THE PLEISTOCENE OR GLACIAL PERIOD. 405 



of post-glacial submergence are reported at still greater heights in 

 Labrador. ~~~ 



The Loess. 



The term loess is used with not a little latitude, both as a text- 

 ural and a formational name. Lithologically, loess is a variety of 

 silt intermediate between the finest sand and clay. In general, it is 

 free from stones of all sorts, except the concretions which have been 

 developed in it since its deposition. In the exceptional cases where 

 stones occur in it, they are confined to its extreme basal portion. \ At 

 its base, too, it is sometimes interst ratified with sand, especially where 

 it is thick. 



The composition 1 of the loess is significant in that it contains 

 angular, undecomposed particles of the commoner carbonates, calcite 

 and dolomite, and silicates, such as the feldspars, the amphiboles, 

 the micas, etc. Even the rarer silicates, such as epidote, apatite, 

 tourmaline, zircon, etc., have been identified. Magnetite also is a 

 common, though never an abundant, constituent. These constituents 

 strongly suggest that the material of the loess was derived from the 

 flour of the glacial mill. In color it is predominantly buffish brown, 

 but in not a few places it has a bluish cast a few feet below the surface. 



By virtue of its peculiar mode of adhesion and of its porosity, the 

 loess often stands with vertical faces (Fig. 523) for long periods, where 

 sand or clay would be degraded into slopes. Roads on the loess tend 

 to assume the form of miniature box canyons, because the loess of 

 the road-bed is washed or blown away, while that on either side stands 

 up with steep or even vertical slopes. Its porosity seems to be due 

 in part to the size, shape, and arrangement of its grains, and in part 

 to vertical tubelets that usually affect it, and which are supposed 

 to have been caused by rootlets. Weathered faces of the loess often 

 show a rude columnar structure (Fig. 524), the columns being one 

 to several feet in diameter. The loess often shows no stratification, 

 but in its coarser phases there is often some suggestion of such structure, 

 and when the loess proper is inter bedded with sand, this suggestion 

 becomes distinct. 



The best known portions of the loess in America and Europe are 

 associated with glacial formations, though the loess extends far beyond 

 1 Sixth Ann. Rept. U. S. Geol. Surv., pp. 244 et seq. 



