In a translational slide, layers or numbers of layers on a sliding surface slide over an existing plane of weakness. This can be a thin layer of low stability deep underground, a sloping rock substratum with heavy clay cover or a layer of soil directly below the root horizon, for example. These areas often cause water to accumulate so that they become saturated and can act like a sliding surface. This type of slide can extend from square metres to square kilometres. It often occurs on flysch, calcareous marl shale or metamorphic shale.