The Fourmile Canyon Formation was named for exposures along Fourmile Canyon, in T. 27 N., R. 48 E., in the southern Cortez Mountains (Gilluly and Masursky, 1965).

Graptolites from the Fourmile Canyon Formation are Early Silurian in age. Gilluly and Masursky (1965) speculate that the somewhat younger and coarser grained Elder Sandstone may be part of the same depositional assemblage now separated by thrusting.

The Fourmile Canyon Formation is composed of chert, siltstone, argillite, and shale with a few thin beds of sandstone. In general, a higher percentage of chert seems to be present in the lower portion of the formation with more sandstone and shale in the upper portion. Most of the formation is interbedded dark-gray to black, massive chert, and tan to brown weathering shaly siltstone which commonly show small-scale current-type cross-bedding and are locally graded (Gilluly and Masursky, 1965). The cherts are organic-rich and essentially indistinguishable from those in the surrounding Slaven Chert and Vinini Formation. The diagnostic feature in the cherts of the Fourmile Canyon Formation is the presence of dolomite nodules up to 4 inches across which are not found in the surrounding cherts (Gilluly and Masursky, 1965). The cherts and siltstones grade into one another laterally.

The fine grained siltstone and sandstone are composed of about 75 percent quartz, 20-40 percent dolomite rhombs, 10 to 15 percent orthoclase and 5 percent muscovite (Gilluly and Masursky, 1965). Most of the quartz and dolomite are angular grains. There are only a few thin shale interbeds which have not been altered to argillite, and these fissile siliceous shales contain the graptolites which date the formation as Silurian.

Both the upper and lower contacts of the Fourmile Canyon Formation are structural. Gilluly and Masursky (1965) estimated the thickness of the Fourmile Canyon to be between 4,000 and 6,000 feet.

The Fourmile Canyon Formation is exposed only within the southern portion of the Cortez Mountains.

The details of depositional setting are poorly understood for the allochthonous Fourmile Canyon Formation. Poole and others (1977) suggested that the Fourmile Canyon Formation along with the Elder Sandstone represent inner-arc basin deposits. The depth of deposition is questionable for the Fourmile. The fine-grained current cross-bedded and graded siltstones, graptolitic argillites and shales, and dolomitic cherts probably represent deposition in slope environments at moderate water depths.