of the landscape downwards towards the west may have occurred, or the slope may simply reflect the greater proximity of the western granite bodies to the sea and repeated episodes of marine erosion.