204 PHYSICAL GEOLOGY 



ANCIENT GLACIERS 



In much of Canada, in the United States east of the Mis- 

 souri River and north of the Ohio, and in northern Europe, 

 the mantle rock consists of a mixture of bowlders, gravel, sand, 

 and clay, ranging in thickness from a few inches to more than 

 500 feet. These materials occur separately in some places, 

 and elsewhere are mixed confusedly in all possible proportions. 

 This mantle rock was not produced by the weathering of the 

 underlying rock, for in any given locality it contains material 

 to which the decay of the bedrocks of that locality could not 

 give rise. This fact is further shown by the contact between the 



FIG. 214. Diagram showing gradual transition from residual soil into the 

 unaltered rock below. (U.S. Geol. Sure.) 



mantle rock and the underlying rock. Mantle rock formed in 

 place normally grades more or less insensibly into the firm rock 

 below (Why? Fig. 214). In the areas in question, however, 

 the surface material gives place abruptly in most places to the 

 unaltered rock beneath as suggested in Figures 229 and 230. 

 The mantle rock of these areas, therefore, was brought to its 

 present position by one or more of the agents which transport 

 materials upon the land. It is known as drift, the term having 

 been applied under the impression that it had been drifted 

 by waters to its present position from outside sources. 



Figure 215 shows a typical exposure of unstratified drift 

 (till). As shown in the illustration, till consists usually of 

 material of many kinds and sizes, and is not in layers. The 

 stones and bowlders are sometimes of kinds which do not 

 occur as bedrock within many miles. Some of them are 

 subangular in form and have flat faces, often highly polished 

 and covered with minute scratches (Fig. 231). The drift 



