The Deadwood Formation thins southward and eastward form about 850 ft (259 m) in western North Dakota to an erosional edge in eastern North Dakota and central South Dakota. Thickness variations reflect pre-Middle Ordovician erosion, preexisting topography of the Precambrian surface, and depositional thinning eastward. The lower Deadwood consists of a clean, quartzose basal sandstone overlain by glauconitic carbonates and sandstones with minor shales. The upper Deadwood consists of less glauconitic to nonglauconitic sandstones and carbonates.

The Winnipeg Group consists of the Black Island and Ice Box Formations and Roughlock Sandstone. The Black Island Formation is composed of a lower red-brown and green sandstone and shale unit with a maximum thickness of about 100 ft (30 m) that is confined to the central basin area. The upper Black Island is primarily a quartzose sandstone with a maximum thickness of about 160 ft (49 m) in the central basin area. It thins to less than 20 ft (6 m) in eastern North Dakota and pinches out southward in southern North Dakota. The Ice Box Formation consists primarily of greenish-gray, noncalcareous shale with a thickness of 110-130 ft (34-40 m) in most areas of North Dakota; it thins southward to about 40 ft in the northern Black Hills outcrops and thins northward as it intertongues with the Black Island sandstone in Canada. The Roughlock Sandstone consists of greenish-gray calcareous shale grading upward into interbedded calcareous shale and argillaceous limestone in eastern North Dakota. It grades into a calcareous siltstone and fine-grained sandstone in south-central North Dakota and extends through north-central South Dakota to the outcrops in the northern Black Hills.

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