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Wood^ Structure of the Thames Yalley. 



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■^ 1-1 



section of one great branch of the valley, 

 up which its gravel extends for many miles 

 above the line of section. 



The deposits of this valley are various, 

 and while only some occur in the portion 

 west of London all are present in the 

 eastern portion, where we best find the 



^ means of testing how far the views above 



I quoted are well founded. 



The meaning usually attached to the 



1 term terrace-deposits is that, on either side 

 ^ of a valley, and at corresponding heights 



in it, deposits occur showing the valley to 

 ^ have been cut down from one or more 

 E higher levels to its present form, and of 

 3 its contained deposits the oldest as a con- 

 1^ sequence stands at the highest and the 

 o:| newest at the lowest level. To see if this 

 S;o is the character of the Thames Valley, let 

 =^ '=5 us take a section from the Essex heights 

 '^ _ to Dartford Heath, in Kent, transverse 

 .| to the direction of the valley at this 

 f^ part. 



tig In this section we have, on the Essex 

 summit and about 400 feet above the sea, 

 a thin and very partial warp-gravel (a;l) 

 r ^ composed of the redeposited material of the 

 «;^ Bagshot pebble-beds; and, occupying suc- 

 ■| cessively two lower levels, and marking 

 g g successive stages in the denudation of the 

 I"* valley, the thin, angular, but extremely 

 §■2 patchy terrace-gravels (a; 2 and a;3) ; then 

 2^ comes the great sheet of the Thames gravel 

 Z {x4:'), occupying, hui on one side of it only, 

 rt" the trough of the valley; resting in its 



more elevated part upon the London clay, 

 g but in the lower upon the Thanet sand ; 

 i down to which sand the valley has in this 

 ^ part of it been cut. The detached mass of 



1 the same gravel {x4:") on the south side 

 g of the river has, as will be shown, been 

 « detached from the rest of the sheet cover- 

 S ing the northern slope by dislocation and 

 ^ denudation. Here we have certainly no- 

 " thing like terrace conditions, for the gravels 

 % {xl, x2, and a; 3) do not occur on the south 

 H of the river, however far the section may 

 "^ be prolonged in that direction ; nor is 



there anywhere east of London, with 

 the exception of the summit of Shooters' 



S -a 



