Greenhorn Formation (Kgr): Gray to light gray calcareous shale and shaly marl with thin beds of limestone. Shale contains white to pink calcareous specks. Marine. Thickness as much as 100 m (30–75 ft). Belle Fourche Formation (Kbf): Gray to black shale with ironstone concretions and numerous bentonite beds. Mosby Sandstone Member (upper part of formation in north- and east-central Montana): brown sandstone locally with chert pebbles, interbedded with gray shale. Big Elk Sandstone Member (lower part of formation in southwest-central Montana): light gray, chert-rich sandstone commonly stained dark red, interbedded with thin, dark gray to black clayey shale. Marine shelf. Thickness as much as 260 m (853 ft).