THE 



JOURNAL OF GEOLOGY 



NOVEMBER-DECEMBER, igoj 



THE AGE AND ORIGIN OF THE GYPSUM OF 

 CENTRAL IOWA. 



The known gypsum of Iowa is confined to Webster county, 

 which lies somewhat north and west of the center of the state. 

 Here it occupies a tract about six miles wide and fifteen miles 

 long, on the edge of which the city of Fort Dodge is located. 

 The original gypsum area has been greatly reduced by the erosive 

 and solvent action of the Des Moines River, which crosses it 

 nearly at right angles, cutting completely through it and deep 

 into the Coal Measures beneath. Some of the earliest geological 

 work undertaken in the state was carried on in this region. The 

 Des Moines River exposes along its banks the indurated rock 

 and gives at times sections of rock and drift 200 feet in thick- 

 ness, which greatly facilitate geological study. The unique 

 deposit of gypsum has been attractive to the student on account 

 of the theoretical problems that it presents, and to others it is 

 interesting on account of its practical value. 



In the year 1849 Owen^ made a hurried trip up to the Des 

 Moines River, noticed the gypsum, and made certain deductions 

 in regard to its origin. After describing its remarkable thick- 

 ness and purity, he adds : 



This plaster stone of the Des Moines does not appear to have been 

 deposited in nests or conical heaps, as in the shales of the Onondago Salt 

 Group of New York, but rather in continual horizontal beds, conformable to 

 the underlying shale. The immense quantity of gypsum of this part of the 



' Geology of IViscofisin, Iowa, and Minnesota, p. 126 (Philadelphia, 1852). 

 Vol. XI, No. 8. 723 



