Extended from MT southward into Powder River basin, northern WY. Thickness is 8 ft in Shell Oil no. 1 Clear Creek well, NW/4 SE/4 sec 11, T57N, R78W, Sheridan Co, WY and 35 ft in Shell Oil no. 1 Crow Tribal well, NE/4 NE/4 NE/4 sec 36, T9S, R37E, Big Horn Co, MT; both wells are in Powder River basin. These wells are about 40 mi south of previously mapped southern erosional limit of Silurian rocks. Conformably overlies Stony Mountain Formation; unconformably underlies Duperow Formation. Core descriptions. A lenticular unit 3-10 ft thick directly above shale bed at top of Bighorn Dolomite and unconformably below Jefferson Formation at Little Tongue Canyon section, secs 27 and 22, T56N, R87W, Sheridan Co, WY, is tentatively assigned to Interlake. Late Ordovician and Silurian age.

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

Raised to group rank and divided (ascending) into: Strathclair, Fife Lake, Guernsey, Cedar Lake, Grondale (new), Mendenhall (new), Missouri Breaks (new), and Sherven (new) Formations in the central Williston basin subsurface of ND. History of nomenclature chart. Group rank assigned because each formation is "lithologically distinct," and each is a "non-uniform sequence of genetically disparate facies." Overlies Stonewall Formation of Bighorn Group. Underlies Ashern Formation of Elm Point Group. Of Silurian age.

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