The angle of friction denotes the angle that an inclined plane assumes with respect to the horizontal when a body lying on it just starts to move (static friction angle) or slides slowly at constant speed (dynamic friction angle, sliding angle). The angle of friction θ corresponds to the arctan of the coefficient of friction μ.

Typical values of friction coefficients for different rock types:

Class / Friction angle (dry) / Associated rock types

Low friction / 20° - 27° / shale (high mica content), slate, marl.

Medium friction / 27° - 34° / sandstone, siltstone, chalk, gneiss, slate

High friction / 34° - 40° / basalt, granite, limestone, conglomerate

The values apply to dry stone surfaces. If the surfaces are wet, the friction angle decreases sharply.