Vol. 6S.'] GLACIAL OmaiN OF THE CLAT-WITH-rLINTS. 207 



the original escarpment was considerably denuded at the time of 

 formation of the gravel which now passes over it. East of the Colne 

 the escarpment forms a prominent feature. 



The sequence of events we believe to have been somewhat as 

 follows. In Pleistocene times, when the cold was approaching 

 its maximum, a lobe of the ice-sheet surmounted the Chiltern Hills 

 and the clean top-ice tore up the ' head ' of Chalk and Eocene 

 waste which had been accumulating for ages, producing from the 

 materials the Clay-with-Elints. The Eocene material would be 

 relatively more and the Chalk material less abundant towards the 

 south-east, and this accounts for the Clay-with-Elints passing into 

 gravel, made up of Reading-Bed materials, in this direction. 



The Thames offered an obstacle to the advance of the ice- 

 sheet, and above Eourne End proved an effective barrier, for the 

 Glacial Gravels do not cross here. Similarly, between Rickmans- 

 worth and Watford the ice seems to have been stopped by the old 

 Thames ; but between Bourne End and Eickmausworth the pressure 

 of the ice was so great that it forced its way across the river, filling 

 the old channel and eroding the old Eocene escarpment which had 

 been the southern boundary of the river-valley. 



The result of plugging the old Thames valley would be to force 

 the river to rise, until it found a col over which to escape. This 

 seems to have been found at Bourne End, and the water rapidly 

 eut the valley between Bourne End and Maidenhead. 



The Boulder Clay of Einchley appears to be a detached part of a 

 mass of Drift which would cross the line of the old Thames at some 

 point, perhaps near Hatfield, and block its channel. There would 

 then be a section of the river, between Eickmausworth and the 

 Eastern Drift, without an outlet, until the water rose high enough 

 to find a col. A way of escape was along the eastern margin of 

 the ice-plug, and the Colne was formed — running southwards from 

 Eickmausworth, and so reversing the direction of drainage of the 

 old river. 



The ice-sheet, by the time that it had reached the Thames, had 

 nearly approached its southern limit : for Bourne End is in the 

 latitude of Highgate, aud there would be floods of water from the 

 melting ice, which spread out the Eluvioglacial Gravels. The 

 materials of these gravels would be mainly derived from the ice- 

 sheet ; but a certain amount, and notably the Triassic pebbles, would 

 be introduced by the Thames in flood-time. Such river-floods might 

 be expected early in the year when the river was swollen by melting 

 ice ; and at such times the stream could return to its old course, for 

 its new valley would not have been cut so deeply as at present. In 

 this manner we account for the belt of Triassic pebbles across the 

 Eluvioglacial Gravels. The belt, as we have seen, widens east of 

 the present Thames, as might be expected on this hypothesis. The 

 occasional Triassic pebbles met with south of this belt are regarded 

 as derived from it. 



On the retreat of the ice a new system of drainage would be 

 instituted on the gravel-flat. We find a number of the small 



