THE SANDSTONE OF FOSSIL RIDGE 183 
strike of this sandstone is uniformly north in the northern part of the 
field, its distance from the foothills varies more or less on account of the 
sinuous strike of the foothills formations, which is due to important 
folds, their failure to materially affect the strike of the Hygiene sandstone 
perhaps being due to the yielding nature of the intervening Pierre shales. 
Both above and below the harder ridge-making portion of the Hygiene 
everywhere are softer sandstones, and a short distance to the eastward 
is a more or less persistent sandstone which forms a marked ridge east 
of Fossil Ridge and Rocky Ridge. 
The fact that this sandstone is continuous from Boulder to Rocky 
Ridge does not render entirely necessary the conclusion that the former 
at Boulder and the latter at Ft. Collins are synchronous, though no 
evidence has been found to the contrary. In this region the Pierre for- 
mation, together with the overlying Fox Hills and the underlying 
Niobrara and Benton, are marine, as shown by the unvarying evidence 
of the fossils contained therein. The base of the Pierre all along the 
plains adjacent to the foothills consists of black shales to a thickness of 
hundreds of feet, the conditions under which they were deposited having 
been quite uniform throughout the region. 
Among the possible changes which could have caused the temporary 
and widespread deposition of sandstone instead of shales may be men- 
tioned: (a) an unlift of the adjacent land surface, so that coarser mate- 
rials would be carried seaward by the streams; (0) the cutting away of 
certain formations on the land surface so as to expose to erosion materials 
of different character; (c) the shallowing of the sea, either by uplift or 
sedimentation, and consequent increase of the power of waves and 
along-shore currents; (d) the increase of currents from any other 
cause. 
That the sandstones were laid in somewhat quiet water is evident 
from the unbroken and unworn condition of the fossils, even such fragile 
shells as Anomia retiformis Meek. The bivalves also are mostly found 
with the valves in juxtaposition. We have seen very few specimens 
which showed evidence of having been broken before being buried, the 
only really noteworthy case being a bryozoan on a fragment of Placenti- 
ceras in such position as to show that it lived on the shell after it was 
