506 ME. F. W. HAEMEE ON THE [Nov. I907, 



Lake Pickering was caused by the obstruction of the pre-Glacial 

 drainage by the North-Sea ice, the level of the water rising until an 

 outlet across higher land was established through the Malton gorge 

 into the Yale of York. In like manner, on the view here taken, 

 the advance of the Chalky Boulder-Clay glacier to Buckingham and 

 beyond must have impounded the drainage of the Oxford plain, 

 causing an accumulation of water which eventually escaped across 

 the Chiltern Hills into the basin of the Lower Thames, at first 

 possibly at more than one point : finally the whole of it passing 

 to the south through the Goring gap. 



Both of the valleys mentioned are of a comparatively recent 

 character ; they have been excavated across ridges of high land 

 transversely to the alignment of the latter and to what may have 

 been the original direction of the drainage, and they have been cut 

 down to the level of the plains on either side of the hills which they 

 intersect. 



IX. The Pleistocene Eeosion. 



It would be instructive to study earefully and in detail the 

 physiography of this region. The first thing that strikes one 

 is that it has been subjected to abnormal erosion, many of the 

 valleys being remarkably straight, as if cut by a considerable volume 

 of swiftly-running water. The Ordnance-Survey map shows the 

 100-foot contours only ; but, if they were given for each 2b feet, as 

 in some other parts of England, the erosion would be still more 

 apparent. 1 The valleys and gorges by which the district is ravined 

 are all of the same type, although a few of them cut through the 

 crest of the escarpment and the majority do not. They must have 

 originated, I think, more or less contemporaneously, under meteoro- 

 logical conditions of great intensity. 



Prestwdch's remarks as to another region (Quart. Journ. Geol. 

 Soc. vol. xlvi, 1890, p. 169) which he says was exposed to 



' the heavy rainfall, snow, and ice of the Pre-Glacial and Glacial Periods, when 

 torrents scored the flanks of the hills, and carried clown the harder debris to 

 the lower grounds at their base,' 



may be justly applied also to this district. 



Dealing with the denudation of the Chalk-range as a whole, 

 we may notice a marked difference between those portions which 

 were covered by the ice-sheet, and those beyond its farthest 

 extension. The crest of the White Horse and Chiltern escarp- 

 ments, except for the great gap at Goring and the trench-like 

 valleys before mentioned, is continuous and unbroken, presenting 

 an exceedingly steep face towards the lower ground on the north- 

 west, while the valley- denudation to which the dip-slopes have 

 been subject is phenomenal. Beyond Dunstable, and thence to 



1 Evidence of similar denudation occurs near Salisbury ; see PI. XXXII. 



