Wood— Structure of the Thames Yalley. 105 



. 2. Tliat the original valley, formed subsequently to the deposit of 

 the Upper Drift or Boulder-clajr, and by denudation during emer- 

 gence from the Upper Drift sea,^ began (after passing through the 

 Bagshot sand and London clay alone, between Windsor and London) 

 to cut down on the east of London to the Lower Tertiary sands 

 in the central line of its trough ; and that the line there curving 

 south-eastwards, and passing across the extreme south-west of 

 Essex, made its way to the sea in the direction of the eastern 

 half of the Weald; the London clay forming only the higher 

 slopes of the trough on this side of London. 



3. That in the trough so formed, and after its complete excavation, 

 the fresh-water deposit of the Lower Brickearth (x 4/) was accumu- 

 lated, having a very limited extension. That after this the valley 

 underwent a considerable depression, by which the sea was admitted, 

 rising up the sides of the valley to a height very far above the 

 limits of the Lower Brickearth, and covering that deposit, by which 

 an uniform sheet of closely bedded fine gravel (cc 4/^) (composed of 

 material brought mostly from other areas than those draining into 

 the valley,) was deposited ; while changes of level occurred d;iring- 

 this depression that gave rise to the bands of clay and sand, yielding 

 fresh-water shells, which occur in a few places near the margins of 

 the deposits, and are intercalated in the gravel. And that, after the 

 completion of this formation, a change ensued by which the sources 

 furnishing the gravel were cut off, the mud brought down by the 

 surrounding streams — instead of being, as heretofore, carried off by 

 the tide that brought the gravel-freighted ice — ^being intercepted and 

 allowed to subside in deposit.^ 



4. That the south side of the valley was then subjected to violent 

 convulsions, which broke up the surface, and caused an extensive 

 denudation of all the three deposits ; eventually converting the 

 valley north and west of Grays into land upon which a forest sprang 

 up ; while at Grays, and south of it, the denudation, cutting through 

 the Thames gravel and subjacent Thanet sand of Dartford Heath 

 and Grays Hill down to the Chalk, formed the valleys of the Cray 

 and Darent, the waters of which j)assing by Purfleet and Grays, 

 flowed to the sea, then occupying the Chalk area of the Wealden 

 denudation, and in which the later gravel (a; 5) and the fluviatile 

 deposit of Grays (x 5') were accumulated. 



5. Then, but after what interval cannot be shown, probably a long 

 one, a part of the forest was subjected to a downcast, and the swamp, 

 with its peat, formed over it ; and, finally, the river Thames found 

 its way through the swamp, covering the peat with its mud, and 

 having an access to the sea provided for it by the downthrow forming 

 Sea Eeach ; a downcast which, although then completed to the extent 



1 I confiae the term Drift to the English Glacial beds that are older] than the 

 valleys existing in strata which are newer than the Trias, and lie south of Flam- 

 borough Head ; using the term Post-glacial for such as are newer. 



2 That the climatic conditions producing gravel were not changed, is showti by the 

 formation of the later gravels {x 5) ; and I have seen in the Upper Brickearth at 

 "Wanstead large angular chalk flints, apparently ice-borne. 



