Talus cones on north shore of Isfjord, Svalbard, Norway, showing angle of repose for coarse sediment This angle is equal to the arctangent of the coefficient of static friction μ s between the surfaces. The term has a related usage in mechanics, where it refers to the maximum angle at which an object can rest on an inclined plane without sliding down. Material with a low angle of repose forms flatter piles than material with a high angle of repose. The internal angle between the surface of the pile and the horizontal surface is known as the angle of repose and is related to the density, surface area and shapes of the particles, and the coefficient of friction of the material. When bulk granular materials are poured onto a horizontal surface, a conical pile will form. If a small amount of water is able to bridge the gaps between particles, electrostatic attraction of the water to mineral surfaces will increase the angle of repose, and related quantities such as the soil strength. The angle of repose can also be affected by additions of solvents. The morphology of the material affects the angle of repose smooth, rounded sand grains cannot be piled as steeply as can rough, interlocking sands. The angle of repose can range from 0° to 90°. At this angle, the material on the slope face is on the verge of sliding. The angle of repose, or critical angle of repose, of a granular material is the steepest angle of descent or dip relative to the horizontal plane on which the material can be piled without slumping. Sandpile from the Matemateca ( IME-USP) collection

Wallace stegner angle of repose review