THE ROLE OF ROCK SLIDES IN THE DEVELOPMENT OF 'ROCK TOWNS' ON OUTCROPS OF PENNSYLVANIAN SANDSTONE

‘Rock Towns’ atop Lookout and Pigeon mountains in northwest Georgia and northeast Alabama are believed to have formed by the gravitational sliding of jointed blocks of Pennsylvanian sandstone over interbedded shale weakened by weathering. To better understand this process, we have investigated the outcrops at Cherokee Rock Village, Sand Rock, Alabama, based on images provided by Google Earth supplemented by aerial photography flown by drone. Vectors drawn for various blocks show a complicated pattern of movement influenced by original dip, local topography and joint directions. Structural dip is northwest at a gentle angle (± 5 degrees) but the attitude of contacts may be complicated by fluvial channeling at the base of the sandstones. The locality lies up-dip at the crest of a southeasterly facing escarpment. Two sets of joints are present, trending 045 and 085. While some blocks have migrated downdip, movement to the northwest is constrained depending whether or not beds ‘daylight’ on the dip slope. The majority of movement is to the southeast and therefore controlled by topography. Shale beds are apparently being preferentially weathered and eroded on the escarpment (sapped) allowing the sandstones to camber and slide southeast, eventually tumbling over the scarp. Depending on the position of adjacent elements some blocks seem to have followed the strike of joints. Some blocks have rotated and others have toppled onto their sides, but in general, blocks retain their original attitudes, giving the impression of street planning. They are readily reassembled, jigsaw fashion.