Geoscience Reference
In-Depth Information
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 sol-
uble 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 Dart-
moor 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 loc-
al streams, and associated with minor amounts of sand, gravel and peat-like organic de-
posits of lignite. This sediment fill continues to be a very important material for ceram-
ics, 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 subsid-
ence 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 act-
ive 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 lar-
ger 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 quar-
rying, 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 res-
