During the summer of 1981 the Laramie Energy Technology Center conducted a post-burn coring program at the Hanna III Underground Coal Gasification site, Hanna, Wyoming. Detailed geologic studies were conducted on the altered and unaltered overburden as well as an analysis of the burn cavity. The overburden consists of about 46m of Paleocene Eocene Hanna formation above the Hanna No. 1 coal seam used in the burn the overburden contains two basic lithologic units: Unit A consists of very fine grained sandstones, siltstones, and claystones deposited as a lacustrine delta. Unit A is immediately above the Hanna no. 1 coal. Unit B is above Unit A and consists of carbonaceous shale's and mudstones containing isolated ventricular and tabular sandstone bodies deposited in a meandering fluvial system. The Hanna no. 1 coal accumulated in a poorly drained swamp that was subject to clastic flooding from an adjacent fluvial system.