216 dr. a. jowett on the [June 19 14, 



continued to operate until the ice-barrier in the neighbourhood 

 of Burnley sank below 775 feet above O.D., the lowest altitude 

 of the watershed in the gorge. 



The third great feature is represented by a series of deep channels 

 between the Rossendale highland and the Irish Sea. Their 

 position and magnitude indicate that, after the Cliviger overflow- 

 channel was abandoned, and before the natural outlet of the 

 Bibble was unsealed, they conveyed the surplus drainage of 

 Ribblesdale into the Irwell-Mersey basin. 



There were thus two great episodes in the history of the 

 glacial drainage in this area. During the first, the western 

 drainage crossed the Pennine watershed by the Cliviger and Walsden 

 overflow-channels into Calderdale ; and during the second, it was 

 entirely disposed of west of the Pennines. 



It has been shown that, at the culmination of the Grlacial Period, 

 this region was practically all ice-covered up to, and generally 

 across, the Pennine watershed. The development of extensive 

 systems of glacier-lakes and overflows could therefore only take 

 place during the advance of the ice-barriers towards their maximum 

 extension, and after the ice had retreated sufficiently to expose a 

 certain amount of land-surface sloping towards the ice-barrier. 

 Consequently, we fail to note here the close connexion between the 

 margin of the Drift and the complete series of lakes and overflow 

 valleys that is found where considerable unglaciated areas projected 

 above the surface of the ice-sheet at its maximum extension. 



The most obvious condition that a number of overflow- channels 

 must satisfy, in order to be considered as being in operation at the 

 same time and thus marking the position of the ice-barrier at a 

 particular stage, is an unbroken decrease of altitude from the 

 highest to the lowest. In the same direction there should also be 

 a gradual increase in the capacity of each channel, as measured by 

 the area of its cross-section on the watershed through which it 

 cuts — because each lake had to pass on to the next, not only the 

 water that it received from the preceding lake, but also the surplus 

 drainage of its own basin. Moreover, as the ice-sheet retreated, 

 the increase in the amount of water draining from the land as a 

 result of the increased area not covered by ice naturally tended to 

 make the overflow-channels at lower altitudes greater than those 

 at higher altitudes, ccBteris paribus. In some cases, however, 

 instead of a single overflow-channel between two others, two or 

 more may occur near together, their altitudes lying between the 

 two extremes, and their joint capacities also being intermediate 

 between those of the overflows next above and next below in the 

 sequence. 



The overflow-channels on the south and west of the Rossendale 

 highland fulfil these theoretical requirements to a remarkable 

 degree, the distance between one ' aligned sequence ' and the next 

 being generally sufficient to separate them completely, and thus, 

 with few exceptions, leaving no doubt as to the set to which any 

 particular channel belongs. 



