Named Hayden group. These rocks were previously assigned to Division III of the Hartville formation [not used in this report] in earlier reports. May be up to 120 feet thick. Composed of: beds of medium light-gray to greenish-gray, silty, massive, dolomitic, fossiliferous limestone; medium dark-gray, calcareous shale; medium dark-gray, ochre to buff, thinly laminated mudstone; and fewer beds of gray, calcareous, argillaceous siltstone; dark-gray to red or pink irregular chert; and medium- to light-gray, laminated to cross-bedded, calcareous sandstone with worm tubes. Identified in two measured sections: (1) near Lake Guernsey; (2) at pump station in Hot Brook Canyon, west of Hot Spring, South Dakota, on the Chadron arch. Overlies Roundtop group (new); underlies Meek group (new). Could be a tongue of Fountain formation. Correlated with Marmaton group of Des Moines series. Age is Pennsylvanian. Report includes measured sections.
Named from Hayden Cliff, sec. 22, T. 27 N., R. 66 W. (the type locality), Platte Co., WY, in Denver basin.

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

Combined with overlying [/]Meek-Wendover groups to form a subdivision of Hartville formation in the subsurface, along the east side of Powder River basin in Niobrara, Weston, Crook, and Campbell Counties, Wyoming. Overlies [/]Reclamation-Roundtop groups; underlies Broom Creek group, all considered subdivisions of the Hartville formation. Hayden group correlated with upper part of the lower division of Casper formation in the Laramie Mountains, Wyoming, and with the upper part of Tensleep sandstone of the Bighorn Mountains, Wyoming. Age is Middle Pennsylvanian (Desmoinesian).
[The use of a hyphen between undifferentiated [groups] is not considered proper notation (CSN, 1933).]

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