GEOLOGY 



valley suggest such an explanation. In advocating a granite origin for 

 the Bovey valley clays, and combating Maw's opinion to the contrary, 

 Pengelly alluded to the occurrence of derivative crystals of felspar in one 

 of the beds. From the relative positions of the gravels on the heights, 

 and the gravels, sands and clays in the valleys, we may infer that they 

 represent successive stages in the denudation effected in Bagshot times, 

 subsequent to the Tertiary planing of the Cretaceous surface. 



In Devon no proofs of Pliocene deposition have been found, although 

 some of the gravels at high levels may belong to this period. The 

 evidences of Pleistocene denudation are confined to the results of a long 

 period of fluviatile erosion. Patches of gravel, marking stages in the 

 excavation of the present river valleys, are of more frequent occurrence 

 on the newer rocks than the older. The raised beaches rarely indicate 

 a depression of more than 40 or 50 feet, but that they were elevated 

 many feet above their present position is proved by the evidences of 

 submerged forests on the foreshore in Torbay, Salcombe Estuary and 

 Bigbury Bay on the south coast, and at Westward Ho on the north. 

 Intermediate between the beach formation and the forest growth, prob- 

 ably at a time when the elevation of the beaches reached its maximum 

 limit, a quantity of talus was shed from the high land over the land- 

 ward margin of the raised beach platform. This old talus or Head is 

 found in pinnacles on the old beach platform, insulated from the main 

 cliffs by recent marine erosion in several places between Start Point and 

 Plymouth Sound. In inland localities, especially on higher lands and 

 near stream sources, there are flats or gentle slopes of marshy land with 

 clay soil and fragments and boulders of local rock which may date to 

 this period. We may also ascribe to it the ' head ' or white surface 

 clays of the Bovey valley, which contain Betula nana^ Salix cinerea, S. 

 myrtilloides^ identified by Professor Heer. 



The evidence of a colder climate furnished by this flora, and by the 

 more rapid disintegration of exposed rock surfaces leading to the forma- 

 tion of talus fans or screes in favourable situations, is led up to by proofs 

 of the prevalence of a somewhat colder climate during the formation of 

 the old beaches. Hunt cites the shells Trophon truncatus and Pleurotoma 

 turricula obtained from the raised beach on the Thatcher rock in evi- 

 dence of this. An additional proof is afforded by the rare occurrence of 

 large worn boulders of extraneous rocks on the old beach platform. Of 

 this the granite boulder overlain by the raised beach at Saunton 

 (Barnstaple Bay) is the familiar example. Two boulders, also of large 

 dimensions, occur on the old beach platform on the east side of the 

 Prawle. These boulders seem to indicate transport by floe ice, or by 

 casual bergs drifted on shore many miles away from their sources. The 

 ; forest growth must have flourished over a much more extended foreshore, 

 when the raised beaches stood high above sea level, and their growth 

 was circumscribed during the succeeding period of subsidence, which 

 allowed the sea to gain access to its old cliff bounds in the older rock 

 area and to encroach beyond them in the newer. 



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