IOWA ACADEMY OF SCIENCES. 89 



the same or approximately the same horizon over a wide stretch 

 of country, the legitimate inference would be an extpnsive 

 retreat and readvance of the ice. In the nature of things, how- 

 ever, it is impossible often, if ever, to apply this test alone, and 

 in general forest beds, except where they show tropical or 

 temperate floras, have little independent value. 



Buried Soils. — This term is intended here to cover only the 

 black soil proper; the clay mixed with humus. Under ordinary 

 circum stances this is not deep, and in general over the Wiscon- 

 sin drift it is about 8 to 12 inches in thickness. Since the soil 

 comes from the successive growth and decay of vegetable mat- 

 ter, and since but a small fraction of the latter is usually pre- 

 served, a soil calls into consideration an important time factor. 

 It is true there are instances of rapidly formed soils but such 

 may in most cases be discriminated. The peat represents the 

 forms of relatively rapid vegetal accumulations and six inches 

 of true soil means a much longer time period than the same 

 thickness of peat. It is important, however, to carefully dis- 

 criminate true soils from apparent soils found by the washing 

 in of soil material. At Port Dodge the Des Moines valley is 

 apparently post- Wisconsin; yet, in some drift exposed down in 

 the valley, is a soil as deep, and as well marked as that over 

 the upland drift. The explanation seems to be, not that the 

 lower drift is older and separated from the Wisconsin by an 

 interval as long as post- Wisconsin time, but that the exposure 

 represents a buried terrace, and that the soil was not altogether 

 developed in situ. When, however, the soil is clearly devel- 

 oped in situ, it has a considerable significance. In such cases it 

 will, with certain rare exceptions, be accompanied by concord- 

 ant phenomena and occasionally the latter afford the only 

 means by which its genuineness may be proven. 



Leached Horizons. — That the drift contains a large amount of 

 mechanically pulverized material has been abundantly shown. 

 In Iowa one of the most abundant materials is pulverized lime- 

 stone, and it is pertinent to remark that this material is quite 

 abundant even in the material covering the Des Moines forma- 

 tion, our most important series of beds relatively free from 

 limestone. One of the first processes becoming active in the 

 formation of a soil is that of leaching. The soluble materials 

 begin at once to go into solution and drain out of the upper 

 portion of the soil. As a result acid finds little to dissolve in 

 old soils and much in new drift soils. In the process of time 



