A small number of sites in Pennsylvanian-age shale bedrock in Ohio, West Virginia, and Pennsylvania contain joints lined with rock-like material, similar in appearance to infilling material. Three unstable slopes, one each at Belle Valley, Ohio, East Steubenville, West Virginia, and Hopewell, Pennsylvania, were selected to investigate engineering properties of the lining material and the adjacent shale bedrock, as well as the role of lining material in contributing to slope instability. A detailed line survey was conducted at each site to determine the orientation and continuity of all joints intercepted by the line, and to examine the nature of the material lining the joints. The principal joint sets from each site were analyzed using stereographic projection techniques to determine potential modes of failure. The engineering properties determined for the lining material and the adjacent bedrock included Atterberg limits, grain-size distribution, slake durability, unconfined compressive strength, and shear strength parameters. The lining material was also examined in thin sections and by X-ray diffraction. This study shows that the rock-like material lining the joints consists of original shale bedrock altered by precipitation of hematite (Fe2O3), pyrolusite (MnO2), and goethite (FeOOH) from groundwater flowing through the joints. The lining material is stronger and more durable than the adjacent shale bedrock. The friction angle between the lining material and shale bedrock is higher at the Belle Valley site and lower at the East Steubenville and Hopewell sites. Wedge failures are the dominant mode of failure at all three sites.