146 Professor J. W. Gregory — The Chiltern Wind Gaps. 



raiglit expect corresponding evidence for both lakes. On the contrary, 

 the assumed outlet of the Thames to the Wash is very doubtful. 

 The lake deposits on the floor of this basin are insignificant, and can 

 be explained by local pools which would not have answered the 

 requirements of Lake Oxford; and though the Brightwell gravels 

 to the north of the Goring Gap are referred to by Mr. Harmer 

 as possibly " littoral accumulations of a lake" (1907, No. 1, p. 503), 

 there are no clear remains of lake terraces or deltas. 



The strongest argument for Lake Oxford is the existence of the 

 gorge of the Thames at Goring and of the wind gaps through the 

 Chilterns ; but re-examination of the evidence confirms my original 

 impression that these gaps were cut by pre-Glacial rivers. They 

 present difficulties, which appear to me insupei'able, to the theory 

 that they were cut by the overflow from a glacial lake. The second 

 figure in my paper of 1894 (No. 3, p. 106) shows that these gaps 

 stand at very diff"erent levels. The floor of the Hampden Gap is 

 at about the height of 700 feet, and the escarpment beside it riSes 

 over 800 feet. The deepest of the gaps, that at Tring, has been cut 

 down to the level of 430 feet. The Hampden Gap could only have 

 been made by a lake which was at the level of more than 800 feet 

 above the sea. No such lake could have existed unless the broad 

 opening between the Berkshire Downs and the Chiltern Hills, on the 

 floor of which the Goring Gap has been incised, was post-Glacial. 

 The existence of Triassic drift down to the level of below 300 feet 

 above Wallingford and near Pangbourne shows that the Thames 

 Valley at Goring had been cut down to below 300 feet above sea-level 

 in pre-Glacial times.® 



The top of the gorge at Goring is at about the level of 450 feet, 

 and even if the whole gorge below that level had been cut after the 

 existence of the assumed Lake Oxford, the water in that lake could 

 not have stood much above that level. The overflow from the lake 

 might have deepened the gaps at Tring, Wendover, and Saunderton, 

 which have been cut down respectively to 430, 503, and 461 feet; 

 but even these three deep gaps could onlj^ have been initiated by the 

 lake if its surface had stood over 800 feet above the sea ; and that 

 Lake Oxford could ever have stood at that level seems impossible. 

 If these gaps had been deepened from about 540 feet, which seems to 

 me the highest possible level of the lake, then they would have beeL 

 in existence before Lake Oxford, which would explain only the 

 deepening and not the origin of the wind gaps. The conclusion that 

 the Chiltern gaps were cut by rivers when the Chalk extended farther 

 to the north-west, and are a series of pre-Glacial valleys of which the 

 rivers have been beheaded by the Thame, still seems to me the most 

 probable explanation. 



Mr. Harmer's objection to this origin of the wind gaps is based on 

 their modern appearance and their occurrence as more or less 

 rectilinear channels (1907, p. 503) ; but there seems no very marked 

 difference between them and some of the valleys in the Wealden 

 Downs, which would appear to be certainly pre-Glacial. It is 

 important to remember in connexion with the form of the valleys 

 that in all probability the Chiltern Hills were never covered with ice. 



