350 The Glacial Theory of Prof. Agassiz. 



where it is soon frozen, and in freezing expands, according to a well 

 known law. The upper strata, imbibing more water than the lower, 

 dilate in a greater degree, but the lower strata, in dilating, carry the 

 upper with them, and thus produce rents or crevasses. Again, the 

 flanks of the glacier imbibe more water than the middle, and by their 

 greater expansion give a curved form to the crevasses ; and the lower 

 end imbibes more water than the upper, in consequence of the more 

 frequent rains and alternations of frost and thaw. Besides, as the upper 

 end of the glacier, in expanding, pushes the rest before it, the accu- 

 mulated effect of the whole expansion falls upon the lower end, which 

 is found to travel quickest. The motion, too, is most rapid in summer, 

 and nearly ceases in winter, in consequence of the water being then 

 constantly frozen. From the effect of this internal movement of its 

 parts, the glacier creeps along slowly but surely. In 1827, M. Hugi 

 constructed a hut on the glacier of the Aar, at the foot of a fixed rock 

 called Im Ahschwung. It was found that the hut had receded 2200 feet 

 from the fixed rock in 1836, and 4400 in 1840, showing that it had 

 advanced about 250 feet per annum in the first nine years, and 550 in 

 the four last. Taking summer and winter together, its motion had been 

 about eight inches per day in the first period, and eighteen inches in 

 the second. In glaciers which are much inclined, the motion is more 

 rapid than this. 



Polished and Grooved Surface of Rocks. — The glacier in its course 

 downward carries with it the fragments of rock, gravel, and sand which 

 lie under it. These adhere to the ice, or are embedded in it, and as 

 the mass glides slowly along, they abrade, groove, and polish the rock, 

 and the larger masses are reciprocally grooved and polished by the 

 rock on their lower sides. The effects of this abrasion on the bottom 

 of the valleys may be conceived from the pressure applied. A cubic 

 yard of sandstone weighs two tons, and if we assume the average 

 density of glacier ice to be two-thirds of that of common river ice, the 

 pressure upon each square yard of rock at the bottom of a glacier 100 

 feet deep, will be equal to about sixteen tons, or the general pressure 

 will be as great as would be produced by a bed of sandstone twenty 

 four feet thick. Thus the various materials under the ice are pressed 

 against the rock with an enormous force, while an equally great force 

 of another kind, produced by the congelation of water, propels them 

 downwards. The sand, acting like emery, polishes the surface ; the 

 pebbles, like coarse gravers, scratch and furrow it ; and the large stones 

 scoop out grooves in it. Portions of these substances, and of the rock 

 too, are ground to the state of fine clay, and the whole of the movable 

 matter, stones, pebbles, sand, and clay, are in course of time thrown 

 out at the lower end of the glacier, where they form the terminal 

 moraine. 



