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The entire Ordovician, Silurian, Devonian, and upper Mississippian sections pinch out around the southern margin of the Williston basin. On the southeastern edge, the Pennsylvanian, Permian and Triassic also pinch out beneath the pre-Sundance unconformity.
Upper Cretaceous sands, whose source is to the east, pinch out on the west flank of the basin.
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