478 MR. F. W. HARMER OX THE [NOV. I907, 



under the normal process of subaerial erosion. It cannot have been 

 caused by a river originating on or near the Ironbridge isthmus. 



Unless it is older than the Cheshire plain, which seems improbable, 

 its excavation must have commenced during the existence of a lake, 

 the waters of which, held up towards the north by ice, had attained 

 a level of between 400 and 500 feet. 



As in the case of Lake Pickering, an outfall from the Cheshire 

 plain, for such there must have been during the advance of the ice- 

 sheet, may have taken place at first in more than one direction, as 

 for example by the valley of the Dane and the Rudyard gorge into 

 the basin of the Trent. Mr. G. Barrow & Mr, C. B. Wedd have 

 indeed suggested that the latter was formed in comparatively recent 

 times by the erosion of water escaping rapidly from a Glacial lake, 

 remarking that no gathering-ground capable of supplying a stream 

 of sufficient importance to have eroded it could have then existed 

 to the northward. 1 



The Pennines are broken, moreover, near Madeley, by some 

 trenches which connect with the Trent basin by way of Stafford ; 

 and in this direction also, as suggested by Mr. Walcot Gibson & 

 Mr. C. B. Wedd, water may have escaped during the existence of 

 lacustrine conditions in the Cheshire plain. 2 



It should be noticed in these cases that the lower contour-lines 

 of the hills facing the site of the supposed lake are continuous, 

 while the higher contour-lines cut through the crest of the escarp- 

 ment. Prof. Kendall points out that, in the Cleveland district, 

 this feature is an important characteristic of overflow- channels. 

 The notches in the escarpment leading to the Madeley-Stafford, and 

 the Budyard cols, bear no resemblance to obsequent valleys. 



Unless the girdle of high land fringing the Cheshire plain were 

 more continuous between Market Drayton and Newport in pre- 

 Glacial times, there might also have been an outfall at first into the 

 basin of the Trent between Newport and Stafford, and into that of 

 the Lower Severn to the south-east of Newport. While such outlets 

 remained open, no overflow-channel could have been excavated 

 across the higher ground of the Ironbridge isthmus. 



The advance of the Irish-Sea ice-sheet towards Wolverhampton, 

 however, together with an accumulation of ice from the Pennine 

 valleys farther to the east, would eventually have closed these gaps, 

 restricting the area of the Cheshire lake, and raising its surface-level. 

 The drainage then seeking an escape farther to the west, the erosion 

 of the Ironbridge canon would have commenced. 



Since this paper was written, Prof. Watts has informed me that 

 Prof. Lapworth has for some time held the opinion that the drainage 

 of the Upper Dee was diverted into the Severn Valley in Glacial 

 times. 3 



1 ' The Geology of the Country around Macclesfield, &c.' Mem. Geol. Surv. 

 1906, p. 81. 



2 ' The Geology of the Country around Stoke-upon-Trent ' Mem. Geol. Surv. 

 2nd ed. (1905) p. 64. 



3 I understand that Prof. Kendall expressed a similar view some years ago. 



