Ordovician and Silurian rocks, mainly dolomitic limestone and dolomite, underlie the eastern half of Montana, nearly all of North Dakota and a large portion of South Dakota. They reach a thickness of about 2,000 feet near the basin center (where the cover exceeds 12,000 feet) but are bevelled radially by pre-Devonian and later surfaces of unconformity. Only Ordovician rocks escaped removal from areas of present-day outcrop at the southern rim of the basin.

For historical reasons the outcrop and subsurface nomenclature of the stratigraphic column differ, but a series of sedimentary units which can be traced in basin-wide stratal continuity exists. The Bighorn dolomite of the Pryor-Bighorn Mountains in Montana is demonstrably continuous with and equivalent to the combined Red River, Stony Mountain and Stonewall formations of the northern part of the basin. Similarly, the Whitewood dolomite of South Dakota equates with a small section of the lowest Red River.

Sedimentary thickness and distribution maps reveal former contiguity of the Williston and Nebraska-Iowa-Illinois depositional areas. During pre-Stony Mountain sedimentation the northern Nebraska and northeastern Iowa areas lay on the southern flank of a Red River basin centered in North Dakota.

Ordovician sedimentation (post-Beekmantown Deadwood formation) commenced with deposition of a sandstone on an erosion surface cut across Cambrian and Precambrian rocks. It continued with a shale and sandstone formation (Winnipeg, Simpson) which overstepped the initial sandstone and behaved as a basal deposit to the overlying carbonates of the Red River (Viola), and Stony Mountain-Stonewall (Maquoqueta) formations.

The lower three-quarters of the Red River comprise an uninterrupted succession of marine fossiliferous limestones, highly dolomitized at their periphery, but the upper quarter is a strongly rhythmic carbonate-evaporite sequence. These evaporites spread over most of the basin interior, the earliest having the greatest extent.

An influx of shale in the lower part of the overlying Stony Mountain abruptly smothered the evaporite rhythms. Evidence of facies variation within the lower Stony Mountain, coupled with a shift in the center of maximum accumulation of later formations points to appreciable re-disposition of sedimentary influences. Evaporite rhythms returned after the shale incursion and lasted through the remaining Stony Mountain and the Stonewall formations.

Silurian rocks, the Interlake group (Hunton), are almost wholly dolomite, characteristically pale in color and micrograined. Evaporitic anhydrite beds occur locally but have either the spread nor the rhythmic succession of those in the Ordovician. Correlation and subdivision of Interlake group sections is much aided by the existence of several persistent sandy non-sequential

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beds. The Interlake is variably eroded, but everywhere affected by the great pre-Devonian unconformity.

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