30 GEOLOGY OF THE DENVER BASIN. 



rocks that continued from time to time during the Tertiary era, and which 

 formed an important and characteristic feature of that era. 



The dynamic effects of this movement, as seen in the Denver Basin 

 region, show that it was more intense than previous movements and was 

 produced by forces acting at right angles to the foothill region and to the 

 general strike of the strata, instead of nearly parallel, as were those which 

 produced the various uplifts of the Golden and Boulder arches. It was 

 apparently in the nature of a powerful compression along the base of the 

 foothills, or the contact of the later sedimentary beds with the basement 

 complex of crystalline rocks, accompanied here and there by a certain 

 amount of thrust-faulting, which tended to push the higher horizons forward 

 toward the mountains and over the lower ones. A most important and 

 somewhat singular effect of this movement, which Mr. Eldridge's explana- 

 tion of the Golden and Boulder arches renders necessary, was the flatten- 

 ing out of these arches, so that the present line of contact of the lowest 

 exposed sedimentaries with the Archean, which, since the Laramie move- 

 ment, is a line of strike, is a comparatively straight line; whereas the line 

 of strike of the higher beds, which were deposited over the arch, have now 

 a decided curve inward toward the mountains, and must, before being- 

 upturned, have been compressed into something like a synclinal trough, as 

 explained graphically in a later chapter. 



It seems possible to the writer that some of the curve in the strike line 

 of the upper (Laramie and Fox Hill) beds, or, in other words, of the irregu- 

 lar overlapping of the strata by the Montana and Laramie formations, may 

 have been produced by overthrust faulting along a line making but a slight 

 angle with the bedding, which would naturally have taken place in the great 

 clay horizons of the Middle Cretaceous, where little traces of the shearing 

 would be left. 



The general upturning of the Mesozoic strata along the foothills, which 

 has produced the characteristic phenomena of the hogback ridges, must 

 have been inaugurated by this movement, but it was not completed, as the 

 succeeding Arapahoe and Denver formations have also been upturned at 

 steep angles in the foothill region, and there is some reason to assume that 

 the forces which produced this upturning have been acting in comparatively 

 recent times. 



