Vol. 70.] GLACIAL GEOLOGY OF EAST LANCASHIRE. 219 



south-east of the watershed has also been much altered by torren- 

 tial waters, which have obstructed it with ridges of detritus derived 

 from its precipitous sides since it ceased to be an overflow-channel. 

 Plains of alluvium occur above these mounds, and the diminutive 

 Calder has cut its way through the latter, forming small gorges. 



The watershed in the Cliviger gorge is now just below 775 feet, 

 but it may have been somewhat lower before the landslips men- 

 tioned above. The last overflow-valley on the ridge to the north 

 also cuts below 775 feet, from which we may infer that the 

 union of the great lake north of this ridge with the lake in the 

 Cliviger valley north-west of the watershed took place about the 

 time when the Cliviger gorge ceased to operate as an overflow- 

 channel. The abandonment of the Cliviger gorge could only 

 result from the opening of some other outlet for the water at 

 a lower level than the watershed at Calder Head. Now, all 

 the overflow-channels mentioned above indicate a gradual fall of 

 the ice-barrier southwards, which suggests that there was an 

 effective, though diminishing, pressure of ice from north to 

 south in this locality, and presupposes an ice-stream between the 

 Pendle chain and the Pennines, extending southwards as far as 

 Hurstwood, when the Cliviger overflow ceased to operate. This 

 being the case, there was no possible means of escape for the 

 waters of these lakes below 775 feet across the Pennines towards 

 ~the north. Nor does any gap exist across the main watershed of 

 iihe Ko~ssendale highland below that altitude, until we come to the 

 Brinscall gap 4 miles north-east of Chorley. It is, therefore, 

 practically certain that the retreat of the ice-barrier, by allowing 

 the water impounded in the Cliviger valley above Burnle\ r to escape 

 through the Brinscall gap, led to the abandonment of the Cliviger 

 overflow. This also marks the time of definite separation between 

 the ice-stream from the north and that from the west, for now a 

 lake existed in the basin of the Lancashire Calder, extending, 

 broadly speaking, from Blackburn to Burnley, and confined between 

 two ice-barriers, one a short distance west of Blackburn, and the 

 •other at Burnley. 



The gap in the Pendle chain at Whalley is not an important 

 factor in the existence of this lake, for it was doubtless obstructed 

 by ice in the Kibble Valley at this stage ; and, even if this were 

 not the case, the ice that acted as a barrier between the Pendle 

 chain and the Kossendale highland west of Blackburn must have 

 been still higher at the outlet of the main valley of the Kibble 

 farther north ; hence, so long as the North- Western ice formed an 

 effective barrier here, any drainage from the Kibble basin could 

 not have escaped northwards by way of the Whalley gap, but must 

 necessarily have ultimately escaped southwards by the overflow- 

 channels west of the Kossendale highland. It was only when the 

 main Kibble Valley was unsealed, that the drainage of the Lanca- 

 shire Calder could escape by the Whalley gap into the Ribble. 



