THE THAMES PEOM GUILDFOED TO NEWBUEY. 41 



If this is Glacial Gravel, it is also clear that Tilehurst is close to 

 the original boundary of that formation ; for all around, excepting 

 on the north, are plateaux nearly the same height ahoYe the sea, 

 but capped with Southern Drift, which is older than the Glacial 

 Gravel, and would have been covered by it had it extended so far. 

 Thus, about 3^ miles to the west is the pit 310 feet above O.D. near 

 Bradfield ; across the Kennet, to the south, only 3| miles off, are the 

 pits on Burghfield Common 314 feet above O.JD., and some 7 miles 

 away on the south-east is Bearwood, 274 feet above O.D. All these 

 localities have been described above, and at none of them have I 

 found a single red quartzite-pebble. 



Not only are the red quartzite-pebbles absent from the plateau- 

 gravels, but except in the immediate neighbourhood of the Thames 

 they are not found in the valley- gravels. This seems to me a very 

 serious difhculty in the way of those who argue for the presence of 

 the sea in this part of the Thames Yalley in Glacial times ; indeed, 

 none of the facts just given seem to harmonize with what I may 

 call " the Thames Straits theory " advocated by Dr. Irving.^ 



lY. Gravels op the Yallets, Terraces, and Minor Plateaux. 



It is probable that the rivers flowing from the Wealdeii area and 

 from the Chalk country west of it, to which the Southern Drift is 

 due, continued to exist after the plateaux above described were 

 formed, and that some of the gravels of the terraces and minor 

 plateaux were also formed by them. Indeed, if I am right in hold- 

 ing that the sea has not flowed over this part of the country since 

 the plateau-gravels were deposited, this must be so, the present 

 rivers being in some complicated manner truly descended from the 

 original streams which began their course as the country was for 

 the last time raised out of the sea. 



The division between plateau-gravels and river- and valley-gravels 

 is consequently an arbitrary one, but in part of the area at least it 

 has a real value, for the streams which formed the gravel of the high 

 plateaux drained an area in the Weald, outside the drainage-system 

 of the present rivers of the district. In the west, however, all the 

 materials forming the gravels of Silchester and Bucklebury Common 

 might be obtained from the Chalk and Tertiary formations of the 

 present drainage-area of the Hiver Kennet. 



As a rule, however, the gravels of the terraces, valleys, and minor 

 plateaux have been derived from the immediate neighbourhood. 

 This is very clearly shown in the following example on the eastern 

 side of Chobham Eidges, where there is an excellent series of 

 sections on the Lightwater Eoad. The diagram (fig. 3, p. 42) 

 shows the relative position of these sections. It extends from the 

 great plateau of Chobham Eidges down one of the lateral valleys to 

 the stream named the Hale Bourne, which flows through Chobham 

 village. At the top of the ridge a road-cutting 400 feet above O.D. 



^ 0]i. cit. p. 561. 



