Named as an informal unit, a bed in the interbedded shale and limestone unit in the middle part of the "Forestgrove member" (new, informal) [see Forestgrove member] of the Heath Formation. Named for Cox Ranch, T14N, R21E, Fergus Co, MT on the Central Montana uplift. No type locality designated. Is a 3.4 to 10.5 ft thick oil and metal-rich fossiliferous black shale interbedded with black fossiliferous limestone. Shale may be calcareous, pyritic. Limestone may be laminated. Upper and lower contacts are arbitrarily placed at top and base of interval that has a Fischer-assay oil yield of 7.5 or more gal per short ton [may not be feasible boundaries for geologic field mapping]. May extend over a larger area than study area. Recognized over 120 sq mi. Contains brachiopods. Of Mississippian age. Geologic map. Stratigraphic diagram. Measured sections.

Source: GNU records (USGS DDS-6; Denver GNULEX).