Vol. 68.1 GLACIATIOX OF THE BLACK COMBE DISTRICT. 40' 



III. Pre-Glacial Conditio?^ of the District. 



With the exception of the drift plain, the main features of the 

 district had been developed before the advent of the Ice Age. As a 

 result of the glaciation, some of these features were accentuated, 

 others were subdued, and some new ones were introduced. 



The cleavage-dip of the slate accounts in part for the smooth 

 northward-facing slopes of Black Combe, and the more craggy south- 

 ■eastern and south-western slopes. These features, however, were 

 certainly accentuated by glaciation. Over-steepening of the seaward 

 slope of the mountain accounts for the precipitous descents upon 

 that side. The sides of the Whicham Valley are similarly affected, 

 and the valley-bottom has been somewhat over-deepened. In the 

 volcanic tracts the rugged features, which now appear to be so 

 prominent when one is looking in a direction opposed to that of 

 the ice-movement, would, in pre-Glacial times, have been prevalent 

 everywhere. The chief alterations in feature of the granite tract 

 would consist in the destruction of ' tors ' and the removal of a 

 ^Teat quantity of rotten rock from the lower ground. Some of the 

 drift-deposits of the plain descend below sea-level : hence it is quite 

 possible that the pre-Glacial sea may have touched the foot of Black 

 Combe ; but, even if this were the case, it would not account for the 

 smoothed and oversteepened slopes, cut-away spurs, and hanging 

 valleys which occur at higher elevations. 



In the mountain districts the main pre-Glacial drainage-lines were 

 similar to what they are to-day. L^'pon the advance of the ice some 

 of the smaller channels were choked with drift ; but in most of these 

 the post- Glacial streams follow the old lines, and are removing the 

 'extraneous material from their valleys. 



IV. The Maximum Glaciatiox. 



Evidence as to the direction of movement of the streams of ice 

 which invaded the district during the maximum ^ glaciation is fur- 

 nished chiefly by (a) the composition and distribution of the drifts, 

 {b) the erratics, and (c) the grooved, smoothed, and moutonne 

 surfaces, seen to perfection in the volcanic tract. Minor evidence 

 is yielded by striae and by scoured or steepened valley- sides, cut- 

 away spurs, and hanging valleys, and, during the stages of retreat, 

 by the trend of moraines and of marginal drainage-channels. 



My observations tend to show that, during the flood-tide of 

 glaciation, the whole district was completely overridden by an ice- 

 sheet formed by the confluence of the edge of the Lake-District ice- 

 cap with the great Irish-Sea Glacier, one of the points of contact 

 between the two lying about a mile west of Black-Combe summit. 



^ The anomalous distribution of some of the Lake-District boulders may have 

 been due to valley-glaciers reaching the sea before its invasion by the Scottish 

 ice, and launching bergs which scattered in various directions and strewed rock- 

 fragments over the sea-bed. 



Q. J. G, S. m, 271. 2 & 



