RIVER DEVELOPMENT. 63 



of the Glacial Gravels it seems to have been much further south. 

 We have seen that at Richmond Park and Wimbledon materials 

 from north and south mingle after their descent from the 

 Chilterns and North Downs respectively ; here, therefore, we 

 must have the valley-bottom at the period of formation, some 

 159 to 180 ft. above the present water level. This valley was 

 occupied by a river flowing, roughly, from west to east like the 

 present Thames, but whether its sources lay within the Chalk 

 rim, on the Jurassic outcrops, or in the Welsh mountains, as 

 various writers have suggested, cannot be discussed now. That 

 the tributaries of this river corresponded with those of the 

 present Thames may be inferred, since, on the south side at any 

 rate, the distribution of the Plateau gravels points to the 

 existence of gaps in the Chalk escarpment near Guildford, 

 Dorking, Croydon and Shoreham. 



The main river now deepened its valley by some 80 ft. and 

 then deposited the Upper Terrace Gravels. Henceforward it is 

 possible to trace the positions of the main and tributary streams 

 to various periods and to indicate with some confidence the 

 changes that have taken place. Wherever river-deposits occur, we 

 have a relic of the floodplain on which the river meandered at 

 that time, and wherever such relics are bounded by higher 

 ground we have a limit to that plain. Thus, in Upper Terrace 

 times the northern limit of the Thames plain ran from Iver to 

 near Acton Station, and again from near Wanstead to Romford 

 and Hornchurch, while the southern bank is preserved from 

 Richmond through Roehampton and Clapham to Camberwell. 

 Near Dartford there must have been a loop to the south through 

 Erith, Crayford, Dartford Heath and Wilmington, and thence 

 north of east to Stone Castle. On the same principle, we see 

 that the larger tributaries have, for the most part, kept to the 

 same general courses, but the Lea has shifted its course steadily 

 eastward and now hugs the eastern bank of its valley, and the 

 same is true to some extent of the Wey and the Mole. The 

 Wandle, whose upper waters originally occupied the Purley 

 valley, laid down vast masses of gravel when it reached the 

 Tertiary plains at Croydon; these gravels deflected the course 

 of the stream further and further west, till it now flows due west 

 as far as Wallington and then turns north again along the 

 western margin of the gravel spread. 



Certain important changes in the courses of the rivers can, 

 however, be detected. Between Maidenhead and Richmond the 

 Thames takes a great loop to the south, partly interrupted by 

 the Chalk inlier of Windsor; the greater part of this loop is 

 included in Sheet 3 of the map, of which it occupies nearly half. 

 From the neighbourhood of Maidenhead, where it leaves the 

 main Chalk area, the river seems, in Upper Terrace times, to 

 have flowed nearly due east, the limits of the valley being 

 marked on either side at Baling and Richmond respectively; 

 on its northern bank the Colne entered, bearing with it large 

 masses of gravel derived from the great spread of glacial deposits. 



K 17412 C 3 



