390 JOHN L. TILTON 



of the disturbance were a factor in determining surface distribu- 

 tion well across the state. Investigation has not, however, by the 

 writer been extended in the field to this part of the state. 



AGE OF THE FAULT 



Since the Dakota sandstone lies on both sides of the fault plane 

 and rests on the truncated surface of various strata of the Missouri 

 stage with no evidence of a fault scarp detected beneath the Dakota 

 sandstone, it appears that the faulting began in the interval 

 bet\veen the deposition of the Missouri stage and the deposition 

 of the Dakota stage, that the fault scarp was well removed before 

 the subsidence that accompanied the deposition of the Dakota 

 sandstone, and that any additional faulting has not been very 

 pronounced since that time. That there has been some later 

 movement is possible, since at Crystal Lake shale included between 

 the sandstone strata is found to dip in the general direction of the 

 dip of the limestone (Missouri) . If there was any movement along 

 the fault plane any escarpment that formed at the surface of the 

 sandstone has since been removed by erosion. 



Variations in dip of the sandstone that correspond to variations 

 in dip of the limestone along the fault plane have not been detected. 

 It therefore appears from this argument also that about all faulting 

 with accompanying disturbance^ was completed before the Dakota 

 sandstone was laid down. In this interval of time there was in 

 the interior and eastern United States one marked period of 

 disturbance, the Permian. In distant regions (Appalachian and 

 Ouachita) the faulting was of the reversed type. Here it is of the 

 normal type. The reversed faulting of the Permian may have 

 been accompanied or followed by relaxational movements in the 

 same or the next period. To this interval of time (Permian- 

 Triassic) it at present seems necessary to refer the major part of 

 the faulting, and perhaps all of it. 



^ From a study of the joint planes it also appears that the jointing in the Missouri 

 stage is related to the faulting and to the upUft toward the northeast (Permian- 

 Triassic), while the jointing in the sandstone is related to the uplift toward the 

 northwest (post-Cretaceous), though affected by the presence of the fault plane and 

 the joint planes, and also affecting those planes. 



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