

Yol. 70.] THE GLACIAL GEOLOGY OP EAST LANCASHIRE. 215 



The Thickness of the Ice-Sheet. 



The extent of the Local Drift suggests that the upper layer of 

 clean ice was from 200 to 300 feet thick, whence we may infer that 

 the actual surface of the ice-sheet at Winter Hill probahly 

 reached an altitude of 1750 feet above O.D. 



From the maximum altitudes attained by the North- Western 

 Drift, from Winter Hill to the Pennines, the surface of the 

 ice-sheet has been estimated to have fallen eastwards between 

 4 and o feet in a mile. Hence, assuming this gradient to be 

 continued westwards to the middle of the Irish Sea, the altitude 

 of the surface of the ice-sheet would be there approximately 

 2100 feet above present sea-level, in the latitude of Winter Hill, 

 at the culmination of the Glacial Period. 



V. Systems of Glacial Drainage. 



Prof. Hull 1 described several watersheds in this region as 

 occurring in valleys, and called attention to the fact that these 

 valleys were generally broad and level. The Walsden gorge . . . 



' is one of the deepest and most perfectly formed valleys in Lancashire .... 

 The change of inclination in the ground on crossing the saddle is almost 

 imperceptible to the eye, and the bottom of the valley presents a smooth 

 surface, which, though not at present containing any stream, has clearly 

 been levelled by the action of water.' 



In his memoir on the Glacier Lakes of Cleveland. 2 Prof. P. F. 

 Kendall expounded the principles underlying such phenomena, and 

 examples have been recognized in various parts of the British 

 Isles, but in few areas are they better developed than in East 

 Lancashire. 



Among the dominating features that acted as controlling factors 

 of the drainage, at different stages during the retreat of the ice 

 and in different parts of this area, the great Walsden gorge is 

 the most important. The altitude of its floor on the watershed 

 between the North Sea and the Irish Sea is about 610 feet above 

 O.D., and on the western side of the Avatershed are many drainage- 

 channels above that altitude which were, one after another, 

 tributary to the Walsden Gorge as the ice-barrier decreased in 

 height. Broadly speaking, the area drained by this gorge ultimately 

 included the whole of the Irwell basin that was not covered by 

 ice, up to the time when the ice-barrier in the Roch Valley 

 fell below 600 feet above O.D., together with a number of small 

 areas at the heads of valleys on the northern side of Bossendale, 

 the drainage of which was reversed so as to enter the Irwell 

 basin instead of that of the Eibble. 



The feature next in importance is the Cliviger gorge, between 

 Burnley and Todmorden. Towards it are directed a number of 

 drainage-channels south of Boulsworth Hill, indicating that it 



1 ' Geology of the Burnley Coalfield ' Mem. Geol. Surv. 1875, pp. 6-8. 



2 Q. J. G. S. vol. lviii (1902) pp. 471-569. 



Q.J.G.S. No. 278 Q 



