88 PEOCEEDIXGS OF THE AXX ARBOR MEETIXG 



formed these sediments as well as the older ones. The foothills monocline was 

 developed at this time. The date of the post-Denver orogeny has not heen 

 definitely fixed. In the Wet Mountains area, directly south of the Colorado 

 Front Range, R. C. Hills found evidence of pronounced movements succeeding 

 the Eocene Bridger. It is possible that there were two or more Eocene uplifts. 

 Some students of Rocky Mountain geology believe that a peneplain was devel- 

 oped in the Front Range in mid-Tertiary time. In the Cripple Creek district, 

 Lindgren and Ransome report evidence of a prevolcanic plateau upon which 

 the fragmental debris of the Cripple Creek volcano was spread. If a mid- 

 Tertiary peneplain existed, a renewed uplift of the range must have taken 

 place in late Miocene or early Pliocene time. Near the close of the Tertiary 

 this erosion surface was bowed upward. The late Tertiarj- uplift inaugurated 

 the present cycle of stream erosion. There is evidence that preglacial erosion 

 proceeded much more rapidly in the foothills area than within the higher por- 

 tion of the range. The present relief features of the foothills undoubtedly 

 resulted very largely from preglacial erosion, since the glacial gravels, as 

 shown by their present relationship, partly buried a topography very similar 

 to the present one. 



Read by title in absence of authors. 



STRUCTURAL FEATURES OF THE COLORADO PLATEAU AXD THEIR ORIGIX 



BT RAYMOXD C. MOORE 



(AJ)st7-act) 



The Colorado Plateau is an elevated region, comprising portions of Utah. 

 Colorado, Arizona, and New Mexico, which is mainly composed of essentially 

 flat-lying sedimentary formations ranging in age from Cambrian to Tertiary. 

 Aside from gentle warping, which has affected all of the region, the stratified 

 rocks of the plateau province are affected in various places by three types of 

 structural deformation: (1) persistent, sharply defined monoclinal folds. (2) 

 localized steep arching in association with laccolithic igneous intrusion, and 

 (3) faults. 



The monoclinal folds exhibit an axial trend which is dominantly north- 

 south, and in practically all cases the inclination of the steep limb of the fold 

 is to the east. The dip in this direction averages about 40 degrees, but ranges 

 locally to 70 degrees. From the crest of such a fold the strata are commonly 

 very gently inclined to the west. The structures are therefore very asym- 

 metrical anticlines. The rocks involved in this type of deformation range in 

 age up to the youngest Cretaceous of the region, but do not include any of the 

 Tertiary (Eocene) formations. From the fact that some of these folds, as 

 beneath Aquarius Plateau, in southern Utah, and in the Chuska Mountains of 

 northeastern Arizona, were truncated by erosion before burial by the earliest 

 Tertiary of the region, it appears that the age of the folding is post-Cretaceous 

 and pre-Tertiary. 



The deformation associated with the laccolithic intrusions consists of sharp 

 upturning and arching of the strata immediately adjacent to the intrusion. 

 Upper Cretaceous beds are involved in this disturbance, but in no case have 



