Subsequent movements can be attributed to a succession of slips, becoming smaller with time, in response to winter rainstorms. Degradation and softening of the slip mass would rapidly lead to secondary slips and initiation of the earthflow. The landslide probably started about 3600 years ago as a sudden large slip in the steep hillslope. By contrast the lower, earthflow sector, which is up to 18 m in thickness, has spread downslope by at least 400 m with little disturbance of the original ground.
The upper, slump sector has moved about 160 m on a curved slip surface within a shear zone of brecciated clay the shear zone, 2 m thick and in places at a depth of 30 m, lies above hard mudstone of the Edale Shales weathered mudstone and any superficial deposits, and several metres of unweathered mudstone, having been removed by shearing and incorporated in the slide debris.
It has a length of 1000 m, a maximum width of 450 m, and an average slope of 12 degrees from the toe to the foot of the back scarp. The ancient but still active landslide at Mam Tor, in Namurian mudstones, is a massive example of a slump-earthflow.
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