110 IOWA ACADEMY OF SCIENCE 



discernible. This slight fault has a throw of about six feet. 

 It is suggestive of the possibility of the greater displacement 

 being of the distributive order instead of being a single simple 

 break in the stratigraphic continuity. 



At the old stone wagon bridge over Soldier creek, one-half 

 mile above the lower viaduct, and immediately north of the Rock 

 Island railway station, there is a singular physiographic sug- 

 gestion of notable faulting. For a distance of several miles be- 

 fore reaching this bridge the creek flows in a deep, narrow gorge. 

 At the point where the bridge spans the waterway the latter cuts 

 sharply into the hard St. Louis limestone, so as to form a small 

 canyon thirty feet wide and twenty feet in depth. The abut- 

 ments of the bridge are the two walls of the canyon. Less than 

 one hundred yards below the bridge the limestone, although 

 standing thirty feet above the creek bed, abruptly disappears. 

 The Soldier gorge opens out into a 'broad, flat-bottomed amphi- 

 theatre a thousand feet wide and half a mile long, the flat form- 

 ing an area sufficiently ample for utilization by the railroad 

 for its local yard purposes. The amphitheatre is excavated en- 

 tirely in the friable sandy shales which overlie the gypsum. No 

 sign of the St. Lonis limestone is to be seen save the point on the 

 north side where the creek debouches from its canyon. 



On the west side of the Des Moines river, opposite the mouth 

 of Soldier creek, new and important data of an exact kind are 

 now available bearing upon the points in question. The exten- 

 sive excavations of the Fort Dodge Clay Works, the construc- 

 tion of the Omaha extension of the Chicago Great Western Rail- 

 way, the drilling of numerous deep wells, and the opening up 

 to inspection of many other sections, disclose a number of in- 

 structive facts which supply the long missing links in the solu- 

 tion of the gypsum puzzle. On this side of the river the gypsum 

 plate retains the same gentle slope to the northward, as it does 

 on the east side of the stream. 



It is shown by drill-holes and by excavations that the gypsum 

 bed, fifteen to twenty feet in thickness, lies between seventy-five 

 and ninety feet beneath the upland prairie surface. This over- 

 burden is composed chiefly of glacial till. Beneath the gypsum 

 layer are sixty to eighty feet of shale — the coal measures ; then 

 the St. Louis limestone. The great thickness of the shale sec- 

 tion carries the limestone a considerable distance beneath the 

 Level of the water in the Des Moines river, a mile and a half 



