TIIE BOULDER VALLEY REGION. 131 



feet by the Ostrea Led characteristic of this horizon, this overlain by tin- 

 usual succession of shaly and lignitic strata, which arc in turn followed 

 Lv the higher sandstones, and a thin layer of the higher clay series above. 

 The whole is capped by a heavy wash of Quaternary. West of the 

 triangulation station the mesa rapidly falls in height, the result of erosion 

 and dislocation and sinking of the strata — beds which to the cast have 

 been in full exposure now gradually passing beneath the level of the 

 tabic at the foot of the mesa — the lower sandstones, coal, and upper 

 sandstones one after another disappearing. The strata north of the fault 

 for the western two-thirds of its extent are the basal sandstones of the 

 Laramie; along the eastern third, the Fox Hills. 



'The stratigraphic throw near the eastern end of the break is sharp, 

 amounting to about 180 feet; it gradually decreases westward, until in the 

 vicinity of Foxtown it passes into the north fracture. 



The movements involved in the production of this fault must have 

 been complex and were probably somewhat as follows: Toward the eastern 

 end of the fault the strata to the south form a part of the relatively undis- 

 turbed mass of country: the beds of the interfault Mock north rose; toward 

 the western end. although the movement of the interfault block may have 

 been diminished in vertical range, the beds south of the middle fracture 

 have themselves here suffered motion and sunk, being cut off from the main 

 body of the strata Lv tin- southern branch of the fault system. In these 

 movements the interfault block north of the middle fracture has been 

 especially affected, for in the restoration of the equilibrium between 

 the greater and more unyielding masses upon the north and south it lias 

 apparently accommodated itself to their movements and assumed such space 

 and shape as was permitted it in the general crush which took place. The 

 position, as shown in the figure, is one of uneven elevation; the block may 

 be regarded as hinged upon the main mass of beds to the east at the sharp 

 fold forming the rim of the Davidson svncline, ami. having been subjected 

 to the extraneous pressure developed at the time of the disturbance, as 

 having relieved itself from the strain by elevation and crumpling. 



The southern fracture is simple, with a depression or Lowing down of 

 the beds to the north of it from either end toward the center, with a gentle 



