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SUMMARYCertain observed characteristics of scree slopes; namely concavity of profile, straight slope angle less than the angle of repose, and good size sorting of particles, are not consistent with an angle of repose model for accumulation. An alternative model is proposed based upon rockfall and surface stone movement and is tested against experimental data of particle movement in the field. It is found that the mechanical model of stone movement generated adequately explains the motion of particles on scree slopes and that it is in keeping with the characteristics of many screes. The static features of some Isle of Skye screes were also measured and the straight-concave slope form with good downslope sorting of material, characteristic of the rockfall process, was found to be present.

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Rotating drum experiments on the repose angles of mixtures of glass spheres have shown that φr (angle of shear) is strongly influenced by the proportions of the mixture. It was found that φr reached a peak value for the minimum porosity mixture; where the pore spaces between the large particles were just filled with small material; which was attributed to increased dilatation on the shear plane during avalanching. The geomorphic significance of these observations, in terms of slope development, is discussed. Secondly, the results of the experiments showed that, although more constant than φi (limiting angle of repose), φr was subject to some variation. Thus φr, as measured in a rotating drum, is not a true constant and can not be exactly analogous to φ'cv (angle of internal sliding friction at constant volume) as measured in a shearbox test—as has been previously suggested. It is tentatively suggested that at least some of the variability in φr is attributable to the magnitude of the immediately preceding value of φi, in that an unusually high value of φi, favours a lower value of φr due to the greater amount of kinetic energy released on failure.