Between the Laramie Mountains and the Medicine Bow Mountains is a complexly downfolded area, the trend of which roughly paralles the sweeping arc of U.S. Highway 30 and the Union Pacific Railroad from Laramie to Rawlins. The area is comprised of the Laramie Basin and the Hanna Basin. The basins, which are shallowest to the south, reach a maximum depth in the northern part of the Hanna Basin north of the coal mining community of Hanna. The Hanna Basin is quite small as intermontane depressions go – only about 35 miles by 20 miles – but it is unique because of the great depth to which the sedimentary rocks are depressed. The floor beneath the sedimentary rocks lies about 30,000 feet below sea level.
The basins contain a thick sequence (up to 23,000 feet) of Upper
Cretaceous and Tertiary clastic sedimentary rocks derived in part from adjacent
uplands. The Hanna and Ferris Formations contain thick coal seams. Coal was
originally mined underground at old Carbon and later at Hanna and was used to
fuel steam locomotives on the Union Pacific Railroad; now coal is extracted by
underground and surface mining methods and is used as fuel in
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