104 Wood— Structure of the Thames Valley. 



Could the wliole subject of tlie denudation of the Upper and 

 Middle Drift be discussed here, it could be shown, by a variety of 

 collateral evidence, that the Cliffe fault, forming Sea Eeach, was 

 part of a series of movements subsequent to the formation of the 

 original valley in which the Lower Brickearth was deposited. The 

 sections given will, however, show that this fault is due to a line of 

 rectilinear movements that has thrown down the bed of the old 

 forest at Plumstead, and brought up, by faults, the gravel of Dart- 

 ford Heath, and the range of the Erith, Bostol, and Plumstead Hills ; 

 and which has isolated the Wicldiam Lower Brickearth behind that 

 range/ To these movements, and to the denudation contemporaneous 

 with them it is, that the south-easterly extension of the Thames 

 gravel has been destroyed, except the fragments isolated at Dartford 

 Heath and Higham. While we have thus evidence of the former 

 extension of the gravel in this direction, we have also evidence of 

 its being shut off from the sea on the east by a tract of land several 

 miles broad, and we are thus driven to trace its extension to the sea, 

 at its eastern end, in a direction that carries us over the now-elevated 

 Chalk country lying north of the eastern part of the Weald of 

 Kent. So also, on the west of London, we are met with evidence 

 of its original extension by another channel in the direction of the 

 same elevated Weald country. 



It is quite impossible, within the compass of a short paper, to 

 show adequately the movements which brought into existence the 

 original valley, and those which afterwards so materially altered it, 

 and dislocated the formations within it ; far less to show the relation 

 which that valley and its deposits bear to the general question of the 

 formation of England since the elevation of the Upper Drift. As to 

 the former, the sections given must, as far as they go, speak for 

 themselves ; and if the reader will (after marking the northern 

 boundary of the gravel given in the note to page 100) carefully 

 follow their direction in the map, and, while bearing in mind that 

 one great branch of the valley and its gravel cuts through the line 

 joining the Upper Drift outliers on the Finchley and Havering hills, 

 correllate these sections with each other, they will — while at variance 

 with any extension of the Thames gravel to the present sea ; with 

 the formation of the Brickearth by floods, commencing at the period 

 of the highest valley gravels, and continued down to that of the 

 lowest, — ^bear out, I think, the following conclusions, viz. : 



1. That there is no loidespread set of hill gravels within the area 

 of the Thames Yalley ; the only gravels older than the Thames 

 gravel (a? 4'') being the extremely partial and thin denudation, 

 gravels, xl, x2, and a; 3.^ 



• If the places of the Plumstead fault in Section 3, and of the Cliffe fault in 

 Section 5, be marked on the map, and a ruler laid through them, it will indicate the 

 direction of a rectilinear line of throw extending from the Cliffe fault, and passing 

 exactly through the faults of Little Thurrock, Purfleet, Erith, Plumstead, and Lower 

 Charlton. 



2 The extensively-spread gravel of the Middle Drift, which partially underlies the 

 Boulder-clay, lies without the brow of the Thames Valley, except for a small distance 

 in the western part of it. 



