Читать книгу Southern England - Peter Friend - Страница 39

Bodmin Moor (A6) and Dartmoor (A7) are the most southerly large upland areas in England and, in each case, the granite bedrock has resisted landscape erosion to produce the high ground. The highest point of elevation in this Landscape is High Willhays (a1) at 621 m above sea level on Dartmoor. Evidence of the ongoing nature of this landscape erosion is provided by the contrast between the high moorland, with bogs, steep valleys and exposed tors, on one hand, and the surrounding low farmland on the other.

In the general section of this chapter I have outlined some main features of the southwest granites, such as their resistance to erosion and the valuable minerals associated with them. They have also provided excellent strong building stone for the buildings of the Region.

As in the granite areas of West Cornwall (Area 1), mineral mining activities have had a strong impact on the area, and derelict tin mine buildings are scattered over much of the landscape. The china-clay industry has also produced significant changes to the scenery, one of the most remarkable sites being the workings 3 km northeast of St Austell (a2). These pits now house the famous Eden Project, an educational charity providing a ‘Living Theatre of People and Plants’ and attracting over a million visitors each year (Fig. 67).

FIG 67. The Eden Project is situated in a former china-clay pit. (Copyright Dae Sasitorn & Adrian Warren/www.lastrefuge.co.uk)

The present-day pattern of streams and their valleys has evolved from ancestral streams and valleys that carved most of the inland scenery over millions of years. In the general section of this chapter, we have seen the remarkable way that the drainages of the rivers Tamar and Exe flow southwards across most of the Southwest Region to the sea, divided by the high ground of Dartmoor. A general tilt of the Region to the south, and the resistance of the granite domes to stream and valley erosion, appear to have been important factors. Closer examination of the drainage patterns shows that the streams and valleys of the Bodmin Moor granite tend to radiate out from near its centre, but that the distinctly larger Dartmoor granite has eroded down to form two drainage divides, one in the north and one in the south. This may simply be a matter of the different size of these two granite areas, which has allowed a more complex drainage pattern to develop through time over Dartmoor.

The parallel groups of incised valleys that are common in the granites of Area 1 are not clearly developed on Bodmin Moor and not visible at all on Dartmoor. It is intriguing that the fault system that was responsible for the parallel valleys further to the west is not present in these larger eastern granites. This may tell us something about the greater depth of weathering and erosion experienced by the eastern granites.

There are a number of gorges resulting from the deep incision of rivers and streams into the granites and their surrounding materials. Around the Dartmoor granite, the valleys of the River Dart to the east and the Lydford Gorge to the west (a3) are examples of these. South of the Bodmin Moor granite, the River Fowey also has a spectacular and well-known gorge at the Golitha Falls (a4).

Tors are remarkable features of both the Dartmoor and Bodmin Moor granite areas (Fig. 68). They provide a focus for visitors in granite scenery that is often otherwise rather featureless and empty, and there are well over a hundred tors on Dartmoor alone. Tors tend to look like heaps of granite blocks, but a closer inspection shows that they are not jumbled but rather blocks that ‘belong’ next to their neighbours. These linked blocks are relict volumes of a much larger volume of granite, most of which has disintegrated and been removed by weathering. Tors are very much features of granite weathering, suggesting that the coarse interlocking crystal texture and general lack of layering have caused these remarkable landforms to appear.

Many tors occur on the most elevated parts of the scenery, looking like man-made cairns. Others occur on the slopes of valleys, but it is clear that tors will only form where down-slope processes, driven by gravity, can remove the weathering debris from around them. Cracks in the granite (technically called joints) give tors much of their distinctive appearance: near-vertical joints produce towers and pillars, while roughly horizontal joints give the rocks a layered, blocky appearance (Fig. 69). Most of the joints seem to have formed during the arrival of the granite material from below (intrusion), either due to contraction from cooling of the newly solid material, or due to other stresses acting shortly after solidification. The flat-lying joints (horizontal on hill tops, and parallel to slopes elsewhere) may also be due to the erosion of the present scenery, allowing the granite to expand and fracture as the weight of the overlying material is removed.

FIG 68. Hay Tor, Dartmoor, looking southeast. (Copyright Dae Sasitorn & Adrian Warren/www.lastrefuge.co.uk)

FIG 69. Hound Tor, Dartmoor. (Copyright Landform Slides – John L. Roberts)

The slopes round tors tend to be covered with loose granite blocks (often referred to as clitter), generally angular and obviously derived from the tors (Fig. 70). Finer-grained, crystal-size gravel or sand of quartz and feldspar is another weathering product and is locally called growan or sometimes head. It is clear that much of the alteration of the granite that has resulted in the appearance of the tors must have been strongly influenced by the climate, vegetation and soil-forming conditions existing at different times and in different scenic settings. Much of this has been compared to the weathering and down-slope movement that is seen in high-latitude cold climates today, and so is explained as a result of the cold climate conditions experienced repeatedly during the Ice Age. However, weathering of granites is much faster today in the warm, tropical jungle areas of the world, compared to drier, cooler and less vegetated conditions. Early episodes of weathering of the Southwest Region granites may have taken place under the warm, tropical conditions that are indicated by early Tertiary fossil deposits elsewhere in England.

FIG 70. Mass-flow terrace, looking westward from Cox Tor, Dartmoor. The terrace is interpreted as being the result of down-slope movement under alternating freeze-thaw conditions. (Copyright Landform Slides – Ken Gardner)

Rock basins are low-lying hollows in the granite topography draped with granite weathering products. Sometimes these are dry and their floors are simply coated with granite weathering materials. In other places the hollows are covered by peat, which is often a feature of the higher and wetter parts of the granite hills. Under very wet conditions, the hollows contain deep bogs or mires, with a reputation for being bottomless! How these low hollows were excavated is a puzzle.

Topographic platforms, cut by storm waves during times of high sea level, have been claimed to be present around the Dartmoor and Bodmin granite areas, although they are not as distinctive as those discussed on the Land’s End and Carnmenellis granites of Area 1. The Area 2 platforms are at heights of between 200 and 300 m above sea level, but in the absence of dated deposits similar to the St Erth beds of Area 1, their relevance to sea-level changes is open to doubt. Indeed, as mentioned above, terraces have been recognised around the Dartmoor granite that are thought to be the result of down-slope mass movement under freeze-thaw conditions, rather than due to sea-level changes.