74° VICTOR ZIEGLER 



postulates the presence of extensive strike faults with steep westerly 

 dips and overthrust from the west. This interpretation is shown 

 to be free from the inherent weaknesses of the older so-called 

 arch hypothesis, and to be in perfect accord with all observed 

 geological features. 



Conclusion. — The formations represented in this area of the 

 foothills range in age from Permo-Carboniferous to Paleocene. 

 The Cretaceous formations certainly, the older probably, extended 

 formerly over the entire area of the Front Range. At the close of 

 Arapahoe time and during early Denver time the area to the west 

 of the foothills rose, and the sedimentaries were folded into a normal 

 monocline with average eastward inclination of about 45 . Locally 

 (as at Golden and Boulder) excessive overturning occurred, accom- 

 panied by fracturing, and resulting in extensive overthrusts from 

 the west along steeply dipping fault planes. Minor strike faulting, 

 and dip faulting with hingelike displacements, as well as drag fold- 

 ing, accompanied and followed the major uplift and faults. The 

 natural buckling and wrinkling of strata, where the pressure on 

 the monoclinal fold was not intense enough to cause huge strike 

 faults, resulted in the formation of folds en echelon, with their 

 attendant overthrust faults from the east. The exact time of the 

 uplift cannot be closely determined, except that the basal beds of 

 the Denver are involved in the folding and that the composition 

 of the upper beds of this formation shows that erosion had entirely 

 cut through the sedimentary series over the site of the Front Range 

 at the time of their deposition. 



The sincere thanks of the writer must be expressed to Dr. H. B. 

 Patton for advice and helpful suggestions in the preparation of this 

 paper. 



