Analysis of 21 post-burn cores taken from the Hanna IV UCG site allows 96 m (315 ft) of overburden to be subdivided into four local stratigraphic units. The 7.6 m (25 ft) thick Hanna No. 1 coal seam is overlain by a laterally discontinuous, 3.3 m (11 ft) thick shaley mudstone (Unit A') in part of the Hanna IV site. A more widespread, 30 m (90 ft) thick well-indurated sandstone (Unit A) overlies the A'unit. Unit A is the roof rock for both of the Hanna IV cavities. Overlying Unit A is a 33 m (108 ft) thick sequence of mudstone and claystone (Unit B), and the uppermost unit at the Hanna IV site (Unit C) is a coarse-grained sandstone that ranges in thickness from 40 to 67 m (131 to 220 ft). Two elliptical cavities were formed during the two phases of the Hanna IV experiment. The larger cavity, Hanna IVa, is 45 x 15 m in plan and has a maximum height of 18 m (59 ft) from the base of the coal seam to the top of the cavity; the Hanna IVb cavity is 40 x 15 m in plan and has a maximum height of 11 m (36 ft) from the base of the coal seam to the top of the cavity. Geotechnical tests indicated that the Hanna IV overburden rocks were moderately strong to strong, based on the empirical classification of Broch and Franklin (1972), and a positive, linear correlation exists between rock strength and volume percent calcite cement. There is an inverse linear correlation between rock strength and porosity for the Hanna IV overburden rocks. 28 refs., 34 figs., 13 tabs..