SSS GEOLOGY 



and oxidation, 1 in some places. Loess occurs in isolated spots even 

 as far west as Washington and Oregon. 2 



Age. The relations of the loess to the several drift-sheets make 

 it clear that it was accumulated at different stages of the glacial 

 period, but within the glaciated area the accumulation at one of 

 these stages exceeds that at all others, both in volume and a real 

 extent. The loess deposited at this stage is often referred to as 

 "the loess/' and is usually correlated in time with the lowan drift. 

 It is at least later than the Illinois sheet of drift which it mantles, 

 and earlier than the Wisconsin drift which overlies it. Locally, a 

 thin mantle of loess overlies the Wisconsin drift, even its later 

 parts. 3 No considerable body of loess older than the Illinois drift 

 is known. 



Thickness. The loess of the Mississippi basin rarely attains a 

 thickness of more than a score or two of feet, and this only along the 

 main valleys; but exceptionally its thickness approaches 100 feet. 

 Thicknesses of 10 feet are much more common than greater ones. 



Accessories. The loess contains characteristic accessories of 

 two kindc, namely, concretions and fossils. The concretions are 

 of lime carbonate and iron oxide. Many of the former are irregular 

 and of such shapes as to have been called "petrified potatoes"; 

 but many of them have other shapes. The ferruginous concre- 

 tions take various forms, one of which is the "pipe stem," perhaps 

 formed about rootlets. The fossils are chiefly gastropods (Fig. 5 ( . )'J ) , 

 almost exclusively of land species, or of such as frequent isolated 

 ponds. 4 There is, however, a lowland silt formation classed by 

 some as loess, in which fresh-water fossils are found. The other 

 fossils are bones and teeth of land mammals. 



Origin. There has been much diversity of opinion as to the 

 origin of the loess, the fundamental question being whether ii is 

 aqueous or eolian. There is little doubt that the loess-like silts 

 which occur in the terraces of rivers are of fluvial origin; but some 



1 Report on Crowley's Ridge, Ark. Geol. Surv., pp. 224-235. 



2 Jour. Geol., Vol. IX, p. 730. 



3 Jour. Geol., Vol. IV, pp. 929-937. 



4 Shimek, Geol. Surv. of la., Vol. IV, pp. 929-937, and Loess Papers, 

 Bull. Labr. Nat. Hist., Univ. Iowa, 1904. 



