Mountain peaks constantly crumble, dropping rock fragments that pile up below. Most of the rubble pours from gullies and spreads out in cones that often merge into one another, forming a broad band of broken rock between valley greenery and the peaks. These fans can also alternate in vertical strips with forest. Talus consists of the larger fragments, usually big enough to be stepped on individually.

Talus slopes build gradually over the ages. On the oldest slopes, soil fills the spaces between rocks, locking them together to create smooth pathways. But talus can be very loose on volcanoes and younger mountains, where vegetation hasn't filled in the spaces.

Climbers on talus slopes need to keep alert because it's easy to knock rocks loose. Put your weight onto each rock slowly, ready to change your step if the rock starts to move. Keep close together so a rock set off by one climber can't gain dangerous momentum by the time it reaches other team members.

Adapted from Mountaineering, Freedom of the Hills, 6th Edition