In a small area in Sec. 6, T. 22 S., R. 44 W., Prowers County, Colorado, there are three roughly circular areas from 100 to 200 feet in diameter in which collapsed and brecciated masses of the Hays limestone member and Smoky Hill marl member of the Niobrara formation occur in contact with the uppermost part of the underlying Carlile shale. They are probably exhumed sink holes formed as a result of the development of solution caverns within the Greenhorn limestone or within soluble horizons in deeper lying rocks. As a result of repeated roof subsidence such caverns may have worked their way upward through the overlying rocks by a process of natural stoping, probably during Pleistocene time. The exposures now observed represent cross sections of the natural stopes at levels somewhat below the height they originally reached.