RECITAL OF CERTAIN OUTSTANDING FACTS 289 



Only a few ontstanding facts relative to the later history of the Eocky 

 Mountains need be mentioned in this connection. First, it is beyond 

 dispute that since the relatively small nplift at the end of the Cretaceous 

 period the Eocky Mountain region has never been submerged, although 

 parts of the continent at a distance from these mountains subsided 

 beneath sealevel in Tertiary time; second, there were periodic uplifts 

 during the Tertiary and Quaternary periods, accompanied by warping 

 of the surface and extrusion of igneous rock. Much of the evidence of 

 the earliest Tertiary movements has been destroyed by erosion, but the 

 later uplifts and cycles of erosion are recorded in the old peneplains, one 

 of which is now found in northern Colorado at an altitude of about 

 12,000 feet, and another at 10,000 feet. Doubtless remnants of other 

 persistent plains will be found. These plains are raised somewhat more 

 in some places than in others and show some warping, but on the whole 

 they suggest vertical uplift. In brief the mountains have been peri- 

 odically rising from the close of the Cretaceous period to the present 

 time. 



Some of the minor features are important in indicating the manner in 

 which this rise occurred. Throughout the mountain region overthrust 

 faults are rare and normal faults are numerous. With few exceptions, 

 displacements along the foothills may reasonably be attributed to drag 

 in the rise of the mountains, and to such secondary action as sag follow- 

 ing a period of uplift, and to local lateral crowding at the edges of the 

 sagging mass. Lest I be misinterpreted, let me reiterate that I am' 

 speaking of the Southern Eockies alone and not of parts of the Wasatch 

 Mountains and other places still farther to the west and north, where 

 great overthrusts have occurred. 



There are few so-called rock folds in the Southern Eockies of such 

 character and magnitude as to enter seriously into the problem of moun- 

 tain-building. These are the hogbacks, formed by the upturning of the 

 sedimentary formations on all sides of the mountains and the large down- 

 warped or less uplifted areas, as one chooses to think of them, in North, 

 Middle, and South parks, and other less conspicuous features, commonly- 

 classed as folds, such as the en echelon folds of the foothill region and 

 the less conspicuous undulations of the sedimentary rocks of the plains,, 

 which, in my opinion, should be expressed as warping rather than as 

 folding under lateral compression. It is a common custom to connect 

 the upturned beds of the foothills with the isolated remnants of the same 

 beds high in the mountains, in such a manner as to make it appear that 

 they once extended in a continuous undulating but unbroken surface 

 over the range. This practice may be justifiable for certain purposes,. 



