THE BOULDEE VALLET REGION. 



129 



between the lines varying from 1 t<> 4 miles. The stratigraphic throws, 

 with one or two exceptions, are between 150 and 250 feet. The faults are 

 nearly nil strike faults, and include not <>nlv the normal but also the 



reversed type. 



The Marshall subsystem. This IS CHI 1 1] II ISI •( 1 of & HKlill fractlU'C a]l<l tWO 



lateral branches given off from its southeastern side. The three max be 

 designated the main or northern, the middle, and the southern. The 

 southwestern end of this system of faults is at the point of the mesa, a 

 half mile southwest of the town of Marshall, and lies in the sharp fold of 

 the strata, whereby the series of beds, noticeably the basal sandstones 

 of the Laramie, an- reduced from a nearly vertical position with a X. 7 \V. 

 strike to one but slightly inclined with a strike of X. .'}<> to .in E. and 

 a gentle dip, rarely over L5 , southeast. The northeastern extension of 

 the main fracture is obscure, bul is mapped in accordance with the weight 

 of evidence gathered in the field, and not beyond the actually observed 

 occurrence. Of the three, the northern apparently ends by a gradual 



FlG. hi Section through northern and middle fault*, Marshall system. 1. Ooiicretionurv layer of calcareous 



sandstone. 'J. Summit sandstone of I'm Hills. 8. Sandstone A. i. Sandstone B, A and Bforin the basal sand 



bbi i eu ii' A thin carbonaceous Bhale usually separates A and B. A coal bed li><':illy rests upon B. 5. Ostrea bed 



6. in f the coal seams usually present between s:nuisti s B and C, ocourring in a scries ..I shales, v. Sandsl C. 



B. Quaternary cap of mesa. 



diminution of the throw; the middle and southern terminate on the 

 northern and southern side, respectively, of the Davidson mesa, in the 

 sharp bend of the strata which forms the western rim of the Davidson 

 syncline. 



The main or northern fracture is the most extensive of the three and 

 is probably the westernmost break of the Boulder Valley system, although 

 the Montana clays west of South Boulder ("reek would naturally conceal 

 any further occurrences of fractures, owing to the homogeneity of their 



material. The fracture itself could be seen at the time of examination at 

 but one point in its length, namely, in the Marshall No. ."> mine, but it is 



MON XXVII- 



-11 



