Southern England

The western part of the St Austell granite (A5) lies within Area 1, and again its resistance to landscape erosion is shown by the high ground that it occupies. The remarkable feature of this granite is the way it has been altered by the circulation of hot fluids. Much of the feldspar in this granite has been altered to the mineral kaolinite, which is a member of the clay mineral group that is the key component of china clay. The result of this is that the St Austell granite has been quarried, particularly in its western part within Area 1. The kaolinite has been extracted from the rotted granite by high-pressure water jets, which leave large volumes of quartz and feldspar grains that are heaped up in enormous and obvious spoil heaps.

Most of the original heathland and moorland on the St Austell granite has been destroyed by the mining industry. More recently, the Eden project redevelopment of one quarry complex (in Area 2) has brought many visitors to the area.

Landscape B: The Lizard

Lizard Point is the most southerly headland in Britain, part of a wider Lizard landscape comprising a flat heathland plateau bounded by dramatic cliffs 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.

FIG 65. Natural and man-made features of Area 2.

FIG 66. Area 2, showing Landscape A to D and localities (a1, a2 etc.) mentioned in the text.

Landscape A: Granite areas

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.

Landscape B: Killas and other Devonian bedrock

Apart from the granites, Devonian sediments make up the bedrock of the southern and central part of Area 2. They consist largely of slates and mudstones with some sandstones, and are known generally as killas to distinguish them from the granites and other younger, less altered sediments. In a few localities around Plymouth (b1) there are Devonian limestones, similar to the well-known limestones around Torquay (d1) and Chudleigh (d9). The settings in which these Devonian sediments may have formed are illustrated in Figure 38, in the general section of this chapter.

The youth of the coastal scenery of this Landscape combines with the vigour of many of the processes operating to make it much more distinctive and dramatic than the inland scenery. In the west of Area 2, cliffs characterise the Cornish section of the south coast and often intersect deeply incised valleys that are clearly older features (Fig. 71).

Cornwall and Devon are separated from each other in this Area by the River Tamar, and this meets the south coast in a large drowned valley system around which Plymouth (b1) has grown (Fig. 72). Plymouth Sound is one of the best natural harbours in the Southwest and the historical naval importance of this city is the result. Similar, but smaller, valley systems (sometimes called rias) are common all along this stretch of coast, as they are further west in Area 1. Flooded valleys form the estuaries of the River Fowey, east of St Austell, and farther east still at Salcombe (b3) and Dartmouth (b8).

FIG 71. Polperro, on the south Cornwall coast. (Copyright Dae Sasitorn & Adrian Warren/ www.lastrefuge.co.uk)

FIG 72. The Tamar and Brunel Bridges, between Plymouth (Devon), to the right, and Saltash (Cornwall). (Copyright Dae Sasitorn & Adrian Warren/www.lastrefuge.co.uk)

The headlands from Bolt Tail (b2) to Start Point (b5) are made of some of the most highly altered and probably oldest bedrock in Devon, although the age of their deposition as sediments is not known. They have been changed locally to mica-rich and hornblende-rich schists that must have been altered (metamorphosed) several kilometres below the surface, before being pushed upwards during the Variscan mountain-building event. The local resistance of these schists to erosion has led to a particularly intricate pattern of small but sharp headlands and tight small bays. The slope map (Fig. 78) reveals a strong east-west orientation of slopes in this area that must be a reflection of folding in the bedrock. Three separate coast platforms, the highest at about 7 m above present sea level, are very clear at Sharpers Head (b4). Each platform represents an episode in the retreat and relative lowering of the sea before the latest Flandrian rise.

FIG 73. Slapton Sands (Fig. 66, b7). (Copyright Dae Sasitorn & Adrian Warren/www.lastrefuge.co.uk)

Just north of Start Point (b5) lies the ruined village of Hallsands (b6), which vividly illustrates the damage people can unwittingly do in changing features of the coastal zone. From 1897 to 1902 over half a million tonnes of gravel were removed from the bay off Hallsands to construct an extension to the dockyard at Plymouth. Engineers believed that natural storm currents offshore would replenish the material they had taken, but this did not happen. Instead, the removal of the shingle left the beach open to intense storm erosion, and in January 1917, some 15 years later, the lower part of the village and a sizeable section of coastline were removed by a combination of storm and tide conditions.

Further north, the 3.5 km long barrier beach of Slapton Sands (b7) is another shingle barrier kept active by storm waves from the southeast (Fig. 73). The freshwater lagoon behind it, Slapton Ley, is a nature reserve, home to many rare species of plants and animals. It is under threat from erosion and breaching of the shingle barrier, causing flooding by salt water, and from silting up because of ploughing and deforestation of the inland landscape.

This Landscape of Area 2 also includes a short section of the north Cornish coast around Tintagel (b9), which was an important trading centre on this difficult coast and became the site of a twelfth-century Norman castle, linked to the legends of King Arthur (Fig. 74). The coastline here is often sheer and rugged, and the bedrock contains sharp folds, fracture surfaces and multiple surfaces of mica-rich cleavage, providing evidence of extreme compression during the Variscan mountain building. Many of the cliffs are flat-topped, because erosion has been controlled by relatively flat-lying surfaces in the bedrock, which contrasts sharply with the hog’s-back or whaleback cliffs of other coastal stretches.