54 NATUKE AND PEOPERTIES OP SOILS 



This drift is often merely ground-up rock, at other times 

 the original soil is mixed with foreign detritus, while again 

 the variable mixtures may be wholly reworked and consid- 

 erably stratified. Besides this the streams of water, which 

 issue from under the ice, may be instrumental in distribut- 

 ing sediments for miles beyond the ice front. Glacial lakes, 

 when in existence for sufficiently long periods, furnish basins 

 for the deposition of materials derived from the erosive and 

 grinding influence of the ice. The ice may also provide a 

 large amount of detritus so fine as to be susceptible to wind 

 movement, and thus seolian influences as well as alluvial and 

 lacustrine may be concomitant to a great ice invasion. 



During the Pleistocene northern North America, as well 

 as part of Europe, was successively invaded by ice sheets, 

 which exerted the influences above described and, while the 

 central ice caps in Canada probably never wholly disap- 

 peared, the regions to the southward certainly experienced 

 alternate glaciation and interglaciation. At least five in- 

 vasions are evident in eenti^al United States. Debris from the 

 last, called the Wisconsin, now covers wide areas. The in- 

 terglacial periods are shown by forest beds, accumulations 

 of organic matter, and evidences of erosion between the drift 

 deposited by the successive ice sheets. Some of the inter- 

 glacial periods evidently were times of warm, and even semi- 

 tropical, climate. Just what was the exact cause of the ice 

 age is still under dispute.^ That it was due to a change in 

 the carbon dioxide content of the air seems as probable as 

 any of the numerous hypotheses that ha\e been advanced. 



The area covered by glaciers in North America is estimated 

 as 4,000,000 square miles, while at least 20 per cent, of the 

 United States is either directly or indirectly influenced by the 

 debris. The greatest southward extension of the ice is marked 



* Hnmplireys, W. J., Factors of Climatic Control, Jour. Franklin Inst.. 

 Vol. 189, No. 1, pp. 63-98, Jan., 1920. 



