THE QUATERNARY PERIOD 341 



sometimes having risen like great fountains because of pressure. 

 Such deposits are now actually in process of formation along the 

 edge of the great Malaspina glacier of Alaska. 



During the ice retreat glacial lakes were numerous, particularly 

 where the north-sloping valleys were dammed by the ice thus 

 ponding the waters in the valleys. Some materials were directly 

 deposited from the glacier in those lakes, but more was brought 

 in by debris-laden streams flowing from the land already freed 

 from the ice. Such glacial lakes and their deposits were common 

 and of unusual interest, but they will be described under a sub- 

 sequent heading. 



In conclusion we may say that the deposition of glacial mate- 

 rials, like glacial erosion, has not changed the major topographic 

 features of the glaciated region. The general tendency of ice 

 deposits has been to fill, or partially fill, depressions and thus to 

 diminish the ruggedness of the topography. 



The Loess Deposits 



Loess deposits are widespread over much of the region from 

 eastern Nebraska, across Iowa, Illinois, and Indiana. Its distribu- 

 tion is pretty largely independent of topography. Typically it is 

 a soft, buff to yellowish-brown, very fine grained, sandy clay which 

 seldom shows signs of stratification. Its thickness usually varies 

 from 10 to 100 feet. Where eroded or cut into, the loess exhibits 

 a remarkable tendency to stand in perpendicular cliffs, sometimes 

 with suggestions of a sort of columnar structure. For this reason 

 it was once known as the Bluff formation. It is remarkably free 

 from coarse materials, except for certain carbonate of lime and 

 oxide of iron concretions and fossils, the latter being chiefly shells 

 of land Gastropods. Most of the loess was deposited during the 

 Iowan Glacial stage, because it rests upon the eroded and weath- 

 ered surfaces of older glacial deposits and often passes under the 

 later or Wisconsin deposits. 



The question as to whether the loess was of aqueous or eolian 

 origin has long been discussed. "In part the loess seems to have 

 been washed from glacial waste and spread in sluggish glacial 

 waters, and in part to have been distributed by the wind from 

 plains of aggrading glacial streams" (W. H. Norton). 



