260 
ELDRIDGE. 
Structural Development of the Area. 
Introductory .—The abnormal conditions which have been 
noted in the relations of the several formations to each other 
are directly traceable to a series of non-conformities that 
exist at the particular horizons at which these conditions 
occur. Excluding the higher ones of general occurrence 
along the base of the mountains in this portion of Colorado— 
that is, those between the Laramie and Arapahoe, and the 
latter formation and the Denver beds—there are still to be 
found four which are in some respects peculiar to this locality: 
one between the Archaean and Trias, of special development 
in this area; a second at or near the close of the Trias; 
a third at the top of the Jura, and a fourth in the Cretaceous 
at the close of the Colorado. 
Entering most prominently into the history of these non¬ 
conformities are as many folds, all of which occurred prior to 
the general uplift of the Rocky Mountains, and hence, with 
the erosion going on at the time, represented a completely 
different topography for the region from that of the present 
day. When the great uplift of the Rocky Mountains brought 
the beds into the position they now have, all hills, the result 
of previous folding, were changed in their individual posi¬ 
tions from one in which the plane of their bases was hori¬ 
zontal to one in which it became vertical, or at least inclined 
at a high angle, and parallel to the direction of the mount¬ 
ains. In the subsequent erosion of the region, therefore, 
what would originally have peen a profile section of the 
strata constituting these folds now appears in plan on the 
present surface of the ground, all originally north and south 
dips becoming present north and south strikes—in some 
cases slightly altered in character by incidental variations 
in the amount of folding in the general uplift of later times. 
The detailed character and the contours of these ancient 
elevations cannot be determined, the two dimensions given in 
the profiles being naturally the only ones admitting of obser- 
