Abstract : This work is a contribution towards understanding the complex phenomenon of sliding of sedimentary versants on bedding planes. We have combined the geological and mechanical approaches. Our purpose is to analyse the experiences and concepts that influence directly or indirectly this occurrence. A sedimentary versant is the result of a series of natural phenomena (deposition, diagenesis, epigenesis, tectonic stresses, erosion, etc.). These facts have determined or modified the slope's mechanical properties and characteristics (heterogeneity,. anisotropy, discontinuity, deformation, inclination, etc.). Here, it is most important to consider the scale effect. On the other hand, sliding of a versant occurs only if geological and mechanical conditions are favorable. It is the objective of a stability study to determine these conditions. Usual formulae and methods to compute the stability of rock masses are not a good model of versants. The size of these structures (10 4 to 10 8 m3) is at the origin of strong heterogeneity in the resistance to sliding and the equilibrium conditions. Tectonic deformation along joints may invalidate formulae usually employed to calculate shear resistance, particularly between two layers whose deformation modulii are very different. The evaluation of resistance to sliding on a bedding plane must consider, in addition to geologicaL conditions, the materials involved, the morphological characteristics, and water influence. It is concluded, that the stability analysis of a rock slope, should focus to determinate the influence of the variation of the different factors in control of equilibrium. The «security coefficient» reveals itself as lacking a useful intrinsic meaning. We present a back-analysis of "Claps-de-Luc", historic collapse, from which a significant part of the sliding surface is still visible. A topographic reconstruction is described. Finally, the probable cause of sliding is deduced from the analysis of the influence of several factors on the stability.