64 LONDON DISTBICT. 



These gravels had the effect of pushing the main stream gradually 

 southwards, the Colne being broken up into a number of ' dis- 

 tributaries/ as in a delta. In Middle Terrace times the most 

 southerly point reached by the Thames was Hampton, but 

 subsequently it almost reached Weybridge, the most southerly 

 point of its course at the present day being the backwater near 

 Shepperton Lock. This southerly encroachment by the main 

 river has materially altered the courses of the southern tribu- 

 taries, which must obviously have been longer, when the Thames 

 was further north, than now. It seems likely that the Wey 

 followed a north-easterly course to join the Thames near Baling ; 

 the deposits of that terrace there contain masses of sand 

 resembling Bagshot Sand, and a lenticle of highly glauconitic 

 material, apparently derived from the upper part of the Brackles- 

 ham Beds, has been found by Mr. F. N. Haward. While this 

 former course of the Wey has been eroded away and is conse- 

 quently hypothetical, a part of the valley of the Mole has been 

 preserved to the south of the Richmond and Wimbledon 

 plateau. On the southern bank of the dry valley between 

 Kingston and Merton relics of a high river -terrace occur, and 

 the bottom is occupied by large spreads of sand and gravel 

 which appear to belong to the Middle Terrace; over much of 

 the area, however, these deposits are concealed by a layer of 

 clay washed down from the hills practically indistinguishable 

 from London Clay in situ-, excavations have revealed the 

 existence of the river deposits at a number of localities where 

 they are not shown on the map. It is certain that this valley 

 was formed by a river draining a tract of Bagshot Sand, flowing 

 west to east to join the Wandle ; it laid down deposits belonging 

 to the Upper and Middle Terraces and encroached on its left 

 or north bank. After the Middle Terrace period this river 

 ceased to flow. It seems a reasonable inference that it was the 

 Mole, and that by its southward encroachment the Thames broke 

 laterally into the valley between Middle and Low Terrace times. 

 That part of the valley which is now unoccupied is drained in 

 part by the Hogsmill, which may have been a southern tributary, 

 and in part by the Beverley Brook. The latter probably had its 

 original source in the Glacial gravels of the Wimbledon Plateau. 



Another example of ' capture by lateral encroachment * 

 occurs near the western margin of our map ; the gravel-filled 

 valley parallel to and south of the railway through Chertsey is 

 now dry ; it represents the former course of the Chertsey Bourne, 

 which has been diverted into the Thames flat a little further 

 west. This instance of diversion is remarkably clear and has 

 been described in detail elsewhere. 1 



There have evidently been considerable changes in the course 

 of the Thames near our eastern margin. At the period of the 

 Upper Terrace the river came as far south as Dartford Heath; 



1 * Summary of Progress for 1911' (Mem. GeoL Surv,), 1912, pp. 74 77; 

 Proc. Geol. Assoc., vol. xxiv, 1913, pp. 331-314 ; ' Geology of the Country around 

 Windsor and Chertsey ' (Mem. Geol. Surv.), 1915, pp. 88-90. 



