angle of repose

The steepest angle at which a pile of loose material — sand, gravel, talus, snow — can rest without sliding. Each material has its own angle: dry sand around 34 degrees, angular gravel steeper, round pebbles shallower. Exceed the angle and the slope fails. The concept applies to everything from sand dunes to avalanche-prone snowfields to the rock piles beneath cliffs. Wallace Stegner borrowed it for a novel title, and the phrase has resonated beyond geology ever since — the angle at which things come to rest, the maximum slope a life or a society can sustain.