THE BOULDER VALLEY REGION. 137 



hill in northwest, north, and northeast directions, and met in the body of 



the hill l>v a luld in the opposite direction, the dip being to the south. 

 The strata forming this interfault block have a general tall of 2 down- 

 stream. The block to the cast of this affords particular evidence of the 

 compressive forces acting in an east-and-west direction oxer the prairie 

 region, in the sharp fold which appears in the sandstones of the Fox Hills 

 and Laramie. The beds have been thrown into a well-defined, unsym- 

 metrical anticline. Its long slope of about 15° is to the west, the short 

 one of 65° to the east. From the latter dip the strata again rapidly assume 

 an approximately horizontal position, or one in which the dip is between 1° 

 and 5° to the east. The center of this arch furnishes the finest collecting 

 ground of upper Fox Hills fossils in the Denver held. 



The lower coai creek fault. — By this is designated the prominent curved fault 

 extending for the greater part of its length along the bluffs on the 

 eastern side of Coal Creek from a point near the entrance of Mock ("reek 

 to tin* northern limits of the map. Though its recognized extent southward 

 i> as indicated by the solid line — to a point 1 \ miles below the mouth of 

 Rock Creek — it is probable that it reaches at least a mile or two farther 

 up tlu- valley of Coal Creek, as shown by the broken line. The northern 

 end lies beyond the limits of the field mapped, and is also concealed 

 beneath the surface deposits of the prairie, which here falls rapidly to 

 the general level of the Coal ('reek bottom. The fault probably belongs 

 to the "normal" clas-, though the evidence is very meager. 



Wherever the line of fracture is seen the shales of the upper Laramie 

 are found upon the east, opposed by those of the Fox Hills on the west. 

 The former hold all positions from horizontal to vertical, though usually 

 the steeper, while the latter are always highly inclined, even to an 

 occasional overturn. Within a short distance of the fault line the strata on 

 either side regain their normal position, those on the west assuming a gentle 

 westerly dip of 111 , prevalent for a narrow strip, those on the east settling 

 to their natural position of shallow dip and gentle rolls. The particular 

 horizons of the two formations opposed to each other at the outcrop are 

 probably: for the Fox Hills, a stratum not over 50 or 75 feet below its 

 summit; for the Laramie, one at least "250 feet above its base. This would 



