1. Map showing contours on top of the upper Cretaceous Mowry Shale, Powder River Basin, Wyoming and Montana Yale Public content 1991. 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.