THE ICE AGE AND ITS WORK. 45 



favorable conditions. In the first place it is evident that ice ero- 

 sion to some extent must have taken place along the whole length 

 of the glacier's course, and that in many cases the result might be 

 simply to deepen the valley all along, not quite equally, perhaps, 

 but with no such extreme differences as to produce a lake basin. 

 This would especially be the case if a valley had a considerable 

 downward slope, and was not very unequal in width or in the 

 nature of the rocks forming its floor. The first essential to lake 

 erosion is, therefore, a differential action, caused locally either by 

 increased thickness of the ice, a more open and level valley floor, 

 or a more easily eroded rock, or by any combination of these. 



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 

 country. 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 lower part of which was already nearly level 

 for several miles, and with a considerable width to the base of the 

 mountain slopes. In the non-glaciated districts of our own coun- 

 try, 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 motion. 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 ingulfed 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 

 Prof. James Geikie to consist, on the average, of as much stony 

 matter as clay, sometimes one material preponderating, some- 

 times the other. The same thing is indicated by the enormous 

 amount of debris often found on the lower parts 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 similarly 

 buried in debris. Dr. Diener found the Milam Glacier in the cen- 

 tral Himalayas completely covered with moraine rubbish; and 

 Mr. W. M. Conway states that the lowest twenty miles of the 



