644 



THE PLEISTOCENE PERIOD 



longed weathering and oxidation, in some places. Loess occurs in 

 isolated spots even as far west as Washington and Oregon. 



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

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



no p q 



Fig. 534. Loess Shells, a-b, Zonitoides minusculus (Binney); c-d, Euconulus 

 fuhus (Drap.); e-f, Strobilops labyrinthica (Say); g, Polygyra clausa (Say); h, P. 

 multilineata (Say); i-j, Sticcinea obliqua Say; k, S. avara Say; l-m, Poly gyro, monodon 

 (Rack); n, Bifidaria pentodon (Say); o, B. corticaria (Say); p, B. muscorum (Linn.); 

 q, B. armifcra (Say). The small figures adjacent to some of the large ones show the 

 natural size of the shells. 



period, but within the glaciated area most of it is younger than 

 the Illinoian sheet of drift which it mantles, and older than the 

 Wisconian drift which overlies it. Locally, loess covers Wisconsin 

 drift in a few places. No considerable body of loess older than 

 the Illinois drift has been identified with certainty. 



