Southern England. Peter Friend

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      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.

       Ice Age episodes

      Ice sheets do not appear to have covered the present land of the Southwest Region to any important extent during any of the major cold episodes of the Ice Age. In the Isles of Scilly, material deposited directly from a grounded ice sheet has been recognised and is thought to be Devensian (last cold phase) in age (Fig. 49). Various giant boulders derived from metamorphic sources are a notable feature of some localities on the North Devon coast, some of which appear to have come from Scotland. However, it is not clear whether they were transported to their present locations by a large ice sheet or by floating ice.

      In spite of the lack of an actual ice sheet, the repeated cold episodes of the Ice Age must have had a considerable effect upon the weathering style of the bedrock, for example influencing the granite tors, mobilising material to move down slopes and changing drainage patterns and the surface blanket of soft materials.

      FIG 49. Map showing the greatest extent of the last main (Devensian) ice sheet across England and Wales.

      AREA 1: WEST CORNWALL

      A remarkable feature of the peninsula of West Cornwall (Figs 50 and 51), as it narrows towards Land’s End, is the contrast between the spectacular coastal scenery and the scenery inland. The rocky coastal cliffs and sharply indented coves reflect West Cornwall’s exposure to the prevailing Atlantic storms, and contrast starkly with the inland scenery of rolling – though often rocky -hillsides, carved into a network of small valleys and streams.

      The main features of the inland landscape appear to have formed over millions of years, and ultimately reflect the bedrock pattern that has been inherited from the Variscan mountain building that ended 300 million years ago. In contrast, the coastal landscape is clearly much younger, and much of it has been produced by changes in sea level that have occurred since the last main cold phase of the Ice Age, some 10,000 years ago. There is some evidence of earlier sea levels but this is more difficult to evaluate, as it has generally been removed by more recent erosional events.

      FIG 50. Location map for Area 1.

      I have divided West Cornwall into three Landscapes (A to C), each with distinctive bedrock geology (Fig. 52).

      Landscape A: Granite areas

      The Isles of Scilly (A1; Fig. 53) are formed by the westernmost significant granite bodies of southwestern England. They lie some 45 km southwest of Land’s End, scattered over an area approximately 20 km by 15 km. Most of the 150 islands are little more than bare outcrops of granite, sometimes largely submerged at high tide. The landscape is windswept and mainly treeless, with heathlands where the ground has not been cultivated. Historically the islanders eked out a precarious existence from crofting, until the nineteenth century, when shipbuilding and the growing of flowers became economic. Today most of the cultivated land consists of small fields of flowers edged with evergreen hedges, and horticultural work, along with tourism, has become the mainstay of the economy.

      The smaller islands are often arranged in rows, separated by ‘sounds’ (areas of shallow water) that tend to have a northwest-southeast orientation. These sounds must have been valleys before they were drowned by the recent (Flandrian) sea-level rise. Their orientation is similar to that of the valleys and faults of the Land’s End granite, discussed more fully below. Numerous sandy bays and beaches reflect the granite weathering and the transport of the weathered sediment, by storms and tides, to more sheltered parts of the island landscape.

      FIG 51. Natural and man-made features of Area 1.

      In the general section of this chapter it has been mentioned that the northern Scillies appear to have been invaded by ice late in the history of the last (Devensian) cold phase of the Ice Age (Fig. 49), and this is surprising in view of their southerly location. It appears that when the Devensian ice sheet had grown to its greatest extent, an elongate tongue of ice, perhaps some 150 km wide, extended for nearly 500 km from the Irish and Welsh ice sheets to the edge of the Atlantic continental shelf. This tongue became so large because it was vigorously fed by ice from the high ground of Ireland to the west, and the Lake District of England and the mountains of Wales to the east. The ice extended across the mouth of the Bristol Channel, well clear of the present north Cornwall coastline, before leaving ice-laid sediment on the northern fringe of the Isles of Scilly. South of the island areas that were covered by ice, the granite has been weathered locally into tors.

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