218 DR. A. JOWETT 0~$ THE [June I9T4,. 



and the ridge. The highest quite unmistakable overflow-channel 

 is about 30 feet deep, on the watershed at the head of Catridge 

 Clough. The floor of the channel south-east of the ridge consists 

 of Boulder Clay, exposed in section b} r the small stream that here 

 takes its rise. The course of the present stream is much steeper 

 than that of the overflow-channel, the latter having suffered con- 

 siderably from post- Glacial denudation. It should be noted that,, 

 while this overflow-channel was in use, the Cliviger gorge on the 

 south-west, or as much of it as existed at this stage, must have 

 been full of ice. The altitude at which the Catridge channel 

 began (1160 feet) corresponds with that of a delta-like feature 

 below the highest channel which cuts through the ridge between 

 Hurstwood Brook and Swinden, so that at this stage a lake pro- 

 bably occupied the upper reaches of Shedden Clough, Cant Clough, 

 and the valley of Hurstwood Brook. At the same time, a lake 

 at a higher level (1200 feet) occupied the head of Swinden, and 

 received the waters of still another lake in Thursden, the waters 

 of which were at a level of about 1260 feet. This lake was formed 

 when the ice retreated from the watershed at Widdop Cross. The 

 highest overflow-channel across the ridge south of Thursden is 

 dry on the watershed ; but, lower down, a small stream has exposed 

 ah excellent section in well-stratified material, consisting chiefly 

 of shale with layers containing grit-boulders and sand and a few 

 pieces of chert. This is evidently the delta of the stream which 

 flowed into the Swinden lake, for its upper surface slopes gently from 

 about 1225 to 1200 feet, an altitude corresponding with that of 

 the highest overflow on the next ridge to the south. We thus have 

 evidence for a chain of lakes, connected by overflow-channels from 

 Thursden to Catridge Clough, which conve3 r ed the drainage of the 

 western slopes of the Pennines south of Boulsworth Hill into 

 the basin of the Yorkshire Calder. As the ice-barrier retreated,, 

 gap after gap was cut at lower altitudes through the ridges between 

 the lakes ; the levels of the water in the lakes were lowered ; and 

 the lakes increased in area, the drainage finding its way ultimately 

 across the watershed by way of the 'Cliviger gorge into the 

 Yorkshire Calder. 



Although space forbids a detailed account of all these overflow- 

 channels, some of which are fine examples with deltas at their lower 

 ends, the Cliviger gorge itself must be dealt with. This valley is 

 400 feet dee-p on the watershed, where, according to Prof. Hull, 1 it 

 is filled with Drift. Landslips, by breaking away from the sides 

 and filling the floor of the gorge, doubtless account for the absence 

 of the characteristic outlines of a Glacial overflow-valley on the 

 watershed : for, although a valley certainly existed here before 

 Glacial times, it must have been further ground out by the ice, as 

 well as deepened by the water, that passed through it during the 

 advance and retreat of the ice-barrier. The floor of the gorge 



1 ' Geology of the Burnley Coalfield' Mem. Geol. Surv. 1875, p. 7. 



