The most notable feature of Mam Tor is the active landslide which invades its southeast side almost to the summit, and interrupts the ramparts of the hillfort, unless its builders used it as part of the defences. This rotational landslide began roughly 4,000 years ago. The toe is a debris flow. The landslide is due to weak shales underlying sandstones, a common phenomenon all around the Dark Peak, notably at Alport Castles, Longdendale, Glossop and Canyards Hills, Sheffield. Indeed, three larger landslides occur on the north side of Mam Tor, one of them cutting the main ridge at Mam Nick which allows a minor road over into Edale; another creates the striking crag of Back Tor well seen from Mam Tor.