An investigation was conducted to provide basic data on some of the characteristics of a modified in situ 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, ad optical microscopy.