THE WORK OF GLACIERS 161 



Thousands of mountain lakes owe their existence to such morainic 

 dams. 



Submarginal Moraines. — Another kind of moraine is formed 

 under the sides of a glacier by the movement of the ice from the 

 center to the sides. This should not be confused with the lateral 

 moraines of the surface (p. 154). In valley glaciers which receive little 

 superglacial debris these submarginal moraines maybe thicker than the 

 surface moraines. The presence of polished and striated pebbles and 

 bowlders in such moraines is abundant proof that the drift composing 

 them had been carried between the ice and its bed. 



Ground Moraine. — A glacier may be so full of debris in its basal 

 portion that it is unable to carry all of it. Under such conditions 

 some of the load is deposited and is overridden. Such deposition takes 

 place (1) where the ice is thinning near the end, as this makes its 

 movement less rapid, so that it is unable to carry all of the load which 

 it has acquired in its progress through a rough valley. Such deposi- 

 tion also occurs (2) after a glacier has passed over a projection in its 

 bed, as the bottom of the ice is then heavily loaded with the debris 

 which it has plucked or abraded from the obstacle. 



In a valley formerly occupied by a glacier there is usually a layer 

 of compact till composed of clay and much-worn pebbles. This 

 deposit is known as the ground moraine and was derived either from 

 the bottom of the advancing ice, as described above, or from the base 

 of the ice upon its disappearance. It is usually thickest near the 

 terminal moraine and thinnest near the head of the glacier, while over 

 portions of the valley it may be entirely lacking. Since conditions 

 in valley glaciers favor erosion rather than deposition, their ground 

 moraines are seldom important, being in contrast in this respect to 

 continental glaciers (p. 171), whose ground moraines are of con- 

 siderable thickness, although seldom attaining the depth of terminal 

 moraines. 



The Work of Glacial Streams. — The streams which flow from 

 beneath glaciers or from their sides are supplied with pebbles from 

 the moraine and an abundance of rock particles derived from the 

 rock ground to fine flour between the ice and its bed. With such tools 

 they are able to deepen their channels as long as they have sufficient 

 velocity. The streams from certain glaciers emerge from their fronts 

 in deep gorges which they have cut in the rock. The Lammer Glacier 

 of Switzerland and the Mer de Glace are examples. It is doubtful, 

 however, if the deepening which is such a marked feature of valleys 



