STRUCTURE OF FORT DODGE BEDS 
115 
shale, with an interbedded sandstone, which is not a constant 
feature, however. The gypsum ranges in thickness up to a maxi- 
mum of thirty feet and the overlying shales and sandstone reach 
a thickness of fifty feet. The conglomerate is not over two 
feet thick. 
The remarkable feature about the Fort Dodge beds, however, 
is their relationship to the beds which immediately underlie them. 
As has been stated they are not conformable with these under- 
lying strata. The evidence seems to show convincingly that the 
Fort Dodge beds were deposited in a basin of erosion cut in the 
strata of Des Moines and Mississippian age, which form the bed 
rock of this general region. In most places the gypsum lies on 
shales of the Des Moines series and is well above the level of 
Des Moines river, which may be taken as a convenient datum. 
The sections given in Plates I and II will show this to be the case 
at the exposures, and a few well records will show similar condi- 
tions elsewhere. Thus, at the County Farm the elevation is 1140 
feet and the base of the gypsum is a hundred feet lower, overlying 
Des Moines series shales. The elevation of water in the river at 
Fort Dodge is about 980 feet above sea level. A mile east of 
Fort Dodge the elevation is about 1115 feet and the gypsum lies 
seventy to seventy-seven feet lower or about 1030 to 1040 feet. 
In the southern part of section 12, Cooper township, about four 
miles northeast of Fort Dodge, the surface elevation is 1120 feet 
and gypsum is present at a depth of eighty feet or 1040 feet above 
sea level. In all cases where the gypsum has been penetrated 
Coal Measure shales have been found just beneath it. 
On the other hand, in some places the erosion of the aforesaid 
basin proceeded far enough to cut through the beds of Des 
Moines age and to reach the underlying limestones, marls and 
sandy beds of Mississippian age — -the Ste. Genevieve and St. 
Louis strata. The surface of the Mississippian rocks on which the 
Des Moines beds were laid was very irregular and this fact ac- 
counts for some of the stratigraphic peculiarities which are to 
be seen in the gypsum area. Des Moines river flows across the 
region in a southeasterly direction and in the northern part has 
exposed the Mississippian strata. Elsewhere the Des Moines 
beds probably underlie the river deposits, except at a few points 
where the Mississippian beds extend up far enough to be un- 
covered. Soldier creek flows southwest and joins the river from 
the east in the northern part of Fort Dodge. In the lower reaches 
