FOOTHILLS STRUCTURE IN NORTHERN COLORADO 731 



older steeply dipping ones, which is not the case. As will be seen 

 from the sections, these faults are of the hinge type, with maximum 

 displacements at Golden. This gradually decreases to zero toward 

 the north and south. 



All facts recorded by Eldridge and all facts observed by the 

 writer accord perfectly with such an explanation. We should 

 expect fault lines of this nature to be characterized by the crumpling 

 and overturning of strata affected. The more sharply a formation 

 is truncated the more evident the effects on its bedding planes should 

 be. Maximum displacement and maximum disturbance should 

 logically go hand in hand. The course of the fault plane and its 

 position would determine whether a formation disappears from the 

 bottom up or from the top down. As a general rule, with dips and 

 strikes as observed, the formation on the west side of this fault 

 plane should disappear from the top downward, while the formation 

 on the east side should disappear from the bottom up. This is 

 actually the case (Fig. 8). 



Upon cursory examination the decided westward curve of the 

 outcrops is somewhat surprising and seems to suggest that the 

 eastern block represents the upthrow side. This is, however, not 

 true, and the inward curve is the combined result of the gradual 

 steepening and eventual overturning of the monoclinal fold as we 

 approach Golden, and the displacement along the westward- 

 dipping fault surface. 



The actual inward sweep of the strata resulting from the writer's 

 interpretation of the structure can readily be approximated as 

 follows : The total thickness of strata actually cut out at Golden is 

 about 10,000 feet. Therefore, the lowest bedding plane on the 

 Fox Hills remaining at Golden must have been located at least 

 10,000 feet higher than its present position on the monocline before 

 faulting. To this must be added the difference in elevation between 

 the top of the Archean and the Fox Hills today (at least 1,200' feet) 

 and a certain amount to allow for folding, hence 12,000 feet may 

 be considered a safe estimate as to the minimum amount of throw 

 necessary to bring the base of the Fox Hills from its original loca- 

 tion in the monocline to its present position. A vertical drop of 

 12,000 feet on a fault plane dipping 55 westward will result in a 



