FAULT SYSTEMS TN IOWA 



103 



CONTROLLING FAULT SYSTEMS IN IOWA. 



CHARLES KEYES. 



With its even plains surface, the infrequency of bed-rock 

 exposures, and the universal presence of thick till or loess man- 

 tles, detailed geologic mapping of the prairie states is attended 

 by many inherent difficulties not met with in more broken 

 country. In consequence of the existence of these unusual 

 conditions the consideration of possible noteworthy geotectonic 

 features in the region is largely neglected. Anything beyond 

 a few of the most obvious local characters completely fail of 

 record. Over a very large part of the Mississippi basin the 

 tectonics are commonly treated as if there were none at all. It 

 seems to suffice to regard the strata as essentially flat-lying and 

 as having no pretense to deformation of any kind. In Iowa, for 

 instance, beyond the general assertion that the foundation rocks 

 dip gently to the southwest no further note is made of the local 

 or broader tectonic characters. 



Lately, both in our own state and in neighboring states, the 

 neglected problems in regional tectonics have been attacked 

 from new and unexpected quarters. Novel data have been ob- 

 tained. Long known but isolated facts have been reviewed, re- 

 interpreted, and recorrelated. The trend of most fruitful in- 

 quiry has been pointed out. In Iowa, especially, results quite 

 surprising have been reached. Attention already has been di- 

 rected 1 to the Triassic mountain-building which took place with- 

 in our boundaries. Particular interest also attaches to the re- 

 cent determination 2 of the distinct synclinorial character of the 

 Iowa coal basin. Now note must be made of another instruc- 

 tive phase of the regional tectonics and the discovery of what 

 appears to be two well-defined systems of faulting on a large 

 scale that has heretofore eluded detection. 



The lines of faulting of the two systems trend nearly at right 

 angles to each other. In the system which prevails in the east- 

 ern part of the state the direction of fracture is northwesl and 



iProc. Iowa Acad. Sci., Vol. XXI, 1914, p. 181. 

 2 Ibid., Vol. XXII, 1915, p. 268. 



