224 dr. a. jowett ox the [June 1914, 



plain along the south of the Rossendale highland to press north- 

 wards up the Irwell Valley. 



The overflow-channels on the eastern slopes of the Darwen- 

 Edgeworth valley indicate that it was occupied by an ice-stream 

 from the north at a comparatively late stage, for the drainage 

 from one of these channels probably entered the Irwell lake while 

 it was still separated from the Rochdale lake. The big channel 

 at Hawkshaw Lane was, however, cut by the overflow from 

 the Edgeworth lake into the Irwell lake, after the ice-barrier had 

 shrunk away from the ridge north-east of Bury. The Edge- 

 worth lake was fed by the overflow from lakes in the Darwen 

 and Hoddlesden valleys north of the watershed, and from a 

 lake in the Belmont valley. In all probability the Edgeworth 

 and Belmont lakes were united before the Hawkshaw-Lane 

 channel was abandoned. The Belmont lake then discharged 

 its waters eastwards into the Irwell lake through a gap below 

 500 feet due west of Bury, and later directly into the Irwell lake ; 

 the latter overflowed south-eastwards, reinforced by the drainage 

 from the Burnley lake by way of the overflow-channels west of the 

 Rossendale highland. It is, therefore, certain that the Cliviger 

 overflow-channel continued in operation at a much later stage 

 than the Walsden channel. The channels west of the Rossendale 

 highland extend in a broad arc, convex towards the west, from a few 

 miles south of Blackburn to Horwich, where the}'' open upon an 

 alluvial plain built of the debris derived from their excavation. 

 The altitude of the ice-barrier corresponding with the highest series 

 fell gradually, from about 750 feet on the north to below 600 feet 

 on the south. The Brinscall channel is cut down below 550 feet, 

 and at the same altitude another series was initiated over a mile 

 away to the north-west : the main channel being about 100 feet deep, 

 and cutting its watershed down to 350 feet above O.D. This series, 

 although much obscured by recent stream-action, may be traced 

 through Chorley to Wigan and Leigh, and marks the last stage 

 during which the Ribble drainage escaped into the Mersey basin. 



The absence of any large and deep valleys of the ordinary type 

 at the western extremity of the Rossendale highland, and the 

 fact that the valleys that do exist open more or less towards the 

 direction from which the ice came, prevented the formation of 

 any but insignificant lakes to break the continuity of the overflow- 

 channels. Though some of the channels are wide and deep rock- 

 gorges, many were wholly or partly excavated in Drift ; and their 

 western walls frequently consisted entirely of ice, or were supple- 

 mented above by the ice -barrier. 



Prof. Boyd Dawkins 1 has shown that the contour of the present 

 surface, here and over the plain to the west and south, is much less 



1 'The Relation of Geology to Engineering ' Proc. Inst. C.E. vol. cxxxiv 

 (1897-98) pp. 269-70. 



