1894. I' HE EVOLUTION OF THE THAMES. 99 



across from Belgium to Kent, and thence to Suffolk, where it con- 

 tinued till Upper Pliocene times. In Kent, this period of submergence 

 was soon brought to an end, before the Crags of Suffolk had been 

 deposited, by the elevation of the great arch that once covered the 

 Weald. As the land there rose the summit was denuded away, and 

 the material was washed down the northern slope as the " Southern 

 Drift." This began in the Upper Pliocene, or at the time of the 

 Norwich Crag, and continued into the Pleistocene. North of the 

 Thames the Pleistocene series began with a marine shingle beach, 

 which extended from Norfolk into Berkshire, and possibly further. 

 The pebble beds in the valley of the Bure, which were included by 

 Mr. H. B. Woodward in the Norwich Crag, and are generally known 

 as the Bure Valley beds, are taken by Professor Prestwich as the 

 type of this marine shingle. He has traced it from the Bure across 

 Suffolk to Braintree, and regards as its southern extension the high 

 level pre-Glacial gravels marked in red on the maps of the Geological 

 Survey. These occur scattered over the higher hills of south-west 

 Essex, Middlesex, Hertfordshire, Buckinghamshire, and Berkshire. 

 The land then rose, and the erosion of the valleys of the Lea, Colne, 

 etc., commenced ; at the same time, the climate became more and 

 more arctic, until it culminated in the " Glacial Period." The Chalky 

 Boulder Clay was then deposited in the district by an ice-sheet from 

 the north. Still later the Chalk escarpment was formed, and the 

 Thame and the Isis flowed northward along the valley at its foot, 

 and entered the Wash. After this the Chalk escarpment was breached 

 at Goring, and the basins of the Thame and Isis were connected with 

 that of the Kennet, and thus formed the Thames. So, according to 

 Professor Prestwich, the formation of the Chalk and Oolite escarp- 

 ments, and practically the whole of the physiography and river systems 

 of the south-east of England, date from Glacial and Post-Glacial 

 times. 



The theory is a brilliant and fascinating one, as it gives a definite 

 historical classification of the drifts, and a working hypothesis with 

 which to determine the relations of a series of gravels otherwise in 

 chaos. Nevertheless, there seems a general consensus of doubt as to 

 the three main positions in Professor Prestwich's argument — viz., 

 (i) The original continuity of the isolated patches of pebble gravel 

 which he has grouped together as the Westleton Shingle ; (2) the 

 recent date of the Chalk escarpment and its breaching by the Thames ; 

 and (3) the submergence of the whole of the South-West of England 

 below the sea in Pleistocene times. In regard to the first, there can 

 be little doubt as to the former continuity of the Westleton beds from 

 the Bure Valley into northern Essex as far south as Braintree. 

 Similarly, the gravel patches in the London district which Professor 

 Prestwich has referred to the Westleton Shingle all agree in being 

 Pre-Glacial in their general composition, and were doubtless deposited 

 by a common agency at approximately the same period. But the 



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