384 JOHN L. TILTON 



much of the area by a thick deposit of Dakota sandstone; and the 

 entire area is deeply covered with drift, through which almost no 

 outcrops of underlying strata appear. Especially is this true of 

 the Missouri stage in the northern half of the area, the area north 

 of the fault plane. Further, the Missouri stage in southwestern 

 Iowa is represented by fourteen different sets of limestone beds 

 separated by intervening beds of shale. The limestones them- 

 selves have shaly partings of various thicknesses, and the shaly 

 members have limestone beds. Through all these beds there are 

 the same species of fossils and the same general assemblage of 

 forms, so that at present, at least, the worker who uses the fossils 

 must associate the assemblage and relative abundance of fossils 

 with the sequence of the beds. In this way a few lines of evidence 

 can be made out that have an important bearing. Detailed work 

 later on the fossil content and the relation of each bed will doubtless 

 yield valuable returns. 



South of the fault plane the varying dip of the strata accords in 

 places with the slope of the stream beds; and no information had 

 appeared in that area that revealed what beds became thinner 

 beneath the drift. 



THE FAULT 



The writer finds that the Thurman- Wilson fault of 300 feet near 

 the Missouri river in Fremont County does not become an anticline'^ 

 toward Stennett, but that it is a normal dip or slightly oblique 

 fault, extending between Fox quarries in the extreme southern part 

 of Cass County and Briscoe (three miles further east), with upthrow 

 on the north side, with displacement of 284 . 5 feet, and with fault 

 plane dipping steeply to the southeast. This line of fracture con- 

 tinues northeast across Adair County, either in a slightly curved 

 line or in a parallel fault or faults (Fig. i). The southeast part 

 of Guthrie County is the limit toward the northeast to which the 

 writer has thus far traced the evidence of faulting in the field. 



At the few places where a measurement of dip was obtainable 

 a marked irregularity of dip was found that in general was at right 

 angles to the strike of the fault plane, and thus at right angles to 



' G. L. Smith, "Carboniferous Section of Southwestern Iowa," loiva Geol. Surv., 

 XIX (1909), 636. 



