GLACIAL EROSION OF LAKE BASINS 101 



If we look at the valley-lakes of our own country and of 

 Switzerland, the first thing that strikes us is their great 

 length, and their situation, usually at the lower end of the 

 valley where it emerges from the higher mountains into 

 comparatively low countrj^ Windermere is over ten miles 

 long, Ullswater nearly eight miles, and the larger lakes of 

 Switzerland and North Italy are very much longer. The 

 first essential condition, therefore, was a valley, the low^er 

 part of which ivas already nearly level fm' several miles, and 

 with a considerable width to the base of the mountain 

 slopes. In the non-glaciated districts of our own country, 

 the Dart and the Tamar are examples of rivers which have 

 cut their valleys down nearly to sea-level while still among 

 the hills ; and in South Wales the Wye, the Usk, and the 

 Severn have a similar character. 



It must always be remembered that glacial erosion is 

 produced by the tremendous vertical pressure of the ice, 

 by its lower strata being thickly loaded with hard rocks 

 frozen into its mass, and by its slow but continuous grind- 

 ing motion over its bed. In the lower part of its course a 

 glacier would be most charged with rocky debris in its 

 under strata, since not only would it have been continually 

 breaking off and absorbing, as it were, fresh material 

 during every mile of its onward course, but more and 

 more of its superficial moraines would be engulfed by 

 crevasses or moulins, and be added to the grinding material 

 below. That this was so is proved by the great quantity 

 of stones and grit in the " till," which is thought by Pro- 

 fessor James Geikie to consist, on the average, of as much 

 stony matter as clay, sometimes one material preponderat- 

 ing, sometimes the other. The same thing is indicated 

 by the enormous amount of debris often found upon the 

 surface of the lower portions of large glaciers. The end 

 of the great Tasman glacier in New Zealand is thus com- 

 pletely hidden for five miles, and most of the other glaciers 

 descending from Mount Cook have their extremities simi- 

 larly buried in debris. Dr. Diener found the Milam 

 glacier in the Central Himalayas completely covered with 

 moraine rubbish ; and Mr. W. M. Conway states that the 

 lowest twenty miles of the Hispar glacier (forty miles 

 long) are " entirely covered with a mantle of moraine." If 



