52 
IOWA ACADEMY OF SCIENCES. 
The geest, the weathered coal measure surface of preglacial 
times, forms no important part of the soil. It is completely 
obscured by drift and loess, excepting where exposed by 
erosion. 
The strata underlying the drift in the eastern part of the 
county belongs to the Des Moines stage of the coal measures. 
The strata underlying the drift in the western part of the 
county belongs to the Missourian stage of the coal measures. 
The dividing line between the surface outcrops of these stages 
may be traced as an irregular line across Madison township, 
the northeastern part of Jefferson, the central part of Douglas, 
Union and Lincoln townships, the eastern part of Scott and the 
central part of Walnut townships. (See figure 1.) The general 
surface of the limestone to the west of this line is higher than 
the surface of the shales east. This difference in elevation, 
together with the presence of preglacial valleys along the 
eastern margin of the Missouri limestone except in the divide 
just south of Patterson, make the limestone form an escarp- 
ment across the county. 
East of the dividing line the strata are generally clayey or 
sandy shale, but there are outcrops of a layer of limestone from 
one and a half to two and a half feet thick, especially important 
in the neighborhood of Truro, hence here called the Truro lime- 
stone. It outcrops along South river at an altitude of seventy 
feet above the river bed, and on both sides of Clanton creek 
valley. It outcrops along the hillsides in Crawford township, 
and appears near the crests of divides between Lee and 
Jefferson townships. Its distance below the base of the 
Winterset limestone is eighty feet. 
While in the Des Moines shales, unconformity is common, 
and in the sandy shales south of Patterson ripple-marks are to 
be found only forty feet below the Winterset limestone, there is 
no unconformity whatever between the base of the Winterset 
limestone and the uppermost Des Moines shales. This gives evi- 
dence that, j ust prior to the time when the Winterset lim estone was 
deposited in the county, the shore line was farther inland (east 
or northeast) of the present limits of the limestone, and, with 
the gradation from sandy shales with ripple-marks, through 
clayey shales to Winterset limestone, sustains the conclusion 
previously advanced by Keyes that the Missourian limestone 
was formed in an advancing sea. 
