An investigation was conducted to provide basic data on some of the characteristics of a modified in situ (MIS) oil shale retort after processing. Samples of retort contents and overburden were obtained from three core holes drilled into Rio Blanco's Tract C-a retort 1 in the western part of the Piceance Creek Basin, Colorado. The retort operation had been completed nearly four years before the coring, and the cavity and mine workings had been flooded by groundwater for almost one year. Cores were characterized by lithologic description, x-ray diffraction, and optical microscopy. Drilling and logging records indicate as much as 35 to 40 feet of roof rock has collapsed into the retort since the burn was terminated. A water-filled attic cavity 46 to 62 feet high exists at the top of the retort. One core hole penetrated 377 feet of rubble in the retort and floor rock with numerous fractures below the retort. Most of the material recovered from the retort consisted of highly altered, fused and vesicular rock. Lesser amounts of carbonized, oxidized and moderately heated-altered oil shale were recovered from the upper and lower parts. Raw shale roof fall at the top and unretorted oil shale rubble at the bottom are also present. Thermal alteration has produced high-temperature silicate minerals from the original mixtures of carbonate and silicate minerals in the raw oil shale. Adequate material was recovered from the retort contents to provide valuable data on the lithologic, mineralogic, and physical characteristics of the MIS retort. 19 refs., 12 figs., 17 tabs.