GROUND-MORAINES. 189 



and if the glacier advances it will upset the old moraine and de- 

 posit another further down the valley. The two last-mentioned 

 moraines indicate the greatest and the least extension that the 

 glacier has ever attained. 



Most of the rocky materials that fall on the surface of the 

 glacier remain there ; but wherever the glacier is traversed by 

 fissures or crevasses the fragments 'of rocks drop down these 

 openings and fall to the bottom of the glacier. Facilities 

 for the dropping-down of debris especially occur at the sides of 

 the glacier, where there is not unfrequently a fissure between 

 the mass of ice and the adjoining mountain. The stones thus 

 deposited and carried along under the glacier form ground- 

 moraines (moraines du fond) . In their onward movement the 

 fragments of rock become rounded and polished by the friction 

 to which they are exposed. The bed of the glacier is also 

 smoothed when the ice is in immediate contact with the 

 rock ; and where stones and sand are frozen into the ice it is 

 scratched by them. The same scratching and polishing takes 

 place on the rocky walls which enclose the sides of the glacier, 

 especially in places where the bed of the glacier is narrowed, 

 and where, in consequence of greater inclination, the mass of 

 ice moves more rapidly downwards. Here the rocks are often 

 regularly polished, and traversed by sharp straight striae, pro- 

 duced by the hard sand-grains and splinters of rock which, 

 being imbedded in the ice, are carried forward with it. In fine- 

 grained rocks these lines are often as sharp as if they had been 

 cut with a diamond; and sometimes they may be traced for 

 several yards. But where the bottom of the glacier is but little 

 inclined, or the mass of ice can spread itself out, the glacier 

 does not possess the power of scoring the underlying rock. 

 Sometimes the glacier is separated from the ground below it, 

 and the melting of the ice causes the formation of galleries and 

 deep caverns. 



The water flowing from the glacier carries off the mud pro- 

 duced by friction, and is rendered turbid by it ; and as the 

 glacier-stream must pierce through the terminal moraine, it will 

 carry away some of the rocky material composing that mass of 

 debris. Hence a glacier-stream will disperse the fragments 

 brought down by the glacier, and in its action will round off 



