Named as upper member of late Eocene Chadron Formation (revised) of the White River Group (revised). To avoid confusion, names proposed informally by Stone (1972, 1973) for his upper two members of the Chadron are retained here and his informal lower Amidon member becomes part of the basal late Eocene (Chadronian) Chalky Buttes Member (new). Named for town of South Heart. Type section is at Fitterer ranch in west face of a north-trending ridge in NW1/4SW1/4NE1/4 sec 7, T137N, R97W, 12 air mi southeast of South Heart and 1/2 mi northeast of section given by Stone (1973), Stark Co, southwestern ND. Forms "haystack" shaped hills. Described as brown to gray-green smectitic claystone commonly with thin lenses of algal crust-ostracod-gastropod-bearing freshwater limestone or marlstone >5 ft thick. Contains conspicuous fibrous calcite concretions as well. At type, smectitic claystone is medium brown to gray-brown; weathers to a "popcorn" texture at surface; drapes over units below, concealing thicknesses; capped by 1-ft limestone. Somber color and rounded, weathered slopes of the South Heart sharply contrast with cliff-forming, dazzling white Chalky Buttes Member below and cliff-forming pink-and-brown early Oligocene Brule Formation (revised) above. Considered excellent marker throughout western ND. Lithologically similar and partly equivalent to Chadron B and C beds of Schultz and Stout (1955) in Big Badlands, SD; "typical Chadron" of Lillegraven (1970) in Slim Buttes, SD. Total thickness at type section is 31 ft. Thicknesses average 10 to 23 ft at Little Badlands; 20 to 60 ft at Rainy Buttes; 8 to 55 ft at small buttes in eastern Stark and eastern Hettinger Cos; reaching maximum of 96 ft at White Butte, Slope Co. Conformably overlies Chalky Buttes Member; conformably underlies Brule Formation of White River Group (revised). Age is late Eocene based on fossil (brontothere) remains. Report includes geologic maps, measured sections, cross sections, and correlation charts.

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