The Haldon Gravels are distinctive deposits that occur above these Cretaceous sediments. They consist largely of flint pebbles and contain sand and mud between the pebbles. Some of the gravels appear to be the result of removal by solution of the calcareous Late Cretaceous Chalk that can no longer be found in its unaltered state so far west. The flint nodules in the Chalk were then left as a layer of much less soluble pebbles. Some of the gravel appears to have been carried to its present position by rivers or the sea, perhaps also with the incorporation of kaolinite clay from the Dartmoor granite. The age of these gravels appears to be early Tertiary, perhaps about 55 million years.
A few kilometres west of the Haldon Hills, northwest of Torquay, the Bovey Formation of early Tertiary age (Eocene and Oligocene, about 45 to 30 million years ago) occurs in a distinct, fault-bounded basin. The formation is more than 1 km in thickness and consists primarily of the clay mineral kaolinite, deposited as mud by local streams, and associated with minor amounts of sand, gravel and peat-like organic deposits of lignite. This sediment fill continues to be a very important material for ceramics, pipes, tiles etc. ranging from high-quality china clay to lower-quality materials. Most of the sediment appears to have been carried into the basin from the area of the Dartmoor granite and its surroundings. The Bovey Basin formed as a result of subsidence along the northwest-to-southeast trending Sticklepath Fault Zone (Fig. 39) which cuts across the whole of the Southwest Region. This fault zone seems to have been active during the accumulation of sediment in the basin and so, at least in this phase of its history, it was much younger than the Variscan structures of the Southwest generally. Further to the northwest along the same fault zone is the smaller Petrockstowe Basin near Great Torrington (see Fig. 38), and, offshore, under the Bristol Channel is the larger Stabley Bank Basin, east of Lundy Island.
About 6 km southeast of St Ives (see Area 1), near the small village of St Erth, a small area is underlain by some soft sands and muds. When first exposed by quarrying, these sediments provided a rich assemblage of fossils that are thought to have lived some 3 million years ago, in latest Tertiary times. The fossils suggest sea depths of between 60 and 100 m, and are now about 30 m above sea level, so they provide a fragment of evidence from a period when the sea was more than 100 m higher than it is now, relative to the land of Cornwall. As will be mentioned below, this deposit is rather similar in its elevation to the most obvious plateau recognised in many of the inland areas, which may also relate to an episode when the sea stood at this level.
Drainage patterns
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.
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