Vol. 70.] GLACIAL GEOLOGY OF EAST LANCASHIRE. 223 



a stretch of canal three-quarters of a mile long crosses the water- 

 shed without a lock. The sides of the gorge rise precipitously 

 to a height of over 300 feet above its floor, Several streams have 

 cut out notches in its walls and deposited the debris upon its floor. 

 The actual watershed is a delta -watershed or ' corrom,' x the sedi- 

 ment being deposited by Light Hazzles Brook, which descends 

 from the east and is deflected southwards by the ' corrom.' 

 A cutting was made in the ' corrorn ' below 600 feet in the con- 

 struction of the canal, and an exposure of alluvium may now be 

 seen in the bank passing below the surface of the water. It is 

 clear that the rock-bottom of the gorge extends considerably below 

 600 feet, and would thus provide an outlet for the drainage con- 

 veyed by the two channels mentioned above, as the ' corrom ' 

 could not be formed until after the Walsden overflow-channel was 

 abandoned. 



A great amount of morainic material occurs in the Roch Valley 

 south and south-west of Rochdale. This is traversed by a number 

 of dry channels directed southwards, in addition to the gorge which 

 the Roch now occupies. The highest of these channels occurs about 

 2 miles south of Rochdale, and is cut down below 550 feet. The 

 opening of this outlet must have been the cause of the abandonment 

 of the Walsden channel. 



(c) The Western Series. 



It has been shown that, near the time when Birtle Dean ceased 

 "bo operate as an outlet of the Irwell lake, the Walsden gorge was 

 abandoned. Hence the drainage from all the overflow-channels 

 now to be described was confined to the western side of the Pen- 

 nines. The channels south-west of Birtle Dean mark the last 

 stages of the separation between the Irwell lake and the Rochdale 

 lake. 



During its later history, the Irwell lake was augmented by the 

 overflow from a lake south-east of Accrington which cut a channel 

 below 800 feet between Baxenden and Haslingden. The retreat of 

 the ice northwards across this watershed removed the last source of 

 supply of ice to the Irwell Valley north of Ramsbottom. West of 

 the Irwell Valley, the Rossendale highland is less uniformly high, 

 and, being traversed by valleys from north-west to south-east, 

 besides projecting farther west, was crossed by considerable streams 

 of ice. The long persistence of the ice-barrier against the ridge 

 north-east of Bury may have been largely due to the convergence 

 of these ice-streams upon the basin-like hollow in which Bury is 

 situated, in addition to the tendency of the ice that poured over the 



1 P. F. Kendall & E. B. Bailey, ' The Glaciation of East Lothian south of 

 the Garleton Hills ' Trans. Roy. Soc. Edin. vol. xlvi, pt. 1 (1907-08) pp. 25-30. 



