THE WORK OF SNOW AND ICE. 



305 



Fig. 261), the rock basins, the lakes (Fig. 261), ponds, and marshes, 

 and the pecuHar topographies resulting from the unequal erosion, and 

 the still more unequal deposition of the drift. Surface bowlders, often 

 unHke the underlying formations of rock, and sometimes in pecuhar 

 and apparently unstable positions, are still another mark of a glaciated 



area. 



GLACIO-FLUVIAL WORK. 



The constant but unequal waste of glaciers has already been referred 

 to. The streams to which this gives rise are usually laden with gravel, 

 sand and silt derived from the ice. Since the mud is often light-colored, 

 the streams are sometimes described as ^' milky." Where the amount 

 of material 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, aggrading streams, and therefore develop alluvial plains, called 

 valley trains (Fig. 281 and 282), or where they enter lakes (Fig. 283), bays, 

 or other streams, deltas. In its transportation, the river-borne drift 



j^^^^^^^^^l 



^I^^^^HjH^^^^^^ 



'^^^^VMSP^ ^^^^^^IB^jH^^^H^^^Hj^^^^^H^^^^^^^^^^I 



^0: 







Fig. 281. — Alluviation by glacial stream: Nicolai Creek, Alaska. (Schrader, U. S. 



Geol. Surv.) 



is assorted; after its deposition, it is stratified. True glacial deposits 

 in the upper part of a mountain valley are, therefore, often continued 

 below by glacio-fiuvial deposits derived from the same source. 



