ACCIDENTS TO RIVERS 33 1 



age. Extensive ice-sheets, by spreading a thick mantle of drift 

 which fills up the valleys, may produce the same effects as lava 

 flows, except that the drift is more easily removed. In the north- 

 eastern United States many streams have been displaced by the 

 sheets of glacial drift, and forced to seek new channels at a com- 

 paratively recent date ; they still preserve all the signs of youth, 

 such as deep, trench-like gorges (see Fig. 32), waterfalls, and 

 rapids. The larger rivers have, for the most part, been able to 

 reoccupy their old valleys, but the smaller streams have generally 

 been compelled to excavate new channels. 



