THE PLEISTOCENE SERIES 641 



sunlstones and cherts, Liassic stones, Red Chalk, and various 

 igneous rocks. The assemblage is in fact that found in the Qlaci.il 

 deposits to the north, and Mr. Pocock is of opinion that the gravel 

 is of Glacial age and is in part a decalcified Glacial deposit. 11 * He 

 admits, however, that at lower levels, 130 to 200 feet above the 

 rivers, it i- sunn-times rudely terraced as if it had been redistributed 

 by lluviatile action. 



In Berkshire and East Oxfordshire the constituents are chiefly 

 subangular flints and materials derived from the Eocene Beds such 

 as flint-pebbles, quartz-pebbles, sarsen-stone, and ironstone. Mr. 

 Osborne White also regards the gravel as of Glacial age, but as 

 belonging to more than one Glacial epoch, and he remarks that 

 "the Plateau and Valley gravels are very often inseparable ; their 

 more level spreads appear to be but the higher and lower members 

 of a single graduated series, the steps and half-landings of a ruined 

 stairway that was never complete." - 



River Gravels. The manner in which disconnected tracts 

 and terraces of gravel, sand, and loam have been formed and left 

 at various levels in a river-valley is explained in all text-books of 

 Physical Geology. They represent different stages in the excava- 

 tion of a valley, those at the highest level above the existing stream 

 being of course the oldest, and the others newer in proportion as 

 they are nearer to the level of the modern alluvium. 



The deposits in the valley of the Thames may be taken as an 

 example of such accumulations. In the higher parts of the valley 

 tracts of gravel are found at various heights, now on one side of the 

 river, now on the other ; three stages or terraces can often be 

 distinguished, the highest being about 40 feet above the level of 

 the river. From this highest terrace few bones have been obtained, 

 but at the lower levels land and river shells, with remains of Elephas 

 primigenius, Elephas antiquus, Rhinoceros tichorhinus, Bison prisons, 

 Rangifer tarandus, Equus caballus, Canis lupus, and Ursus arctos are 

 frequently found. Between Windsor and London the outspreads of 

 gravel become wider and more continuous, but the surface of the 

 older parts still rises to 30, and even 45 feet above the river, and 

 the shells are still of fluviatile species. Hence we may conclude 

 that when these gravels were deposited the Thames had a much 

 longer course than it has at present, and that this pail of the valley 

 was then far inland, otherwise the beds would be at a lower level 

 and the shells would be estuariiu-. 



Below London, at Ilford, Grays, Crayford, and Erith, there are 

 a series of gravels, sands, and loams which rise to 25 or 30 feet 

 above the river, but are nearly 60 feet above the base of the 

 alluvium at Erith. These beds have yielded the mammalian remains 



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