404 PROCEEDINaS OP THE GEOLOGICAL SOCIETY. [Juiie 19, 



through the counties of Leicester, Huntingdon, Cambridge, and west 

 Suffolk and Norfolk, towards Yorkshire, and the latter into the Post- 

 glacial sea indicated by the denuded country stretching over the 

 south of England. 



Although the denudation of these troughs e\idently succeeded 

 that of the arc-crests, which took place during the first emergence 

 of the tracts, it nevertheless appears to have occupied only the early 

 part of the Postglacial period ; for it is into the sea-margin formed 

 by the arc shown in fig. 8, and at a point about 30 miles 

 south-west of the place where this section intersects it, that the 

 lower terrace or fluviatile portion of the great Postglacial deposit of 

 Thames gravel had, as I regard the case, its western outlet ; and 

 the structure of this gravel indicates that it belongs to an early part 

 of the Postglacial period. 



The position of this gravel (the most extensive of the Postglacial 

 series) and that of its allied formations, the East Essex gravel and 

 the gravel of the Canterbury heights, is difiicult of explanation 

 vrithout the assistance of the map accompanying this memoir. The 

 three gravels, I contend, are the deposits of a series of channels 

 penetrating the southern margin of so much of the emerged land of 

 the period as lay to the east of the arm of the Postglacial sea, 

 which is represented by the trough shown in fig. 8. The largest 

 of these channels, that occupied by the Thames gravel, opened out 

 into the western sea around Eeading *, between which place and 

 Maidenhead it followed the narrow and tortuous course now occu- 

 pied by the Thames river. From Maidenhead it expanded until it 

 opened southwards to the sea then occupying the Chalk country of 

 Surrey, at the part now represented by its high terrace of gravel on 

 Richmond and Wimbledon hills, converting the Tertiary beds of 

 northern Hampshire into an island. Erom Wimbledon the channel 

 was again continued by London towards Gravesend, sending in its 

 way a large arm up the Lea valley. East of Gravesend it was barred 

 in from the East Essex gravel by a ridge of lofty land, now cut 

 through by the Thames river, but which, prior to the introduction of 

 that river, was continuous along the east of Essex and north of Kent 

 as far as Rochester. Between Dartford Heath and this dividing 

 ridge it again opened out to the sea occupying the Chalk country. 

 The next channel, that occupied by the East Essex gravel, being 

 divided in this way from the other, is marked by that gravel which 

 ranges along the east side of Essex for upwards of 20 miles, crosses 

 the Thames mouth at right angles, and occupies the western slope of 

 the Medway valley between the ISTore and Rochester, opening out to 

 the sea of the Chalk country about the latter place. This sheet is cut 

 through at right angles by the mouths of the rivers Thames, Crouch, 

 and Blackwater. At each place where the Thames gravel opens 



* The sea to which the main sheet of the Thames gravel opened seems to have 

 been the latest part of that which denuded the arc-crest, — the sea denuding the 

 trough being that into which the lower terrace of the Thames gravel (produced 

 by the elevation and cutting down of the main gravel sheet) appears to have 

 been discharged west of Eeading. 



