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The Southern New England Fold Belt of New South Wales in Australia is characterised by an arrangement of fault bound blocks of mildly deformed sedimentary rocks and interbedded volcanics. The geological blocks, which vary from hundreds of metres to tens of kilometres in extent, are bounded by faults that vary in width from metres to hundreds of metres. The strata may dip at any angle, but typically, the dips are in the range from 5 to 40 degrees. For strata dipping between 10 and 30 degrees, there are several examples of block sliding slope failures, where movement has occurred on surfaces parallel to bedding, allowing joint bounded blocks to separate to form cracks and chasms. In some cases, the areas affected are hundreds of metres wide, and the chasms are tens of metres deep. This paper describes two case studies of such occurrences. It explores the geological and topographical controls on stability and it identifies the relationships between geology and landform that are necessary to accommodate a kinematically admissible movement mechanism. Joints are found to be of similar importance to bedding in affecting rock mass stability. In particular, it is shown that the role of joints is more to act in combination than in isolation, and that the nature of this interaction may vary with position in order to achieve instability. It is noted that the presence of a greater number of different joint sets increases the likelihood of instability, as it facilitates a greater number of possible joint combinations.
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