1. Maps showing coal-split boundaries, isopachs of coal splits, coal resources, and coal quality--Mammoth Coal Bed, Paleocene Tongue River member of the Fort Union Formation, Bull Mountain Coal Field, south-central Montana Yale Public content 1989. 2002-06-01T04:00:00Z. 2002-06-01T04:00:00Z. Geological Survey (U.S.). The Survey ; For sale by Map Distribution. English. Public. Montana and Fort Union Formation.