68 SCIENCE AND SOIL 



tier was practically equaled by the rapidity with which the ice 

 melted at the terminus, and sometimes over broad inter-morainal 

 tracts, where the melting proceeded more rapidly. Many preglacial 

 valleys were filled with the drift (in places 300 feet deep), and com- 

 monly glacial drift was deposited over the general level of the 

 glaciated area to a depth of 10 to 100 feet. 



We are especially interested in four of the important ice sheets 

 that occurred during the glacial epoch. Where the drift from the 

 first of these was not covered by a subsequent glacier, the area is 

 termed the Kansan glaciation; where the drift from the second gla- 

 cier was not covered by a subsequent glacier, the area is termed the 

 Illinoisan glaciation; where the drift from the third glacier was 

 not covered by a subsequent glacier, the area is called the lowan 

 glaciation; and the area covered by the drift from the fourth glacier 

 is termed the Wisconsin glaciation, where not covered by a subse- 

 quent glacier. As will be seen from the glacial map, these respective 

 areas are not confined to the states named. 



It should be understood that, notwithstanding the extensive 

 glaciated regions, glacial soils are not common in the older glaciated 

 areas. The most common soil material between the Alleghanies 

 and the Rocky Mountains, and between the Great Lakes and the 

 Gulf, is loess. 



Loess is a very fine material 1 consisting of grains of quartz, 

 felspar, mica, hornblende, and other granitic or silicate minerals, 

 with more or less limestone, dolomite, magnetite, pyrite, etc., 

 and some clay. Loess has been derived in large part from glacial 

 drift, having been transported by the action of wind and flowing 

 water, probably from deposits of exposed till before it was protected 

 by vegetation (and to some extent from the melting or evapo- 

 rating glaciers), and deposited over all other soil formations and 

 over older glaciated areas. Many of the residual soils in the drift- 

 less, or unglaciated, areas in the Mississippi Basin are now covered 

 with loess. Even the tops of the Ozark Hills of southern Illinois, 

 beyond the most southern point of the glacial lobe, and high above 



1 In 'some places the loess is more or less mixed with the underlying residual 

 or glacial materials, through the action of crawfish, burrowing animals, etc., and 

 occasionally loess deposits are subsequently covered by mixed alluvial materials, 

 which may include sand and gravel with silt, clay, and organic matter. 



