CHAPTER IV 



GEOLOGICAL CLASSIFICATION OF SOILS 



(CONTINUED) 



Ice in the form of glaciers has been, as already stated, 

 a very great factor in soil formation, especially in the 

 north temperate zones of North America, Europe, and 

 Asia. Not only was the old mantle of material swept 

 from the land by the advance of the ice, but a new soil 

 was laid down as drift material. This drift was some- 

 times merely ground-up rock, sometimes rock flour 

 mixed with the original residual soil, and sometimes 

 glacial material wholly reworked and considerably strati- 

 fied by water. Besides this, the streams of water that 

 issued from under the glaciers were instrumental in 

 many cases in distributing sediments a considerable 

 number of miles southward of the ice front. Glacial 

 lakes, also, when in existence for sufficiently long periods, 

 furnished basins for the distribution and deposition of 

 materials derived from the erosive and grinding power 

 of the ice. The ice also furnished a large amount of 

 very fine detritus, which was susceptible to wind move- 

 ment. This material, reworked and deposited as bars 

 in stream beds, was carried many miles by the prevailing 

 westerly winds during a period of aridity following the 

 glacial epoch. It now exists over wide areas, especially 

 in the Middle West of the United States and in northern 

 China, in which places it reaches its best development. 



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