70 
IOWA ACADEMY OP SCIENCES. 
The yellow boulder- clay — stratum number 3 — has boulders 
scattered throughout its thickness, but, as a rule, on slopes and 
near the tops of hills these are much more abundant in the 
upper part, immediately under strata 1 and 2. 
This is strikingly shown in some of the cuts and exposures 
at Forest City, Spirit Lake and Granite. It appears as though 
this stratum had at sometime been much thicker upon the hills, 
forming their barren surface immediately after the recession of 
the glacial sheet. By the action of winds and water the finer 
material at the surface was sifted out and carried away before 
the hills were covered with vegetation, the heavier boulders 
being but little disturbed, excepting as they were undermined. 
As a result the hills were cut down and the boulders were 
brought closer together at the surface. Their accumulation 
retarded the surface disturbances and the vegetation peculiar 
to barren grounds was enabled to gain a foothold. Finer 
material, brought hither by the winds, ^ was retained by this 
vegetation and a new surface soil was formed — the stratum 
number 2 — of which a vegetation more abundant then took pos- 
session. This retained still finer material, mingling with it its 
own decomposed substance, and the present surface soil — 
stratum number 1 — was gradually formed. It may here be 
noted that the finest part of the material from stratum number 
3 seems to be in all respects like our loess. 
The conditions which probably prevailed before the forma- 
tion of strata 1 and 2 are still illustrated by comparatively 
barren prairie hills west of Forest City and in the vicinity of 
Granite, where stratum number 3, or mere indications of num- 
ber 2, form the surface, whose vegetation, as incidentally noted 
in the following paper, is quite different from that of the more 
fertile surrounding prairie. 
The occurrence of the scrub bur oak groves on some of 
these hills is interesting. The plants are chiefly shrubs, sel- 
dom more than five feet in height, and usually not closely 
crowded, and they seem to prosper best on the leeward side of 
the hills and in ravines. 
This is strikingly shown near Granite. The observer may 
stand on one of the hills west of Granite, and looking to the 
* Even such small pebbles as those which occur in stratum number 3 could be rolled 
a considerable distance by winds. The author saw, last spring, an accumulation of 
sand on a hill in the southern part of West Cedar Rapids which completely covered a 
fence fully five feet high. In the deposit were small pebbles, yet the wind had clearly 
formed the stratum full five feet in thickness since the fence had been built. When 
the workmen were removing a portion of the deposit for the purpose of opening a road 
it was observed (by the author) that the sand was quite regularly stratified, the num- 
erous lines following the surface configuration. 
