Kingsbridge. Report from the Geology Section
|Author(s):||Bennett. Jenny and Mather. John||Origin:||Section Conference Reports|
Kingsbridge is in a geologically fascinating area, the town itself situated on the Lower Devonian rocks within the Looe Basin. To the south, a major structural feature, the Start Fault, crosses the South Hams from east to west, faulting the silty banded slates of the Meadfoot Group against the metamorphosed rocks of the Start Complex. The highly deformed and metamorphosed Start Complex rocks occur in two main groups – a finer grained green hornblende schist and grey mica schist. The age of these rocks is unclear, but they are thought to be altered Lower Devonian, though it has been suggested that they may be much older than this. The fault junction between the Start Complex and the Meadfoot Beds can be seen at the Shippen in Outer Hope Cove where there is a mylonite zone (tectonically crushed rock) and also less clearly in the low cliffs just north of Hallsands. At North Sands Bay, the schists and large scale folding can be seen, as well as smaller scale faulting to the north of Salcombe Castle. The schist has been used in the walls here, and shows clear ‘honeycomb’ weathering which has been used to establish weathering rates as the date of wall construction is known.
Prawle Point and the area around it is a key locality for seeing the effects of changing sea levels over the last hundreds of thousands of years in the Pleistocene. Global sea levels rose and fell according to the amount of water that was locked away in the ice caps during the coldest periods. At Prawle, there are two lines of abandoned sea cliffs and evidence of raised beaches left as sea level fell. The most striking feature is the smooth slopes that run down to the sea, formed by freeze thaw processes breaking down the bedrock during the cold periods. These slopes consist of ‘head’ deposits, a mixture of fine material and fragments of the local schists. The extent of these is considerable on the eastern side of the area, where they are protected from the prevailing westerly winds. To the east at North Hallsand, further evidence of changes in sea level is the remains of a drowned forest that may be seen on spring tides if the beach is clear of shingle.
Present day processes are also well represented here – the ruined village of Hallsands is a dramatic example of what can happen when processes are poorly understood. Here, dredging offshore removed protection for the village which resulted in the further loss of its protective shingle barrier beach.(*) Slapton Ley is also enclosed by a shingle barrier, formed mainly of flint pebbles. It is thought that the barrier was emplaced at about 8,000 years ago as the final surge of Holocene sea level rises brought material in from the English Channel. The implication here is that these barriers are not being replenished, causing concern to local residents as a main access road runs along the top of the barrier. Evidence from sediment cores in the Ley also shows that the water has become more brackish over time.
(*) Worth (R. Hansford), 1904. ‘Hallsands and Start Bay’. Trans. Devon Assoc. Advmnt. Sci. (TDA), vol. 36, pp. 302-346.
Worth (R. Hansford), 1909, ‘Hallsands and Start Bay’. Part II, TDA. vol. 41, pp. 301-308.
Worth (R. Hansford). 1923, ‘Hallsands and Start Bay’, Part III, (discusses the destruction of Hallsands), TDA. vol. 55, pp. 131-147.
Jenny Bennett and John Mather