FAULT SYSTEMS IN IOWA 107 



practically no tilting of the strata, but that the lower beds on 

 the north side of a given point abutted higher Layers of the 

 south side. 



The detection of this notable fault-line fully explains why. 

 during the attempt to map geologically Montgomery county, 

 the Cretacic formations were so well exposed throughout the 

 southern half of the county, but were apparently entirely absenl 

 in the northern part. Planation had entirely removed the higher 

 Mesozoic beds 011 the north, but had not touched those to the 

 south, where they were deeply depressed and thus escaped ob- 

 literation. Another hitherto inexplicable fact, which now ap- 

 pears to be satisfactorily cleared, is the abrupt change of litho- 

 logic character which has been long known in the Des Moines 

 river section a few miles north of the city of Des Moines. For 

 more than a generation this had been one of the most perplexing 

 problems in Iowa geology. 



The Fort Dodge fault is particularly noteworthy because of 

 the fact that to it the great Iowa gypsum field directly owes its 

 preservation; and important chalk deposits exist as outliers 

 eighty miles east of their normal outcrops. As recently acquired 

 the details on this dislocation are unusually full and may be 

 with advantage summed up here. They are all displayed in an 

 exceptionally clear manner within the limits of the city of Fort 

 Dodge. 



The abrupt termination of the thick gypsum bed at the Cum- 

 mings quarry, in the south bluff of Soldier creek, in north 

 Fort Dodge, and its replacement at the same level on the north 

 side of the narrow valley by the St. Louis limestone and coal 

 measures calls at once for a more critical examination of the 

 causes therefor than has been hitherto given to the phenomena. 

 In this district there is a general rising of the limestone towards 

 the north; but in the same direction a marked falling of the 

 gypsum. At the mouth of Soldier creek the gypsum layer co 

 down to a level below that of the creek bed. It is this fact 

 mainly that has in the past given rise to the inference that the 

 gypsum finally rests directly upon the 'limestone, especially 

 since the latter crops out in the banks of the creek and in the 

 ravines within a distance of a few hundred yards beyond the 

 last known gypsum exposure. 



About three-fourths of a mile upstream £rom the Cummi 

 locality, in Soldier valley, near the new brick plant, several es- 



