Vol. 70.] GLACIAL GEOLOGY OF EAST LANCASHIRE. 221 



lakes formed when the ice-edge stood close to the watershed south 

 of Burnley. 



In the Avhole of the Upper Irwell basin there is no outlet lower 

 than the gap south-east of Bacup, except the natural outlet at 

 Rawtenstall. The water must, therefore, have escaped in that 

 direction when the channel south-east of Bacup was abandoned. 

 There are no overflow-channels in the Irwell Valley south of 

 Rawtenstall, until we come to the ridge, south-west of Knowl 

 Hill, which is cut by a series of channels — the highest beginning 

 at 950 feet. 



Northwards, however, the watershed is cut below 800 feet by 

 an overflow-channel near Baxenden, but the channel is directed 

 southwards. It is evident, therefore, that the valley at Baxenden 

 Mas blocked by ice from the north, and the main Irwell Valley was 

 filled with ice up to a lower altitude (950 feet) on the south when 

 the Irwell lake overflowed directly into the Roch Valley. The 

 retreat of the ice south-west of Knowl Hill thus led to the 

 abandonment of the direct overflow from the Irwell lake into the 

 Whitworth lake. 



The Irwell lake discharged its waters into the now augmented 

 Naden lake, which again overflowed into the Whitworth lake by a 

 long, Made, curving channel that trenches the watershed from 900 

 to below 825 feet. The first outlet from the Whitworth lake 

 below 900 feet is south of Rushy Hill, where two parallel dry 

 channels occur. The drainage through the higher of these entered 

 the Wardle lake and escaped by a series of overflows into a lake at 

 the head of the Roch Valley and so into the Walsden channel. It 

 is likely that the earliest drainage through the lower channel south 

 of Rushy Hill also escaped by the same route ; but, by the time 

 that the valley had cut to its lowest level, the ice-barrier had re- 

 treated sufficiently to permit of the mingling of the waters of the 

 Wardle lake with those of the Rochdale lake. Later, as the water- 

 level sank a little lower, the two lakes were partly separated by the 

 lateral moraine that stretches across the Wardle valley on the south, 

 and the Wardle lake began to be drained into the Rochdale lake. 



In the fourth stage, the chief factor was the great overflow- 

 channel now occupied by the Cheesden Brook and its connexions. 

 This channel cuts through the ridge between the Irwell and Roch 

 basins at Birtle Dean as a steep-sided gorge over 200 ieet deep. 

 The Irwell lake commenced to drain into it at an altitude of 

 900 feet, and its lowest feeder entered below 750 feet. The water 

 draining through Birtle Dean was distributed by channels at various 

 levels, the earlier portion ultimately passing through the Walsden 

 gorge. The later portion, however, cut a wide channel 30 feet deep, 

 which terminates in a large delta between 500 and 525 feet CD., 

 and therefore could not have drained through the Walsden gorge. 

 This stage was also marked by the draining first of the Whitworth 

 lake, and then of the lake in Naden Dean, and by a great increase 

 in the area of the Rochdale lake. 



