Once movement of a rock perched on the top of a slope
has been initiated, the most important factor controlling its fall
trajectory is the geometry of the slope. In particular, dip slope
faces are important because they impart a horizontal component
to the path taken by a rock after it bounces on the slope or rolls
off the slope.
The most dangerous of these surfaces act as ‘ski jumps’
and impart a high horizontal velocity to the falling rock, causing
it to bounce a long way out from the toe of the slope.