Wood — Structure of the Thames Valley. 59 



HilP (where tke gravel xl occurs), to be found on the south side 

 of the river any equivalent of the gravels x\, x2, and a; 3 of the 

 northern slope. Conversely, on the west of London, although there 

 are gravels on the Eichmond and Wimbledon HiUs at a far higher 

 level than the gravel of the lower ground, yet on the north side 

 nothing occurs there to correspond with them,* as the gravel on that 

 side spreads over the north slope in a continuous sheet. 



Mr. Whitaker^ speaks of partial terraces in the Thames gravel 

 commencing and continuing for a short distance until they are 

 again lost in the great sheet of the deposit, and has so shown them 

 on the Geological Survey Map, such as that of Hyde Park and that of 

 Clapham and Wandsworth Commons. The former of these, however, 

 does not correspond to the quasi^terrace of gravel that covers so much 

 of the Eichmond and Wimbledon Hills, while the latter is on the 

 same side of the valley. These partial terraces (which are in fact much 

 less persistent than they are represented in the Map), although irre- 

 concilable with any cutting down of the valley from higher to lower 

 levels, are in no way at variance with the unequal action of an 

 upthrow, while nothing approaching to a persistent terrace corres- 

 ponding to another on the opposite slope can be shown to exist in 

 any part of the valley between Eichmond and the sea. 



We will now take the formations that are common to both sides 

 of the Thames river. It does not appear to have been suspected 

 that the brickearth beds are divisible into three most distinct forma- 

 tions, but such is the case. The formations are the following : — 



1. The Loicer Briclcearth (a;4^). — The Brickearth of Ilford, both 

 that of the Uphall and that of the London Eoad field, is a deposit 

 underlying the Thames gravel, and unconformable to it. Like the 

 much newer deposit of Grays, it contains Cyrena fluminalis and other 

 purely freshwater shells, and has a thickness at Ilford of nearly 

 twenty feet. It may be seen in the field on the London Eoad 

 resting in one part direct on the London clay, while in another part 

 it has a thin band of shingly gravel beneath it. In the Uphall field 

 its position relatively to the Thames gravel (x 4/') is best shown, 

 the two deposits being unconformable. (See Section No. 2.) North- 

 wards from UphaU field the brickearth disappears under the gravel, 

 which, there denuded to a thickness of 4 or 5 feet, increases to 



1 Shooters* Hill forms an isolated remnant of the original valley, in which the 

 Thames gravel was deposited ; the valley, as will be shown, having on the east of 

 London, with that exception, had its southern slope destroyed by subsequent events. 



^ The gravel occupying the higher ground above the Thames gravel to the west, 

 beyond Uxbridge, is "the gravel of the Middle-drift, a formation older than the 

 Boulder-clay. 



3 Mem. on sheet 7 of Geol. Survey. Mr. Whitaker also (p. 92) points out the dis- 

 crepancy between the section given by Mr. Prestwich, in his " Ground beneath Us," 

 of the position of the gravel of Clapham and "Wandsworth relatively to that of the 

 "VVandle valley and the true position of those gravels, and I agree with Mr. "Whitaker 

 in so far as he says that the Wandsworth and Clapham gravels join in one part with 

 the gravel of the "Wandle valley, for I have traced the "Wandsworth and Clapham 

 gravels on their south-western side into inosculation with that of the "Wandle valley, 

 down Burntwood-lane, as well as on the north side into inosculation with the gravel 

 nearer the Thames that sweeps round into the "Wandle vaUey. 



