Variety of landslide characterized by movement along a concave-up failure surface, so the moving material partly rotates about an axis at right angles to the slope. Typically, the upper part of the block fractures into a series of scarps arranged like steps tilted backward, and the lower part (toe) forms a fan-shaped bulge covered with radial cracks and ridges. Surface water may be retained in the depressed zone. The slump may be caused by basal undercutting, or by the over-steepening of artificial embankments. It is normally found in a uniform, relatively weak material, e.g. clay.
Rotational slip
The semicircular motion of a mass of rock and/or soil as it moves downslope along a concave face. Evidence for rotational slip in cirques comes from the dirt bands observed in cirque ice, which become progressively steeper from the back wall, but then flatten towards the cirque mouth.
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- Posidonius of Apamea (c.135–51)
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