272 



GEOLOGY 



228, and Pis. XVI and XVII). Surface bowlders, often unlike 

 the underlying formations of rock, and sometimes in peculiar mid 



Fig. 229. Perched bowlder, Argentina. (Robin.) 



apparently unstable positions, are still another mark of a glaciated 

 area (Fig. 229). 



GLACIO-FLUVIAL WORK 



The constant but unequal waste of glaciers has already been 

 referred to. The streams to which the melting of the ice gives rise 

 are unusually laden with gravel, sand, and silt derived from the 

 ice. Since the mud is often light-colored, the streams are some- 



Fig. 230. Diagram to illustrate the profile of a valley train, and its relal ions 

 to the terminal moraine (ra) in which it heads. 



times described as "milky." Where the amount of mat (Mini carried 

 is great, much of it is dropped at a slight distance from the ice, the 

 coarsest being dropped first. Glacial streams are, as a rule, 

 ing streams, and therefore develop alluvial plains, called 

 trains (Fig. 230), or, where they enter lakes, bays, or other streams, 



