A PECULIAR BELT OF OBLIQUE FAULTING 603 



the fault lines, practically without exception, are inclined in the 

 neighborhood of 45° to the long dimension of the faulted strip 

 (Fig. i). Many of these faults are over 5 miles in length, while 

 one slightly curving fault plane has been traced for fully 10 miles. 

 A few of the shorter faults, however, are less than a mile in 

 length. As mapped by Hancock, these faults cut through a region 

 surfaced by the Colorado and Montana groups of the Cretaceous, 

 and by the Lance formation of the early Tertiary. 



Hancock has clearly brought out the fact that the belt of 

 faulting is located on the flanks of two conspicuous domes, which 

 are the two dominating folds of the Lake Basin field. These are 

 the Big Coulee-Hailstone dome in the northwest portion of the 

 area studied, and the end of the Big Horn Mountain anticline, 

 which extends into the southeastern corner of the Lake Basin 

 field. ^ Southeast of the Big Coulee-Hailstone dome, in the direc- 

 tion of its principal axis, is a minor circular uplift which the author 

 has called the Broadview dome. Hancock has pointed out the 

 fact that the most intense faulting occurred along the steeply 

 dipping south flank of the Big Coulee-Hailstone dome and around 

 the southeast side of the Broadview dome.^ From the Broadview 

 dome to the northwest end of the Big Horn Mountain anticline 

 very few faults were observed, but on the northwest slopes of the 

 latter uplift they again become numerous. In short, the faulted 

 strip follows the southern flank of the Big Coulee-Hailstone dome 

 (including its satellite, the Broadview dome) and, after crossing 

 an intermediate area where there are fewer faults, continues in 

 nearly a straight line along the northern flank of the Big Horn 

 uplift. 



Both the doming and the faulting, which may perhaps be termed 

 the local structural features of the Lake Basin field, are regarded 

 by the author as in all probability related in origin to the major 

 structures of the general region, and to have been determined more 

 or less by the complex forces involved in the development of these 

 major structures. "The mountain masses whose development 

 has probably been the most active in determining the nature of the 

 minor structural features in the vicinity of the Lake Basin field 



' Ibid., pp. 133-34. ' Ibid., p. 136. 



