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 …