Geology Explained in South and East Devon (John. W. Perkins*)
The Devonian rocks exposed on the foreshore are some 400 million years old and the folding and distortions due to separation of the continents and the uprising of the Dartmoor granite mass can be seen. The formation of Plymouth Sound can be demonstrated by reference to the surrounding hills and valleys.
Rocks around Bovisand
South Devon has much to offer for people interested in how Dartmoor and the varied coastline came into being. Unlike most of England the Ice Age caps did not reach South Devon so the rock formations have not been scoured and littered with debris from the north.
Some 400 million years ago much of south-west Devon would have been meandering rivers in an area of flat mud plains. At this time Europe, Africa and America were joined together in one huge continent and England was close to the Equator.
The Devonian Period
The time from 400-350 million years ago is known as the Devonian period. There are no known rocks older than this in Devon. The Devonian period can be divided into lower (the oldest), middle and upper ages.
During the lower Devonian age successive layers of mud and sand were deposited, but in the middle age the sea level rose and the mud layers were covered by large coral reefs. Then, in the upper age, the sea fell back and further layers of mud were deposited. By this time the earlier levels had been compressed and hardened into beds of slate and the coral reefs turned into limestone.
Starting 290 million years ago dramatic movements of the land masses took place in which our present continents were formed. At some stage the existing beds were squeezed and crumpled into a series of folds resulting in what were horizontal beds being pushed into almost vertical positions in some places.. Also a huge mass of granite extending from the Scillies up to Exeter rose up from the molten interior of the earth.
The foundations of South Devon have changed little since those times. The oldest slates, referred to as Dartmouth Slates, can be seen at Penlee Point and then along the coast from Bovisand to Dartmouth. ?major folds to north, minor folds east/west.
Somewhat younger rocks known as Meadfoot Beds and Staddon Grits run from just north of Kingsand in Cornwall across to Staddon Heights.
The Middle Devonian commences just south of Mount Batten with the slate beds at Jennycliff. The next band is limestone which starts at Cremyll, outcrops in the Hoe extending down to the City Centre, and runs east as far as Yealmpton.
North of the limestone are the Upper Devonian shales (known locally as shillet) which extend round the edges of Dartmoor as far as Tavistock. These shales are rather weak and easily shatter when exposed. There is little trace in the Bovisand area of South Devon of younger rocks such as sandstones although they do occur to the east and there are chalk beds in the sea bed towards the Eddystone reef. It is possible that layers of these younger rocks were deposited in the area but have subsequently been eroded away.
The Ice Age
The next event to have lasting effects on the coast was the Ice Age starting some two million years ago. Although the ice caps did not reach this far south they did cause huge fluctuations in sea level. During the latter part of the Ice Age the sea reached 430 feet, and possibly more, above the current level making Dartmoor an island.
The sea effectively planed off the land surface at the 430 feet level and when the sea retreated an elevated plateau remained. Other levels occur at intervals of about /Cfeet in the South Hams area. During the cold phases of the Ice Age the sea fell to perhaps 1 00 feet below, its present level.
Another effect of the intense frosts of the Ice Age was to shatter the exposed rocks on the cliffs tops resulting in landslides of loose material. These formed intermediate plateaux and cliffs known as heads. These are shown to varying degrees between Bovisand and Heybrook, but especially from HMS Cambridge to Wembury. The fragments of stone were carried down the slopes mixed in with … brown earth and when they settled they pointed down gradient so that they now show end on in the head face.
Plymouth Sound
The Sound was formed by the combined effects of the Tamar and Plym rivers. Both of these were fed by tributaries entering, in the case of the Plym, along valleys such as Forder, Lipson and Tothill. As the sea fell during a warm Ice Age period the rivers would have encountered the east/west limestone ridge noted above. Over the years the Plym eroded the limestone to create a gorge between what are now Cattedown and Oreston.
The Tamar also worked its way through the limestone between Edgecumbe and Devil’s Point., but then encountered harder rock which turned it to the east. Before it could reach the sea some miles to the south it also had to round a volcanic knol now known as Drake’s Island. At this point, just west of Mountbatten, the Tamar met up with the Plym coming from the east after collecting water flowing out of Radford Valley.
The combined rivers were then forced south west by a ridge extending out from Bovisand on which the breakwater now stands. The river valley, as it was then, is now known as Smeaton Pass and is revealed by the course of ships entering Devonport.
Ten thousand years ago the sea rose above it’s current level flooding the river valleys and leaving huge amounts of sand and silt to bury the lower parts of the original gorges and valleys.