1. Geologic map showing thickness of sedimentary rocks from the ground surface to the top of the upper Cretaceous Pierre Shale in the south half of the Powder River Basin, northeastern Wyoming and adjacent areas Yale Public content 1995. 2002-06-01T04:00:00Z. 2002-06-01T04:00:00Z. Geological Survey (U.S.). The Survey ; For sale by Map Distribution. English. Public. Powder River Basin (Wyo. and Mont.). Cretaceous.