OVERTURNS IN THE DENVER BASINS 585 



the discovery of what appear to be Benton shales on the south 

 side of Boulder Creek, disappearing under the apparently over- 

 turned Jura-Trias at the base of Flagstaff Mountain (the axis of 

 the Boulder Arch, described in the monograph before men- 

 tioned) , compelled a re-examination of the subject. In this 

 vicinity the most pronounced overturn is in the Niobrara basal 

 limestone, which is very hard and sufificiently resistant to form 

 a ridge making element. It is normally overlaid by several 

 thousand feet of easily eroded Upper Niobrara and Pierre shales, 

 and underlaid by Benton shales. When these formations are 

 erected to a position approaching the vertical,. the rapid cutting 

 away of Upper Niobrara and Pierre shales must inevitably leave 

 the Niobrara limestone partly unsupported on the east side, to 

 bear the burden of the lateral pressure of the mountain column 

 upon its base. Flagstaff Mountain rises abruptl}- about 1,000 

 feet above the upturned edge of the limestone. Other foothills 

 are still higher, others still are lower and less abrupt, while 

 beyond the foothills the main Rocky Mountain range towers to 

 a height of from 10,000 to over 14,000 feet above the level of 

 the sea. 



There are reasons for supposing that at Flagstaff, as the 

 unsupported limestone gave way and overturned, a break in the 

 underlying Dakota (here very thin) and in Jura-Trias permitted 

 the latter to swing outward at the base and inward and down- 

 ward at the apex, thus executing a partial revolution on an axis. 

 In the meantime, the yielding Benton shales crowded down into 

 the opening thus made, and the broken edges of the Triassic, 

 swinging outward, passed out over the Dakota. Benton, and 

 Lower Niobrara in such a position as now to rest upon the over- 

 turned Niobrara shales, giving the impression at first glance that 

 the Dakota, Benton, and Lower Niobrara had never been depos- 

 ited, and that theTriassic had participated with the Upper Niobrara 

 in the overturn. The following diagram, drawn by Mr. H. F. 

 Watts, of Boulder, Colo., who was associated with the writer 

 in this work, will be an effectual aid to an understanding of 

 what seems to have taken place. 



After solving the problem at this point, it was easy to recognize 



