Large-scale sliding of geological sequences, with little internal deformation, is not a new idea. During the second half of the 19th Century, as the geology of the Alps, north-west Scotland, and Scandinavia was being unravelled, evidence emerged of lateral displacements of blocks many tens of kilometres long in the direction of movement. For example, Tornebohm (1896, p. 194) postulated movement of blocks at least 130 km long on Caledonian thrusts. The main difficulty in these ideas was in understanding the mechanics. The paradox was this: the strength of the rock limits the length of the block that can be pushed along a horizontal surface because, if the force applied to the end exceeds the strength of the material, the block will fail by internal shear at the end being pushed. The strength of rocks is quite inadequate to support the push required to move blocks longer than a few kilometres. On the other hand, if the block slides down a slope under the force of gravity, the previous difficulty is replaced by two others: the coefficient of friction of rock on rock suggests that an angle of about 30° would be required for gravitational sliding — and that also implies a vertical relief of about half the length of the block. The restrictions on relief limit the length of blocks that slide under gravity to a few kilometres.
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- Pore Water and Sliding
Richard E. Chapman
- Springer Netherlands
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