During deposition of the Tensleep Formation, a shallow, semirestricted portion of a major seaway that occupied the geosynclinal area to the west extended into the area of the present-day Bighorn basin. Limiting the transgression of this sea was the Beartooth high on the north and the Bighorn high on the east and southeast. On the western side of the area, a southerly extension of the Yellowstone high restricted circulation. The lower Tensleep Formation (Desmoinesian), characterized by extensive marine influence, was deposited as coastal sand dunes and interdunes over subaerially exposed structural highs. These deposits grade basinward into shoreface sandstones, which in turn grade into sandstones and carbonates of the shelf environment. During deposition of upper Tensleep strata (Missourian through Virgilian), marine waters were less widespread. The Greybull arch, a northeast-trending feature in the northern part of the area, was uplifted, dividing the shallow sea into two parts. The upper Tensleep Formation was deposited as a terrestrial sand sea over the Bighorn high. Coastal dunes and interdunes were deposited seaward of the sand seas and over the Beartooth high, the Greybull arch, and the southerly extension of the Yellowstone high. These deposits grade basinward into clastic shoreface deposits. Following Tensleep deposition,more » the region underwent southward tilting, which caused exposure and erosion of the Tensleep Formation. The resulting unconformity surface was deeply incised by a dendritic drainage system that controlled the thickness of the formation. The Greybull arch and the Bighorn high acted as significant drainage divides, over which very little of the formation was preserved.« less