INTRODUCTION 691 



ence near Red Oak without contributing any data with reference to it; 

 Tilton came across it in 1915, north of Briscoe, north of Bridge water, 

 and farther to the northeast, in work in Cass and Adair counties, first 

 published in 1919; described it as a normal fault downthrow on south 

 side of 281 feet, and this last summer, 1921, located the fault definitely 

 at three additional points : 3 close to the river northwest of Red Oak, at 

 the southwest corner of section 18, Harrison Township, Adair County, 

 and close to the Dallas County line immediately northwest of Earlham. 

 It is the effect of this fault and of erosion along it near Earlham and 

 Stuart that is treated in this paper. 



The area here considered is a critical one, bearing primarily on the re- 

 lation of the Des Moines and Missouri stages in Iowa. The problem is 

 not only of interest in Iowa, but also of interest as related to problems in 

 that historic region of southeastern Nebraska, which area also lies on the 

 upthrow side of the general direction of the Thurmaii-YYilson fault; and 

 is of interest to oil men farther south, in Missouri, Kansas, and Okla- 

 homa, who may wish to know of conditions affecting the boundary of 

 these stages to the north. 



Akea near Earlham and South of Fault 



South and southeast of Earlham are extensive quarries, long known 

 and visited by many geologists. Between the bed of North River, near 

 by, and the top of the quarries may be seen all of the strata from the 

 Hertha limestone to the top of the Winterset limestone. These same beds 

 may be seen outcropping along North River to a point 2 miles southwest 

 of Earlham. They may also be seen outcropping along ravines for about 

 3% miles northeast of Earlham, to the border of the Missouri stage as it 

 has long been mapped. A mile and a half north of Earlham there is a 

 place on the hillside where a reddish sandy shale at the top of the Pleas- 

 anton appears beneath the Hertha limestone (the northwest quarter of 

 the northwest quarter of section 31, in township 78 north, range 29 west). 



Area near Earlham and Northwest of Fault 



A mile and a half west-southwest of this last-mentioned point is a loca- 

 tion in section 35 where 8 feet of a thin-bedded limestone overlies 8 feet 

 of a gray, densely bedded limestone containing Beticularia perplexa, a 

 surprisingly large number of Spirifer cameratus, numerous Com posit a 



3 A report on the "Carboniferous strata of the Missouri stage in southwestern Iowa." 

 by the writer, is now in the hands of the Iowa Geological Survey. It includes further 

 discussion of the effects of the fault, the areal distribution of the subdivisions of the 

 Missouri stage, and of the Des Moines stage in the area considered. 



