492 Mr. J. Ball on the Formation of Alpine Lakes, 



of melting, and is greatest when a glacier descends rapidly from 

 an upper level to a lower one where it encounters a rapid in- 

 crease of temperature. A glacier that had flowed for 23 miles 

 through a nearly level valley would taper gradually to a com- 

 paratively thin tongue. But the ploughing action of the front 

 of a glacier depends, as I believe, much less on the weight of 

 the advancing mass than on its tenacity. Glacier-ice, as we 

 know, will bend to suit the shape of the rocky channel in which 

 it moves ; but some considerable force is necessary for the pur- 

 pose, and that force is partly expended against the sides or 

 bottom of the channel. Thus it happens that when a glacier 

 descends a rapid slope, and then encounters a less inclined sur- 

 face, the front does exert a considerable ploughing action, and 

 shoves before it whatever incoherent masses may stand in its 

 way, until it gradually adapts its bed to the new slope on which 

 it has to advance*. 1 am relieved from the necessity of insisting 

 on the limited power of the front of a glacier to excavate the 

 diluvium, because this is implicitly admitted by both my oppo- 

 nents. M. de Mortillet has, indeed, been good enough to give 

 several sections showing portions of terminal moraine resting on 

 the diluvium, in positions where neighbouring portions of the 

 same diluvium must have remained in their present position 

 under the advancing front of the glacier. 



If we abandon the idea that the glacier of the Tessin during 

 the period of its advance pushed before it fifteen or twenty cubic 

 miles of solid matter that had previously filled the channel of 

 the lake, the next alternative will be to suppose that the glacier 

 advanced until it covered over the underlying diluvium, and 

 that by some agency, not yet explained, it gradually effected the 

 clearing out from the trough of this enormous underlying mass. 



If we had to consider a glacier lying in a valley with a slope 

 of, say 5°, over a bed of diluvium a quarter of a mile in thick- 

 ness, we should infer, from the few observations we possess 

 on the retarding effect of the bed upon the motion of the 

 ice, that the motion of the bottom of the glacier would be less 

 than half of that of the surface, yet sufficient to exert such a 

 grinding action on the subjacent stratum that in the course of 

 ages the whole might possibly be ground down and removed. 

 It would be an essential part of the process that, as the mate- 

 rials were pulverized, the streams flowing beneath the glacier 

 would carry them away to some lower level in the form of 



* I am willing to admit that this action, exerted at the base of consider- 

 able ice-falls, may excavate in the underlying rock basins of slight depth 

 compared to the thickness of the glacier, and may thus have produced 

 some of the tarns seen in high mountain countries. But I believe these 

 cases to be exceptional and unimportant, 



