Rockfall is a form of mass movement or mass wasting in which pieces of rock travel downward through some combination of falling, bouncing, and rolling after they are initially separated from the slope. The sizes of rockfall blocks can range from cubic centimeters to tens of thousand of cubic meters. Although some sliding may occur as the rock is becoming detached, sliding is a minor component of the process. Free fall typically occurs on slopes steeper than 76 degrees, bouncing on slopes between 45 and 76 degrees, and rolling on slopes below 45 degrees. Because slopes are commonly irregular, a rock may alternate between the three modes during its downslope movement. Talus slopes along the bases of cliffs are the products of uncounted rockfalls over thousands of years.

The size of rockfall blocks is controlled by bedding planes, joints, and fractures that form mechanical discontinuities and allow the blocks to become detached...