CRETACEOUS SERIES. 43 
The thickness of this formation, as nearly as could be estimated under 
the unfavorable conditions, is approximately 4,000 feet. 
LARAMIE FORMATION. 
On the comparatively low ridge on the left side of Woody Creek, near 
the point where the stream emerges from the canyon, a heavy bed of pure 
white sandstone outcrops on the west side of the Castle Creek fault, becoming 
yellowish or reddish in places, and forming a bench about 100 feet high. 
This has been taken as being about the base of the Laramie formation.’ 
Below it, on the hillside, a shaft which has been sunk for prospecting 
purposes shows a few feet of solid blue limestone of very fine texture. 
Above this white sandstone come beds of impure brown and green sand- 
stones, thin-bedded and friable, often micaceous and shaly. These carry 
abundant plant remains, which, however, are not sufficiently preserved to 
admit of identification. The series is lithologically like that of the coal- 
bearing Laramie in the Crested Butte and Anthracite regions, but no coal 
seams were noticed in the Aspen district, although some of the layers in 
the impure sandstones above described carry such a quantity of plant 
remains as to become very black and carbonaceous. 
These basal beds of the Laramie form a synclinal basin against the 
Castle Creek fault; outside the rim of this basin the outcrops of the 
Laramie sandstones give place to the underlying Montana rocks. The 
Laramie, therefore, occupies but a limited area, and only the lower portion 
of the beds is exposed, the whole upper part having been removed by 
erosion The actual thickness of the formation as shown here is probably 
-about 500 or 600 feet 
The Laramie beds are the youngest rock formations exposed in this 
district, with the exception of the Glacial and post-Glacial formations, 
which will be considered separately. 
PRE-CRETACKOUS UNCONFORMITY. 
At the close of the formation of the red Triassic sandstones a marked 
break in sedimentation occurred in this and the adjacent districts. This 
break was probably accompanied by a considerable uplift, for the succeed- 
ing beds of the Gunnison formation, which are of late Juratrias age, are 
1Annual Report of the Hayden Survey, 1874, p. 35. 
