CENTURY OF IOWA GP:0L0GY 431 



The sudden appearance and rapid decline of the Siouan Moun- 

 tains on the mid-continental horizon is an incident of a by-gone age. 

 Brief, brilliant, almost pathetic are the succession of chief events. 

 The main uplifting took place during the Triassic period. In the 

 succeding Jurassic and Comanchan times all of the ranges were 

 completely razed to the present plains-level. During Cretacic time 

 the waters of the sea again rolled unbrokenly over the old base- 

 leveled plain, and the bared foundations of the former lofty 

 mountains made up the bottom of a broad epi-continental sea. No 

 great orogenic uplift was ever more rapidly or completely obliter- 

 ated. It was one of the marvelous episodes in the long history of 

 the North American land contest. '° 



SUBEQUAL SPACING OF CRUSTAL RUPTURES 



Profound faulting is commonly associated with mountain de- 

 velopment. In a plains region, and especially in sea-level basins 

 of a continental interior, notable displacement of strata is about 

 the last tectonic feature that one expects to encounter. The recent 

 location in the Upper Mississippi Valley of a number of fault lines 

 of considerable moment is one of the surprises of geologic in- 

 quiry in this region. Singularly Iowa appears to have been the 

 locus of repeated crustal rupturing on a large scale. ■''^ 



Both in our own state and in neighboring states, lately, some 

 of the long neglected problems of regional tectonics have been at- 

 tacked from new and unexpected quarters. Novel data have been 

 obtained. Long known but isolated facts have been reviewed, 

 reinterpreted, and re-correlated. The trend of the most fruitful 

 lines of investigation has been pointed out. In Iowa, particularly, 

 results quite disconcerting have been reached. Atteniion already 

 has been directed to the vast Triassic mountain-building which 

 took place within our boundaries. Especial interest, also, has been 

 attached to the recent determination of the distinct synclinorial 

 character of the Iowa coal-basin. Further, note has been made of 

 another instructive phase of regional tectonics and the discovery of 

 what has appeared to be two well-defined systems of faulting on a 

 major 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 eastern 

 part of the state the direction of fracture is northwest and south- 

 east. The amount of displacement is large. The spacing is wide. 



'"Proc. Iowa Acad. Sci., Vol. XXI, pp. 181-187, 1914. 

 'iProc. Iowa Acad. Sci., Vol. XXIII, pp. 103-112, 1916. 



