THE BUILDING OF THE COLORADO ROCKIES 247 



the depth of the folded block. In certain portions of the Granby, 

 Sulphur Springs, and Troublesome areas, beds of Uinta age occupy 

 several basins below the peneplain level. The position of these 

 beds strongly suggests a sinking of these basins under the accumu- 

 lation of early Tertiary times. Though the extent of the settKng 

 probably was not great, still it may have been sufficient to cause 

 the calculations to assign a somewhat too meager depth to the 

 rather shallow Granby and Sulphur Springs blocks. 



In the foregoing computations only the crustal shortening at 

 right angles to the trend of the ranges has been considered. But 

 folds are properly considered in three dimensions, and some longi- 

 tudinal warping and pitching of the folds is to be expected. 

 Longitudinal warping would involve crustal shortening in a direc- 

 tion parallel to the trend of the range and at right angles to the 

 shortening above considered. The folds measured in the Lyons- 

 Grand River section exhibit pitching in some places, but this did 

 not appear sufficiently pronounced to justify the extra field work 

 necessary to measure the northwest-southeast warping for the differ- 

 ent sections. The warping at right angles to the main folding was 

 most apparent in the State Bridge and Glenwood Springs sheets. 

 This minor warping, like the major folding, causes upswelHng of 

 the folded tract. Hence in practice the effect of ignoring the minor 

 warping in the direction of the ranges is to assign all the observed 

 upswelling to the work of the major folding and thus obtain an 

 overestimate of the depth of the folded zone. 



There are therefore certain subsidiary considerations, such as 

 uncorrected uplifts subsequent to the folding, and the neglect of 

 upswelHng from longitudinal warping whose influence is to cause 

 overestimates of the depth of folded shell. But over against these 

 is the uncorrected sinking of certain areas of Eocene sedimentation, 

 and the assumption of uniform shortening throughout the full 

 thickness of each deformed block, whose probable influence is to 

 cause underestimates of the depth. In all probability each of 

 these is the source of some error. How nearly they offset each 

 other and so cancel out of the equation cannot be told at present. 

 But they probably do not greatly modify the general order of 

 magnitude of the calculated results. 



