THE PLEISTOCENE OR GLACIAL PERIOD 407 



the borders of the drift in some directions, in both continents, and in 

 America it occurs in the driftless area (Fig. 470). In Turkestan, Mon- 

 golia, and China, 1 where loess has its greatest known development, it 

 is not known to be immediately associated with glacial formations, 

 though its age is probably about the same as that of the chief deposits 

 in Europe and America. 



In North America, the loess does not occur east of the Mississippi 

 basin, and has no great development east of the Wabash river, ' The 

 greatest development is in Illinois, Iowa, Nebraska, and the States 

 lying south of them, even beyond the reach of the most extensive ice- 

 sheet. Within this area, its distribution is peculiar in that (1) it is 

 thick about the border of the area occupied bj' the Iowan ice-sheet; 

 (2) it thins out on the inter-stream areas as it is traced away from 

 this border tract; while (3) it retains its thickness along the valleys, 

 especially the larger ones, but thins gradually from them. Especially 

 does it follow the main streams that lead away from the Iowan drift- 

 sheet. It follows the Mississippi nearly to the Gulf, and is especially 

 thick along this stream and the Missouri. Its habit is to occupy the 

 bluffs immediately overlooking the valleys, and it was formerly known 

 as the Bluff formation on this account. In this position,. it has 

 more than its average thickness and coarseness of grain, and grows 

 thinner and finer in grain back from the river bluffs until.it is lost in 

 a vanishing edge, while its material, at the same time, loses its dis- 

 tinctive characteristics. 



In the regions next south of the borders of the Iowan and Wis- 

 consin drift-sheets, it mantles the divides between the main streams, 

 but farther south it is more confined to the valley borders. Within 

 the general area of its occurrence it has little regard for topography. 

 It can indeed hardly be said to have an upper limit. This indepen- 

 dence of topography is one of its significant features. Within the 

 drift-covered part of the Mississippi basin, the loess occurs (1) as a 

 mantle overlying the drift (Fig. 525), and (2) between sheets of drift. 

 Its relations to the drift-sheets make it clear that it was accumu- 

 lated at several different stages of the glacial period, but within the 

 glaciated area the accumulation at one of these stages far exceeds 



1 Yon Richthofen, China. This author early (1877) advocated the eolian ongin 

 of the loess of China, but this explanation has not passed unchallenged. See Skertchley 

 and Kingsmill, Q. J. G. S., Vol.LI, 1895, pp. 238-254. 



