THE PLEISTOCENE OR GLACIAL PERIOD 887 



silt of the road-bed is washed or blown away, while that on either 

 side stands up with steep or even vertical slopes. Weathered faces 

 of the loess often show a rude columnar structure (Fig. 591), the 

 columns being one to several feet in diameter. The loess, as a rule, 

 shows no" stratification, but in its coarser phases there is often some 

 suggestion of such structure, and where interbedded with sand, 

 stratification is sometimes distinct. 



Distribution. The best known loess in America and Europe is 

 associated with glacial formations, though the loess extends far be- 

 yond the borders of the drift in some directions, in both continents. 

 In China and other lands of Asia, 1 where loess has its greatest 

 known development, it is not generally associated with glacial 

 formations. 



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

 basin, and has no great development east of the Wabash River. It 

 is wide-spread in Illinois and the states along the Missouri, and in 

 the states along the Mississippi farther south. Within this area, 

 its distribution is peculiar in that it follows the main streams that 

 led away from the lowan drift-sheet, and is found especially on the 

 bluffs overlooking the valleys. On this account it was formerly 

 known as the Bluff formation. In this bluff-position, it has more 

 than its average thickness and coarseness of gram, and grows thinner 

 and finer in grain back from the river bluffs until it is lost in a van- 

 ishing edge. At the same time, its material loses its distinctive 

 characteristics. 



Just south of the borders of the lowan and Wisconsin drift-sheets, 

 it mantles many of the divides between the main streams; but 

 farther south it is more confined to the valley borders. It has little 

 regard for topography, and can hardly be said to have an upper 

 limit. Within the the drift-covered part of the Mississippi basin, 

 it occurs (1) as a surface mantle overlying drift, and (2) between 

 sheets of drift. South of the drift there are in places distinct sheets 

 of loess, sometimes separated by a well developed soil zone. The 

 surface of the lower sheet shows the effects of prolonged weathering 



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

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

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

 Researches in China, Vol. I, Carnegie Institution. 



