DRIFT 267 



But in the northern states the mantle is usually quite 

 different from the bed rock under it. Thus, we may find 

 a mantle of sandy soil directly over limestone, and clay 

 over sandstone. Moreover, the boulders (and there are 

 often many of them) are of materials that do not exist in 

 the bed rock of the region at all, but must have been 

 transported from a great distance. All mantle rock that 

 has been brought to a region, and not formed where we 

 find it, is called drift. 



The agents that have brought drift, like the agents that produce 

 weathering, are Nature's "common workmen": chiefly air, water, and 

 ice. We have already learned that the wind can shift a great sand 

 bank (cf. 290), and that running water will carry away material from 

 one place, and then, when its velocity is checked, will deposit the 

 material at another (cf. 287 and 290). We do not need to study 

 rivers to learn how running water transports and deposits materials; 

 we can see its action after any rain. On every slope the rills that pour 

 down wear away the earth, and form grooves, or channels. The 

 transported pebbles, mud, etc., are deposited at the base of the slope. 

 A deposit of this sort, whether brought by a rill or by a great river, usu- 

 ally takes the shape of the Greek letter A (our D), and is called a 

 delta. The "Gulf Region" of this country is largely the delta of the 

 Mississippi. 



But the peculiar drift of the northern states (a similar 

 drift is found in Europe) was not brought by air or by 

 water; we have abundant evidence that it was brought to 

 its present position by a great sheet of ice, which covered 

 the land for a time as the interior ice sheet now covers 

 Greenland. The ice brought fragments of igneous rock 

 from Canada, and left them in the United States. It 

 pushed before it great masses of clay filled with stones and 

 boulders, called "boulder clay," or "till." It carried 

 other boulders on its back. By means of great stones 



