Rock slope failures involving secondary toppling and/or slumping of joint-bounded columns due to weathering of underlying materials appear similar to slab-type rock falls which are common in many parts of the world. The writers' experience with such rock falls has been primarily in the Appalachian Plateau of the Eastern United States (Hamel & Adams 1974; Gray et al. 1979). This experience prompts the following comments:

Lateral stress relief accompanying valley erosion (or escarpment development) in flat-lying interbedded strong and weak sedimentary strata produces vertical to sub-vertical tension joints in strong brittle beds (e.g., massive sandstones), diagonal to curved shear joints and fissures in weak deformable beds (e.g., claystones), and mylonites or shear zones at contacts between beds of different strength and stiffness (Ferguson & Hamel 1981). These features define the geometry of rock fall masses and also increase weathering and erosion potential of underlying beds. Ground water flow through tension joints in brittle strata is commonly perched on underlying argillaceous strata of lower permeability. This ground water flow contributes to softening and alteration of argillaceous strata and of mylonite seams, if any, at the tops of these strata. Creep, tilting, and differential settlement of overlying rock blocks is thus enhanced (fig. 4(c) of Ferguson & Hamel 1981).

Within this framework of geology and geometry, rock fall mechanisms typically involve differential weathering and erosion which undercut weak argillaceous beds beneath joint-bounded rock blocks. Whether a rock block topples (with forward rotation) or slumps (with backward rotation) depends on its geometry, the

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