GRASSES OF IOWA. 373 



Toronto beds and arc forced to rely on superposition, the slight topo- 

 graphic difference, and the moraine which generally borders the Wiscon- 

 sin, to discriminate between it and the Iowan. The general character of 

 the country covered by the Wisconsin has already been noted. 



SOILS. 



From the foregoing it is evident that the soils of the state are only 

 indirectly related to the underlying rock series. While the bulk of the 

 drift material is derived from the neighboring strata, there is much for- 

 eign material incorporated as well. The glaciers gathered up the loose 

 material or ground off the harder rocks over a considerable area to the 

 north and dumped a generous share of this heterogeneous mixture within 

 the borders of Iowa. An examination of a bit of the material shows a 

 great variety in the particles. They are derived from nearly all the va- 

 rieties of rock. Some are merely ground up and hence are only potential 

 plant food. Others have been oxidized or are otherwise acted upon by 

 the agencies which prepare the rock for the plant. Mixed with this ma- 

 terial are bits of wood and vegetal matter which, by developing into 

 humus, aid in enriching the soil. 



There do not seem to be many wide and constant differences in drift 

 soils. As a rule they are more sandy than the usual loam. Their chief 

 characteristic is their variability with regard to moisture content. Their 

 heterogeneous texture leads to all conditions from those of drouth to 

 drowning. With careful cultivation, however, this can be very largely 

 controlled. The drift soils prevail throughout the areas of the Wiscon- 

 sin and the Iowan, though often buried beneath a few inches of fine 

 homogeneous material, probably very largely wind blown. 



Much the larger portion of the state has a loess soil. The loess, 

 while composed of the same material as the 'drift, is very fine and is re- 

 markably homogeneous in texture. This gives it, except in gumbo re- 

 gions, the valuable characteristic of an e* r en moisture content. It is in 

 almost every instance abundantly supp . with all the elements of plant 

 food, and, with its ability to absorb and retain moisture, it is among the 

 best of soils. Curiously enough in areas of very thick loess it is not so 

 good as where spread out as a thin sheet over the drift. To some extent 

 this seems to be a matter of the topographic development of its surface, 

 but it is not certain but that there are other factors in the case. 



A third common soil type in the state is the Alluvium which oc- 

 curs so commonly along the streams, particularly in the region of the 

 older drift. Since, however, it differs in no way from Alluvium as de- 

 veloped elsewhere it need not be described. 



