310 



EARTH FEATURES AND THEIR MEANING 



without stratification, or unassorted ; and that deposited by water 

 flowing either beneath or from the ice, and this like most fluid de- 

 posited material is assorted or stratified. The unassorted material 

 is described as till, or sometimes as " bowlder clay " ; the as- 

 sorted is sand or gravel, sometimes with small included bowlders, 

 and is described as kame gravel. To recall the parts which both 

 the glacier and the streams have played in its deposition, all water- 

 deposited ' materials in connection with glaciers are called fluvio- 

 glacial. 



Till is, then, characterized by a noteworthy lack of homogeneity, 

 both as regards the size and the composition of its constituent 



parts. As many as twenty 

 different rock types of varied 

 textures and colors may some- 

 times be found in a single 

 exposure of this material, and 

 the entire gamut is run from 

 the finest rock flour upon the 

 one hand to bowlders whose 

 diameter may be measured 

 in feet (Fig. 335). 



In contrast with those de- 

 rived by ordinary stream 

 action, the pebbles and 

 bowlders of the till are fac- 

 eted or " soled," and usually 

 show striations upon their 



faces. If a number of pebbles are examined, some at least are sure 

 to be found with striations in more than one direction upon a 

 single facet. As a criterion for the discrimination of the material 

 this may be an important mark to be made use of to distinguish 

 in special cases from rock fragments derived by brecciation and 

 slickensiding and distributed by the torrents of arid and semiarid 

 regions. 



Inasmuch as the capacity of ice for handling large masses is 

 greater than that of water, assorted drift is in general less coarse, 

 and, as its name implies, it is also stratified. From ordinary 

 stream gravels, the kame gravels are distinguished by the form of 

 their pebbles, which are generally faceted and in some cases 



FIG. 335. Section in coarse till. Note the 

 range in size of the materials, the lack of 

 stratification, and the "soled" form of the 

 bowlders. 



