Geology of Castle Rock.—Lee. 105 
movement.” Mr. Eldridge estimates that the arch at Golden, 
Colorado, was elevated 9,500 feet at this time. ‘The estimate 
was made from the thickness of the Ft. Pierre shales which 
were deposited against the arch and finally covered it. A sim- 
ilar estimate for the Castle arch would show at_least as great 
an elevation. The width of the belt occupied by the shales in 
Perry park is great. The shales lie at an inclination of 45° to 
go. Their computed thickness is at least as great as the max- 
imum given for the Denver basin (7,700 feet). (I have made 
no attempt to distinguish between the Ft. Pierre and the Fox 
Hills. The latter is inconspicuous in this region. The great 
bulk of the Montana shown on the map is Ft. Pierre.) It 
seems probable on inspection of the present distribution of the 
formations, that the shales never entirely covered the Castle 
arch. It seems safe, therefore, to assume that the elevation of 
the arch, measured from the southern side was at least as great 
as that of the Golden arch at this time (9,500 feet). This, how- 
ever, would be a measure of the amount of subsidence in the 
syncline, to be described later, as well as the elevation of the 
arch. — 
In this region as elsewhere in the Rocky mountains the dis- 
turbance at the close of the Laramie epoch was one of ae 
importance and one by which radical changes were introduced. | 
The Castle arch was destroyed at this time, partly by erustiieet 
and flattening, and partly by faulting. Ridges were thrown up| 
at right angles to its axis (the present foot hills) and the 
mountain region to the west, greatly elevated. This elevation 
and the subsequent period of erosion, previous to the deposi- 
tion of the Arapahoe, has been discussed by Whitman Cross 
in the “Geology of the Denver Basin’ and elsewhere. Some- 
thing of the extent of the movement and the length of the per- 
iod of erosion is indicated by the conglomeratic nature of the 
Arapahoe which contains pebbles from all the older formations 
of the region, and by its position south of Indian creek where 
it lies across the truncated edges of the older formations. 
It is probable that in addition to the flattening of the arch, 
there was faulting in the vicinity of the crest by which the 
southern side-was dropped to a considerable extent, tilting the 
Perry park block to the north and causing the strata in their 
present upturned condition to rest further to the west on the 
