176 TROF. J. TRESTWICH ON A SOUTHERN DRIFT IN THE THAMES 



Polio wing on this period, a tract extending from the east coast 

 and reaching either as far as the district near Bath, or possibly as far 

 as the Bristol Channel, was submerged and covered by the waters of 

 the Westleton sea — a sea bounded by the highlands of the Weald on 

 the south, but of which the northern barrier has yet to be defined. 

 I am led, therefore, to conclude that the early stages of the Southern 

 Drift preceded the Westleton Shingle (in its later stages they may 

 have been synchronous), and either that the Westleton sea beating 

 against a land over which the Southern Drift had spread, took up a 

 jDortion of that Drift with its chert- and ragstone-fragments, or that 

 the streams which still continued to run from off the southern hills 

 carried down thence the Lower-Greensand debris into the Westle- 

 ton sea. Superposition is wanting, and we have only the relative 

 position and the peculiar composition of the two Drifts to guide us ; 

 for although the Southern Drift occasionally contains white quartz- 

 pebbles, they are so few that they may have been derived from 

 Lower Tertiary strata, or from the Lower Greensand and Wealden. 



The Westleton Drift, as already explained, is, in the main, con- 

 lined to the district north, and the Southern Drift to the south, of 

 the Thames. But it is probable that on the southern slopes of 

 the Wealden highlands and their prolongation westward, similar 

 causes were in operation to those we have pictured on the northern 

 slopes, that a similar Drift was there in course of formation at 

 the same time, and that it was there subject to conditions analogous 

 to those experienced by its equivalent in the London Basin. 



As instances of such ourliers, I may briefly mention Alderbury 

 Hill, near Salisbury, the higher hills between Southampton and 

 Winchester, and those in the New Forest between Lyndhurst and 

 Wimborne ; I have, however, now to confine myself to the Thames 

 Basin. I would merely state further that in these districts this 

 drift, like that on the opposite side of the dividing-range, con- 

 tains a considerable proportion of chert- and rarjstone-^Q\>xi% mixed 

 with the angular and subansular flints of the Chalk. 



7. Main Lines of Elevation and Drainage of the South- 

 east of England : Genesis of the Thames. 



Another point remains for consideration, and that is, the effect 

 produced, not only on the configuration of the country, but also on 

 the lines of drainage, by the elevation on the one hand, at an early 

 Pliocene date, of the Wealden anticlinal, and, on the other hand, of 

 the Westleton sea-floor in early Pleistocene times. 



The east and west direction of the anticlinal of the AYealden axis 

 necessarily gave rise to a drainage northwards into the area of the 

 Thames Basin, and plotted broadly the direction of those valleys, 

 which subsequently, with certain modifications, caused by minor 

 disturbances, and by the action of glacial erosion, became the main 

 lines of the transverse valley-system and rivers of the Weald. 



The emergence of the floor of the Westleton sea was, on the con- 

 trary, effected not by any sharply-defined axis, but by flexures 



