GLACIAL SOILS 55 



forms. It advanced first through the valleys, and at the 

 bottom of the mass appears to have been guided in 

 its flow by these channels. The advance probably 

 consumed a long period of years, or even centuries, 

 and the retreat was similarly slow. Along the margin, 

 as in modern glaciers, there were annual fluctuations 

 in the position of the ice front which are indicated 

 by the greater or less accumulation of rock debris, as 

 undulating piles of earth or terminal moraines. This 

 ice picked up immense amounts of material along its 

 way. Most of the original soil overlying the rocks was 

 swept away. Prominences were torn away or planed 

 down, and depressions were filled up. Masses of rock 

 were ground to powder, and boulders were transported 

 to entirely new surroundings. The advance of the ice 

 over the country largely disregarded the rock forma- 

 tions, as it did topographic forms, so that the rocks 

 and soil materials from many sources were mixed and 

 ground together. In this way, the granite boulders 

 strewn over the surface near the southern margin of 

 the ice extension in the United States were derived 

 from points hundreds of miles to the northward, even 

 into northern Canada. The movement was not straight 

 south, but deflected by broad obstructions in the land, 

 so that the source of the soil in any region is determined 

 by the direction of movement in that section. This 

 movement may often be traced by the kind of rocks 

 which have been left, and may lead back to the ledges 

 from which they were derived. 



The relation of glacial soils to the underlying rock 

 depends entirely on the conditions which prevailed 



