1XXV1 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [vol. lxxvi, 



drainage' of the thaw-water; and the erosion of the land outside 

 the receding ice-margin was consequently severe, and was accom- 

 panied by the deposition of much fluvio-glacial material. As a 

 result, the drainage-system of the whole area has been greatly 

 changed, particularly in respect of the main water-partings traver- 

 sing the lowland. It is known that the courses of the Dee, Severn, 

 Trent, and Upper Thames, have all been strongly affected ; but 

 much work remains to be done before the whole history of these 

 changes can be attempted. Indications of temporary lakes are 

 abundant ; and the main channels of discharge must have shifted 

 their position widely as the ice-barriers on the north-west and 

 north-east sank gradually away. 



When mapping tracts in the North-Eastern Midlands in the 

 course of my official work on the Geological Survey, I have re- 

 peatedly met with features in driftless, or nearly driftless ground, 

 for which I could find no explanation in the ' solid ' structure, or 

 in the present drainage-sj^stem. I have in mind particularly some 

 instances of small escarpment-like banks or ridges at the borders 

 of flats practically bare of drift, which closely simulate rock- 

 features, yet bear no relation to the rock-structure. Such features 

 may in most cases be assigned with probability to the exceptional 

 action of Glacial flood-waters, forced temporarily into irregular and 

 aberrant courses. 



In the same region another problem presents itself, in the 

 occasional abrupt ending of thick boulder-clay in steep banks over- 

 looking ground devoid of drift. The most striking example of this 

 kind known to me is the high bank which terminates the bare tract 

 of Lias in the Vale of Belvoir, and rises westwards to the broad 

 upland spur of boulder-clay known as the Nottinghamshire Wolds, 

 where the drift is in places over 100 feet thick. In appearance and 

 structure the bank presents the essentials of an escarpment, and 

 seems, therefore, to denote the wearing-back and the possibly exten- 

 sive reduction of the original plateau of boulder-clay. But I think 

 that the appearance is deceptive in this, as in other cases of the 

 same kind which have come under my notice, and that the 

 main outline of the bank is due to the original concentration of 

 the Glacial deposits in this form, though later denudation has 

 played some subordinate part in the final shaping of the feature. 



Any marginal lowland area sloping away from the ice, and bare 

 at an early stage of the melting, must, of course, have undergone 

 swift erosion, seeing that it had to carry off much extraneous thaw- 



