58 



AQUEOUS AGENCIES. 



ceivable size, from huge bowlders down to fine earth, mixed together 

 into an heterogeneous mass entirely different from the neatly-sorted de- 

 posits from water. It is, therefore, entirely unsorted and nnstratified, 

 and without organic remains. 2. The mass consists of two parts, viz., 

 that which was carried on the top of the glacier, and that which was 

 forced out beneath (ground moraine). The first consists of loose ma- 

 terial containing angular, unworn fragments ; the other of fine compact 

 material containing fragments worn and polished, and scratched with 

 straight, parallel scratches, but in both cases entirely different from 

 water-worn pebbles. In all respects, therefore, the action of glaciers is 

 characteristic and can not be confounded with that of water. 



Evidences of Former Extension of Glaciers. — It is by evidence of 

 this kind that the former great extension of glaciers in regions where 



they now exist, and the former 

 existence of glaciers in regions 

 where they no longer exist, have 

 been proved. We have already 

 stated that during a succession 

 of cool, damp seasons, a glacier 

 may extend far beyond its pre- 

 vious limits. Similar changes 

 take place also in the depth of 

 a glacier. In a word, glaciers 

 are subject to floods like rivers ; only these floods, instead of being an- 

 nual, are secular. Now, as rivers after floods leave floating material 

 stranded on the banks, showing the height of the flood - 

 water, so, in the decrease of a glacier, lines of bowlders 

 are left stranded, often delicately balanced, on ledges 

 high up the sides of the valley. These lines of bowl- 

 ders mark the former height of the glacier. Some of 

 these lines have been found in the Alps 2,000 feet above 

 the present level. Fig. 47 is a cross-section of a glacial 

 valley. The dotted lines show the former level. In 

 the same valleys we find old terminal moraines (Fig. 

 48, a) miles beyond the present limit of the glacier. 

 The characteristic planing, polishing, and parallel scor- 

 ing, have been found equally far above the present level 

 and beyond the present limit of Alpine glaciers. 



Glacial Lakes. — When a glacier retreats, the water 

 of the river which flows from its point may accumulate 

 in great rock-basins scooped out by the glacier, or else 

 behind the old terminal moraines. In these two ways 

 lakes are often formed. 



Fig. 47. — Section across Glacial Valley, showing old 

 Lateral Moraines. 



•.l\ 



OV't 



<'-- 



Fig. 48.— Old Ter- 

 minal Moraines. 



