454 W. A. E. TJSSHER ON THE CHRONOLOGICAL VALUE 



In parts of the Exe and other valleys terraces of old gravel are 

 noticeable, but generally at no great heights above the alluvial 

 valley-bottoms, and in a fragmentary condition. 



Taking into account the large amount of tributary erosion and 

 great subaerial waste during a long period, the fragmentary state of 

 the old river-gravels is not remarkable. 



In this period we must include the formation of the raised beaches 

 and the accumulation of the angular stony loam or " head." 



The nature of these deposits, of which the raised beaches are the 

 oldest (as Mr. Carne pointed out years ago *), furnishes a valuable 

 indication of the physical changes which were in progress for at 

 least a part of this period of fhrviatile erosion. Although certain 

 signs of raised beaches are not discoverable on the Triassic cliff-line, 

 owing to its rapid waste, many of the gravels both on the present 

 Triassic cliffs and in inland localities may be fluviatile equivalents 

 of beaches then forming, and, in some cases, contemporary estuarine 

 deposits, as at Par, in Cornwall, where estuarine gravels contem- 

 poraneous with an adjacent raised beach are shown. 



The gravels shown in the Premington railway-cutting were pro- 

 bably an estuarine deposit of the Taw. It would appear that during 

 the earlier stages of the gravel-period the land was at a much higher 

 level than at present, but gradually sinking till the formation of 

 the raised beaches marked the close of the subsidence, at heights 

 seldom exceeding 20 feet above the sea-level, but locally attaining 

 to 60 feet or more above that level, as evidenced by Hope's Nose and 

 the old beach of Barnstaple Bay. Whether the exceptional height 

 of these beaches may be partly due to unequal subsequent elevation, 

 partly to unequal rise of tide during their formation, I leave for 

 conjecture, as also whether the Lithodomous perforations and high- 

 level marine terraces described by Mr. Pengelly f in the neighbour- 

 hood of Torquay afford satisfactory evidence of the subsidence, in 

 that part of Devon, of heights exceeding 200 feet above the sea- 

 level during the Pleistocene period. 



If such a subsidence took place, it must have been confined to 

 the locality in which the observations were made ; otherwise many 

 of the inland gravels would be marine ; and of this, either in assort- 

 ment or the presence of marine shells, we have no evidence whatso- 

 ever. 



The movement succeeding the old beach-formation was one of 

 elevation, proceeding, at any rate, to the heights at which they are 

 at present found above the sea £. Here two phenomena step in, 

 viz. the " head," forming cliff-faces in some parts of the Palaeozoic 

 coast-line for as much as 1 00 feet in vertical height ; and the sub- 

 marine forests. Whether the Head be due to intense cold and ex- 

 cessive surface-waste, or to a less rigorous climate, its position and 



* Trans. Boy. Geol. Soc. Oornw. vol. iii. p. 238. 



t Trans. Dev. Assoc, part v. p. 82 (1866). 



X Their further elevation, at least near Padstow in Cornwall, is proved by the 

 existence of an old consolidated beach on the Doombar Sands at some feet below 

 high -water mark in the vicinity of a raised beach some feet above that datum. 



