THE PLEISTOCENE OR GLACIAL PERIOD. 409 



On portions of the Great Plains, and in some of the basins of the 

 Western mountain regions, there are deposits called loess, some of 

 which are closely similar to the loess of the drift region, while others 

 are quite different. But there is nowhere a development at all com- 

 parable to that on the borders of the plateaus of Asia, particularly in 

 China. In Washington and Oregon, material which in its general 

 character is quite similar to the loess of the Mississippi basin is 

 widespread. 1 



The loess of the Mississippi basin rarely attains a thickness of more 

 than a score or two of feet, and this only along main streams; but 

 exceptionally its thickness approaches 100 feet. Thicknesses of 10 

 feet are much more common than greater ones. 



The loess contains characteristic accessories of two classes', namely, 

 concretions and fossils. The concretions are of lime carbonate and 

 iron oxide. The former are often irregular and of such shapes as to 

 have received the appellation of " petrified potatoes." Concretions 

 of the sort to which this name is applied are usually though not always 

 hollow. The concretions of lime carbonate are often of other shapes, 

 for example, cylindrical. The ferruginous concretions take various 

 forms, one of which is the " pipe stem/' perhaps formed about rootlets. 



The fossils of the loess are chiefly gastropods (Fig. 526). They were 

 originally reported to include both terrestrial and aquatic forms, and 

 this has much influenced opinion with reference to the origin of the for- 

 mation. According to Shimek, however, the shells in the upland 

 loess are almost exclusively those of land species, or such as frequent 

 isolated ponds. 2 He finds a practical absence of those that frequent 

 rivers and lakes. There is, however, a lowland silt formation, classed 

 by some as loess, called by others loess loam, in which fresh-water 

 fossils are found. The other fossils are bones and teeth of land mam- 

 mals. 



Origin. — The origin of the loess has long been a standing puzzle, 

 and opinion is still divided between an aqueous and an eolian origin, 

 with a growing tendency toward the latter. Some geologists divide 

 the honors between the two hypotheses. There is little doubt that 

 the loess-like silt deposits which occur in the terraces of rivers are 



x Jour. of Geol., Vol. IX, p. 730. 



2 Ibid., Vol. IV, pp. 929-937, and Loess Papers, Bull. Labr. Nat. Hist., Univ. 

 Iowa, 1904. 



