TERTIARY Paleocene Fort Union Formation SB Note: In areas marginal to the main study area, sequence boundaries are Hell Creek Formation unconformities. In the central study area, Maastrichtian in lowstand basins, correlative conformities Fox Hills Sandstone are lateral to the unconformities at several horizons. Bearpaw Shale TSE Judith River Formation Campanian

CEOUS Claggett Shale TSE Montana Group A Includes Telegraph Creek Formation at base Eagle Sandstone/Gammon Shale of Eagle Sandstone Santonian Niobrara Formation Coniacian SB Includes Bowdoin sandstone of subsurface Turonian Carlile Shale SB usage UPPER CRET At Bowdoin dome there is a local Greenhorn Formation unconformity at the base of the Greenhorn Mosby Sandstone Member Equilvalent to Phillips sandstone of subsurface usage Cenomanian Belle Fourche Shale SB Considered to extend to the top of the Muddy Mowry Shale Sandstone in some reports ado Group Shell Creek Shale TSE

Color Includes Viking Formation, Bow Island Sandstone, Muddy Sandstone SB Newcastle Sandstone, and Cyprian Sandstone CEOUS

A Member of the Thermopolis Shale Albian Skull Creek Shale Basal silt unit of Skull Creek Shale Commonly called Dakota silt by drillers Equivalent to First Cat Creek sandstone; Fall River Sandstone WER CRET TSE also called Dakota sand LO Aptian Kootenai Formation Includes Second and Third Cat Creek sandstones SB Probably eroded from Bowdoin dome; Swift Kimmeridgian JURASSIC Morrison Formation Formation may be top Jurassic formation there

Figure 2. Stratigraphic names and unconformities used in this report. Compiled from Rice (1976a), Porter and others (1993, 1997), Dyman and others (1995), Kauffman and Caldwell (1993), and Gries and others (1990). SB = sequence boundary; TSE = transgressive surface of erosion.