GEOLOGY OF SOUTHWESTERN IOWA 77 



CONTRIBUTIONS TO THE GEOLOGY OF SOUTH- 

 WESTERN IOWA. 



GEO. L. SMITH. 



During the last year work on the geology of southwestern 

 Iowa has been continued. The different outcrops in the vicinity 

 of Stennett in Montgomery county have been visited, and im- 

 portant information obtained of the stratigraphy and paleon- 

 ology of the Stennett limestones and the Braddyville and Platte 

 shales. In Fremont county the exposures at Hamburg, McKis- 

 sicks Grove, Mill creek, as well as those from Opossum creek 

 north to Thurman have again been examined, in hopes to defi- 

 nitely locate the break that takes place in the strata between 

 Thurman and the Wilson section. 



The unusual and excessive rainfall of the last summer made 

 the field work disagreeable by the abundance of mud and high 

 water. Erosion was many times greater than in any previous 

 year. In several places overwash and slumping completely cov- 

 ered outcropping strata, while in other places the creek beds 

 were swept bare and clean by high water, affording details of 

 sections not before observed. Erosion has been especially active 

 in the head of ravines in the loess of the Missouri river bluffs. 

 At MeKissicks Grove this has exposed about thirty feet of strata 

 bigher than those already known. 



In tracing the different limestone ledges southward, there is 

 found a marked change in the lithology of the horizon ; the up- 

 per limestone at Nebraska City and Hamburg grades into sand- 

 stone in the state of Missouri, within twenty miles of the Iowa 

 state line. Even at MeKissicks Grove this limestone becomes 

 very arenaceous, and at the most southern outcrop it might be 

 termed a calcareous sandstone. The limestone bottom rock of 

 the Nodaway coal at Carbon in places is five feet thick, south 

 ward at New Market it thins to eight inches, at Clarinda it is 

 only a disconnected layer of nodules, and at Coin it is absent. 

 One of the thin limestone ledges less than two feet thick at the 

 Wilson section increases in thickness to twelve feet in less than 

 a mile to the north. This shows the necessity of caution in cor- 



