The McCallum anticlines, embracing an area about 2 miles wide and 12 miles long, are about 6 miles east of the town of Walden, Jackson County, Colo., on the east side of the Continental Divide. A cover of flat-lying Quaternary gravel obscures the outcrop of Pierre shale at many points on the two anticlines. Wells start in the upper part of the Pierre and penetrate all of the underlying Upper Cretaceous series to the top of the Dakota sandstone, where production of oil and carbon dioxide is obtained. The first prospecting for oil was reported in 1912, but it was not until December 1926 that oil was discovered in commercial quantities.

Two geologic sections across the anticlines, extending to the outcrop of the Dakota sandstone at the base of the Medicine Bow Range, to, the east, disclosed a marked thickening between this formation and a fossiliferous sandstone in the Pierre shale which was used as a key bed. Immediately beneath the Dakota sandstone along ore of these cross sections a sandy volcanic ash resembling that found at the horizon of the Mowry shale in Wyoming was discovered. However, it is considered here the top bed of the Morrison formation.

The impossibility of obtaining dips along the crests, owing to the cover of terrace gravel, made it necessary to construct ten structure sections across these anticlines, using the geometry of conic sections in their development. From the geometric figures developed structure contours were obtained graphically.

The occurrence of carbon dioxide in large volumes in association with petroleum on each of these anticlines provides an unusual problem in oil production. Experimental data relative to the physical properties of this gas offer an approach to the solution of the problem. The applicability of such data to actual operating conditions is yet to be tried, but the work of early investigators in the field of physical chemistry suggests the feasibility of mixing air with the carbon dioxide to assure continuity of production, which heretofore has not been attained in this area.