106 IOWA ACADEMY OF SCIENCE 



its norma] eastern boundary as usually mapped. As it enters 

 Iowa the Cap-au-Gres fault has a displacement of 100 feet, 

 so that here it is still a fracture of considerable moment. 



Another notable fault-line, which merits fuller investigation 

 than has been heretofore accorded it and about which so far 

 as Iowa is concerned relatively little is yet definitely known, 

 exists near Dubuque. It has a throw of fifty to seventy-five 

 feet. One reason for its not being better understood doubtless 

 is the fact of its position for many miles in the channel of the 

 Mississippi river. According to the maps of Illinois this fault- 

 line appears to be really the northwestern extension of the 

 great La Salle fault, which at the town of La Salle has a dis- 

 placement of quite 1,000 feet. Illinois geologists claim that the 

 La Salle fault has a north and south trend ; but the geological 

 maps of the region clearly indicate otherwise, and many other 

 recorded facts fully corroborate the testimony of the maps. 



Between these two great fault-lines are several rather sharp 

 folds which may pass elsewhere into faults. None of these 

 has been examined yet in detail. They may prove to be regu- 

 larly spaced and thus form a part of a definite fault system. 



It is, however, to the remarkable fault system of the western 

 part of the state that attention is here especially directed. Two 

 faults in particular merit full notice at this time because of the 

 fact that their discovery necessitates extensive rectification of 

 geologic boundaries. There are, also, economic bearings 

 which are of great local importance. The two most instructive 

 ruptures of this western system are the ones passing near Red 

 Oak and near Fort Dodge. 



For such a profound fracture, with its maximum displace- 

 ment of not less than 400 feet, the Red Oak fault makes singu- 

 larly inconsequential impress upon the local relief expression. 

 In one direction it appears to extend beyond the city of Des 

 Moines; in the other to Hebron, Nebraska. — a distance of 300 

 miles. Its features are best displayed at its crossing of the Mis- 

 souri river, at Wyoming, a short distance above Nebraska City. 

 When the disturbance was first noted at this point it was 

 thought to represent a sharp monoclinal fold; and it was so 

 interpreted by Professor J. E. Todd. 4 Later investigation on 

 the Iowa side of the river, near Truman, showed that there was 



4 Proc. Iowa Acad. Sci., Vol. I, 1890, p. 58. 



