fenneman] THE BOULDER, COLO., OIL FIELD. 323 
and it is even claimed that he set stakes to mark the proper location 
of wells. A part of Sheet XII of his published atlas has been freely 
used as "Hayden's oil map," and the area of outcrop of his Colo- 
rado formation (including the Pierre) has been largely advertised as 
"Hayden's oil belt." The belief even exists among some investors 
that his survey was made for the purpose of locating oil. 
GENERAL GEOLOGY. 
The Boulder oil held, so far as developed, has its center about 3 
miles northeast of the city of Boulder, Colo. Most of the wells which 
have attracted attention as producers lie in a north-south line a little 
less than 3 miles east of the prominent Dakota hogback which marks 
the eastern limit of the foothill belt. 
Stratigraphy. — Practically the entire Mesozoic group is represented 
in this district, and most of it is of interest in the study of the oil. 
At the base are 2,000 feet of Red Beds of the foothills consisting largely 
of coarse, red, feldspathic sandstones or conglomerates, upturned at 
a high angle and forming a sharp and rugged ridge. The upper 400 
feet are more argillaceous and vary more in color, from light-colored 
shales to dark-colored, ocherous, red shales and sandstones. These 
Red Beds rest upon the uneven surface of the pre- Cambrian and are 
themselves practically free from fossils and carbonaceous matter. 
They therefore form a definite base in which and below which it is 
useless to look either for the accumulation of oil or for its sources. 
All higher formations are associated in some way with indications 
of petroleum at various places in the Rocky Mountains, and in this 
connection will be mentioned below under the occurrence of the oil. 
Overlying these Red Beds is the Morrison formation, consisting of 
about 250 feet of clays and limestones, with argillaceous and calcare- 
ous sandstones. These beds, like the upper ones of the Red Beds, are 
easily eroded, and the line of their outcrop is marked by a continuous 
valley. They are not notably fossiliferous in this immediate vicinity. 
The Dakota sandstone (Upper Cretaceous) overlies the Morrison. 
Its outcrop, forming the well-known Dakota hogback, is, next to the 
crags of the red rocks, the most prominent feature of the foothill topog- 
raphy. The Dakota is here a 350-foot stratum of gray sandstone, some- 
times at the base conglomeratic and often at higher horizons quartzitic. 
Its outcrop would, on the whole, indicate that the Dakota is here as 
elsewhere a porous stratum, though its high dip toward the plains 
soon carries it beyond the reach of the drill. Like the strata below 
this is also poor in fossils. 
In general conformable on the Dakota are the Benton shales, 
dark and bituminous, with a thickness of 500 feet. Locally, as at 
the north end of the field, these shales become dense black limestone. 
Certain horizons are crowded with fossils, especially species of Ostrea 
and Inoceramus. 
