88 
IOWA ACADEMY OP SCIENCES. 
only of loess-like soil is found. Qaite important too is the argu- 
ment furnished by the physical properties of the loess mate- 
rial. This in eastern Iowa is always very easily eroded, so 
much so that upon cleared hillsides it is often impossible even 
for bluegrass to gain a foothold, and failure has been the uni- 
versal result of all attempts to cultivate such slopes. This 
being the case it seems hardly probable that trees, which 
require more time to become established than do smaller 
plants, could have gained a foothold upon these unstable hill- 
tops had they been formed. The organic matter which 
undoubtedly accumulated in these forests gradually decayed, 
mingled with the alluvium brought by the winds, and was 
finaliy consum.ed in leaching iron oxides from the lower strata 
of deposit. 
Other, smaller, vegetation no doubt effected the deposition 
of fine alluvium in the same manner, but to a lesser degree, and 
by the aid of this probably were formed the thin layers of loess 
which sometimes occur in prairie country. 
The element of time still remains to be considered. Without 
an attempt at exact computations, attention is simply called to 
the fact that in eastern Iowa the loess in no place exceeds fifty 
feet in thickness, the average being probably about tenor twelve 
feet, and that if we assume, for example, the deposition of a 
minimum of one mm. a year, the time required for the forma- 
tion of the entire deposit would not be unreasonably great. 
The deposition of loess material is no doubt going on in this 
manner to-day, and the investigation of this phase of the sub- 
ject is worthy the attention of the most careful observers. The 
foregoing statements apply particularly to the loess of east- 
ern Iowa. In the western part of the state and in eastern 
Nebraska much thicker deposits occur, which differ in many 
respects from the loess of eastern Iowa. 
The western loess is thicker, coarser, with more siliceous 
material, and the writer has found it more frequently inter- 
laminated with sand. That it is much less easily eroded because 
of this difference in composition is a well known fact. 
From the general topographical and climatic relations which 
exist between the eastern and western regions to day, it is prob- 
able that during the loess period, as now, the western region 
was drier (a fact also attested by the rather greater abundance 
of dry- region molluscs in its loess), and that strong winds were 
16 A further investigation of the soils in prairie groves of this kind is contemplated 
during the coming summer. 
