Southern England. Peter Friend
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Название: Southern England

Автор: Peter Friend

Издательство: HarperCollins

Жанр: Природа и животные

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isbn: 9780007405923

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СКАЧАТЬ and small coves (Fig. 60). Notice how steep many of the sea cliffs are, and that they show little in the way of well-developed, regular layering or fracturing. Unlike the other upland areas of Cornwall, the Lizard is not underlain by granite. As mentioned in the general section of this chapter, some of the area is underlain by serpentinite, a distinctive, decorative rock that was originally part of the Earth’s mantle, below the crust and many kilometres below the surface (see Chapter 3). Other parts of the Lizard bedrock were originally basalt lavas and minor sheet-like intrusions along with small amounts of sediments, all similar to successions elsewhere that appear to have formed in or below the Earth’s oceanic crust. During the Variscan mountain building, this mixture of distinctive bedrock types appears to have been squeezed up amongst the strongly compressed Devonian killas. Today, the exceptional bedrock chemistry of the unusual Lizard rocks is the reason why the peninsula has such a variety of rare plant habitats. Much of the peninsula is a National Nature Reserve (NNR) or owned by the National Trust.

      As in Carnmenellis and Land’s End, a wave-cut platform has been identified on the Lizard, although its level is rather lower. In fact, the platform actually forms the Lizard Plateau and is remarkably flat, the ground surface varying between 60 and 100 m above sea level over large areas. This relative flatness probably reflects the rather uniform composition of the rock materials involved, and their uniform resistance to weathering and erosion.

      The coast of the Lizard Peninsula is formed almost entirely of steep cliffs, particularly around its southwestern perimeter. A few small beaches do occur in sheltered locations, such as at Coverack (b1), and picturesque fishing villages are scattered along the east side of the peninsula around small coves and gullies.

      FIG 60. The Lizard coastline. Note the contrast between the jagged coastal cliffs and the flat inland landscape. (Copyright Dae Sasitorn & Adrian Warren/ www.lastrefuge.co.uk)

      Landscape C: Cornish killas

      Most of the bedrock of West Cornwall is Devonian sediment, folded, faulted and – locally – altered during the Variscan mountain-building episode (see the general section of this chapter). The Devonian sediments, known to miners and quarrymen as killas, have been less resistant to landscape weathering and erosion than the granites (A) and the Lizard Complex (B), and so have been preferentially eroded to form lower landscapes. All the major bays and estuaries of this Area, such as St Ives Bay (c4) and the Carrick Roads at Falmouth (c7), are situated in killas areas for this reason. The Variscan folding and faulting that deformed the killas has also locally influenced the directions of valleys and their slopes, which have picked out variations in the killas layering, giving an east-west grain to the landscape (Fig. 61).

      FIG 61. Slope map of the eastern part of West Cornwall. The main granite bedrock areas are outlined and important boundaries in the Devonian bedrock indicate the direction of the Variscan folding. Note the circular china-clay workings that are visible in the St Austell granite (A5).

      FIG 62. Complex landscape of the North Cornwall coast, looking eastwards from Crantock Beach, over the Pentire Ridge towards Newquay (Fig. 52, c2) and Watergate Bay. (Copyright Dae Sasitorn & Adrian Warren/ www.lastrefuge.co.uk)

      FIG 63. Headlands, bays and beaches of the Newquay area (Fig. 52, c2), looking eastwards from a point 2 km west of Figure 62. Crantock Beach is visible in the middle distance. (Copyright Dae Sasitorn & Adrian Warren/www.lastrefuge.co.uk)

      The Flandrian sea-level rise, which ended only 5,000 years ago, has also left its mark on West Cornwall. The most obvious legacy is the extensive array of tidal estuaries at the mouths of the main rivers, which are flooded river valleys or rias. The most striking example is the series of branched rias around Falmouth known as the Carrick Roads (c7). These extend northwards across half of the width of West Cornwall and have had an obvious major influence on the road and rail transport pattern of the area. Major branch rias to the west, north and east around the Carrick Roads divide this part of the Cornish landscape into numerous isolated peninsulas. The inland valleys of the killas areas tend to be deeply incised with little widening, and the branching patterns of these valleys are very clear on the slope map. The rias are obviously the direct result of the drowning of valleys of this form by the Flandrian sea-level rise.

      The coastline of the killas landscape of West Cornwall is extremely varied: small, sandy coves alternating with rocky promontories and high cliffs are typical of this part of the north coast (Figs 62 and 63). This irregular coastline is due to local variation in the type and strength of the killas bedrock, with weaker units (often slates) eroding to small bays while the more resistant rocks (often limestones or quartzites) form the headlands.

      The sandy bays of north Cornwall (c1, Padstow and the River Camel Estuary; c2, Newquay Bay; c3, Perranporth and Perran beach; c4, St Ives Bay) are famous for surfing, due to the splendid waves that roll in from the Atlantic Ocean. Apart from Padstow Bay (c1), at the mouth of the River Camel, most of the north Cornwall beaches are not obviously linked to river sources of sand and so must have been filled by sand transported from offshore sources by storm waves. At many famous surfing beaches, such as Perranporth (c3), sand banks built up by winter storms can be eroded in the summer, resulting in dangerous currents sweeping out to sea. The wind-blown dunes of the Penhale Sands, north of Perranporth, are a spectacular example of the way that gales from the west can move beach sand up to 2 km inland. Because of the variation in the killas bedrock, some headlands are long and the inlets are narrow enough to develop fast tidal flows, capable of forming large, regular ripples as seen in the foreground of Figure 62.

      On the south coast, storm-built sandy beaches have also formed, for example at Newlyn (c8), Praa Sands (c5) and at the mouth of Helston valley south of Porthleven (c6). Further east, the coastline is much more sheltered and the scenery is dominated by the drowned valleys and quiet inlets of the Carrick Roads (c7).

      AREA 2: EAST CORNWALL AND SOUTH DEVON

      This Area straddles the boundary between Cornwall and Devon (Fig. 64). In terms of the coastlines of the Southwest, it includes a small stretch of the north coast near Tintagel, and a large section of the south coast from St Austell, via Plymouth and Start Point, to Torquay and Exmouth (Fig. 65).

      In the general section of this chapter, the early geological history of the Southwest Region as a whole has been outlined, particularly the evolution of the Variscan mountain belt. Younger episodes in the region have also been discussed, involving river and valley erosion of the landscape, the effects of the Ice Age and the changes in the coastline that have resulted from the most recent (Flandrian) rise in sea level.

      In the sections below we shall consider more local features of the scenery in this Area, dividing it into four distinct Landscapes (labelled A to D), each underlain by a different kind of bedrock (Fig. 66).

      FIG 64. Location map for Area 2.

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