When a mass of loose, dry, purely frictional material slides down an incline after release at a given velocity, the runout (the distance the centre of gravity of the displaced mass moves from its initial position) depends on momentum transfer within the mass. This can be estimated from the profile of the debris accumulation, which also allows more accurate calculation of apparent angles of sliding friction in rockfall avalanches. The apparent extreme mobility of the Elm and Frank slides, typical rockfall avalanches, is explained by momentum transfer in a loose, dry, purely frictional material with an angle of friction of 30°. Key words: mass wasting, momentum transfer, landslide, accumulation, rock avalanche, runout distance.