The weak layer and its depth below the model surface allowed failure of the sand layers to extend beyond the base of the slope, driving the the toe of the slide up as the head of the slide sank down to produce a headscarp and sag. Again, a weak layer well below the base of the slope causes the slope and a portion of its base to move together, with downward motion of the slope causing outward and upward motion of the toe. Movement of the darker sand layers beyond the base of the slope starts as soon as the top of the slide begins to move downward, showing that the areas are linked along the same sliding surface.