During Paleocene time, the Granite, Wind River, Washakie and Owl Creek Mountains were rising highlands that enclosed the Wind River Basin on the south, west, and north. Fluvial deltaic, lacustrine and paludal sediments of the Fort Union Formation accumulated in the subsiding basin. The streams flowed basinward, as evidenced by paleocurrent data for cross-bed sets in fluvial sandstones that crop out along the southern and western margins of the basin. The streams probably flowed across the southern margin of the present Powder River Basin to empty into the Cannonball Sea.