THE BUILDING OF THE COLORADO ROCKIES 239 



These figures should therefore represent, subject to the above- 

 mentioned quahfications, the degree of upHfting which has resulted 

 from the folding process. 



It will be seen that in the matter of uplift the Grand Hogback 

 sheet falls far short of all the others and appears to be in a class by 

 itself. An inspection of the Grand Hogback section reveals the 

 fact that, while the eastern half of the sheet shows moderate uplift, 

 the western half, on the contrary, indicates subsidence, for the 

 upper surface of the Cretaceous there dips more than 5,000 feet 

 below the datum-plane. Only Tertiary rocks are exposed at the 

 surface in the western portion, and in fact the geological map of 

 Colorado shows that these Tertiary rocks continue to the southwest 

 without interruption for fully 50 miles more — even to within 20 

 miles of Grand Junction. This area west of the Grand Hogback 

 constitutes the southeast portion of the extensive Uinta Basin, 

 which is a structural basin filled with thick Eocene deposits. 

 Throughout this basin of heavy Eocene sedimentation the upper 

 surface of the Cretaceous has been depressed below our datum- 

 plane. 



The Grand Hogback is a sharp ridge of upturned resistant 

 strata, which marks the location of a sharp monoclinal flexure 

 between the uplifted Rocky Mountain region on the east and the 

 downwarped Uinta Basin to the west. A study of the Grand 

 Hogback cross-section sheet raises the question whether subsidence 

 of this basin of Eocene sedimentation may not have been as impor- 

 tant in developing the Grand Hogback monoclinal flexure, as 

 up warping on the Rocky Mountain side. But whether subsidence 

 or uplift has been the more effective in bending the strata, the 

 earlier Eocene beds in any case exhibit essentially as much folding 

 as the underlying Cretaceous, indicating that the flexing for the 

 most part has come after the Laramide disturbance. Later studies 

 may show whether or not this bending was synchronous with the 

 episode of folding which affected the Middle Park beds near Windy 

 Gap. Because the extent of the subsidence suffered by the Uinta 

 basin interferes with any reliable determination of uplifting on its 

 borders, the Grand Hogback sheet will not be included in the section 

 used for estimating the thickness of the folded Rocky Mountain 



