Following truncation of the Mississippian sediments, much of the present State of Wyoming was tilted toward the south. Pennsylvanian seas advanced onto the shelf area of Wyoming both from the southwest and the southeast. In the southeastern portion of the State, a hinge-line developed in the vicinity of the tri-State area. This hinge-line separated a basin deep situated in eastern Colorado from a more stable, restricted bay or gulf which occupied much of eastern Wyoming. An entire sequence of Permo-Pennsylvanian sediments accumulated in this bay, and appears to have occupied an area very similar in outline to the present-day structural configuration of the Powder River basin.

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