354 



The Glacial Theory of Prof. Agassiz. 



The ascent of blocks from the middle or lower part of the ice to the 

 sm'face, explains another curious fact — that though the general motion 

 of the glacier is along an inclined plane downwards, scratches are often 

 found on the rock inclined in the opposite direction ; that is to say, 

 supposing the surface of the glacier to dip at 10 degrees to the north, 

 you will find scratches dipping at 10 or 20 degrees to the south, or 

 even vertical. These are caused, in Agassiz's opinion, sometimes by 

 inequalities in the bottom of the valley, but frequently by enclosed 

 blocks working their way upward by the expansion of the ice, while 

 the glacier is travelling downwards. 



Figure 2 represents the usual form of a lateral moraine in the cross 

 section, and as it would appear on a surface considerably inclined ; ??i 

 the mass of gravel form- 

 ing the moraine ; r k the 

 rock on which it rests. 

 Figure 3 represents the 

 form which, it is assum- 

 ed, a compact medial mo- 

 raine would have if the ice were melted, and the matter left on the 

 surface of the valley. 



Retreating glaciers form a terminal moraine every year, as mention- 

 ed in page 353 ; and in 

 this case we might expect 

 to find a series of mounds 

 transverse to the valley, 

 like X y, figure 5. 



Erratic Blocks. — Sin- 

 gle blocks of huge size 

 are often seen resting on the surface of the glacier, and travelling 

 downwards with it. These are generally angular, and they often stand 

 on pedestals of ice, as in figure 4, where a is a tabular mass of rock, 

 and b the pedestal of ice.* Agassiz describes one he saw on a glacier, 

 which measured 20 feet by 12, and must have weighed 100 tons or 

 more. In accounting for the pedestal h, he observes that gravel, when 

 it rests on the surface of a glacier, being heated through and through 

 by the sun's rays, melts the ice below it, and gradually forms a pool or 

 well in it. A large block, on the other hand, has only its upper sur- 

 face heated, while the inferior mass, remaining cold, protects the ice 

 below — both from the action of the sun's rays, and from the evapora- 

 tion by which ice, like water, wastes away in the open air, and thus. 



Fiff. 5. 



This figure is borrowed from Agassiz's fourteenth plate. Figure three, and 

 all the others, are ideal, and are suggested by his descriptions. 



