THE WORK OF GLACIERS 



179 



Fig. 164. — Kettle holes. (After C. R. Dryer.) 



alluvial slopes are formed from the growth of adjacent alluvial fans 

 (p. 124). Outwash plains differ from valley trains in being shorter 

 and wider. 



Outwash plains are closely associated with terminal moraines. The 

 longer the front of a glacier remained stationary, the more favorable 

 were the conditions for the accumulation of gravel, since the streams 

 then had an abundant supply of debris which they continued to de- 

 posit. The material of outwash plains and valley trains is sand and 

 gravel, the coarser material being nearest the moraine and the finer 

 further away. Outwash plains may be very extensive, and since 

 they are composed of sand and gravel are usually infertile, sometimes 

 to such a degree as to form miniature deserts. The sandy, desert- 

 like plain south of the terminal moraine on Long Island is an outwash 

 plain. 



Outwash plains and morainic plains may be " pitted " ; that is, 

 the general level may be broken by more or less rounded depressions. 

 These pits are called kettle holes (Fig. 164) and were usually devel- 

 oped from the melting of blocks of ice which had been buried in the 

 drift as the ice retreated. In the outwash plain of the Hidden 

 Glacier in Alaska kettle holes are seen to be forming ; and " their 

 development is due to the melting out of ice from beneath the plain, 

 although in no case was the ice actually seen." (Tarr.) 



Terraces. — The valleys of the rivers of Ohio, Indiana, and Illi- 

 nois which carried off the water of the melting ice are now bordered 

 by terraces of stratified drift, and the conspicuous terraces of the 

 Connecticut and Merrimac rivers (p. 127) and their tributaries are 

 remnants of deposits of stratified drift laid down either by over- 



