Earlier studies of factors determining subaerial angles of repose of loose material have produced diverse results. In order to obtain dependable information for use in geomorphic investigations a controlled experimental study was undertaken. Results show that the angle of repose varies (1) inversely with size of fragments in perfectly sorted materials, but directly in those imperfectly sorted; (2) inversely with density of fragments; (3) directly with their angularity, roughness, and degree of compaction; (4) inversely with height of fall of material on free cones; (5) directly with increase of moisture up to the saturation point but inversely beyond that. A slope of repose convex in plan view is gentler than a plane slope, which in turn is gentler than a slope concave in plan view. Irregularities completely buried beneath a pile of loose material have no influence on the angles of repose unless the buried slope continues to a free edge over which the material slumps, when increased slope of the floor decreases the slope of repose of the overlying material.
The angle of sliding friction, like the angle of repose, varies inversely with size and density of fragments and directly with their surface roughness. For the same material, however, the angle of sliding friction is definitely lower than the angle of repose.
As examples of the practical application of these results to geomorphic studies, the following problems are discussed: the weathering back of a mountain front in a hypothetically “rainless” region; the characteristics of talus slopes; the convexity of divides; and the relation of fragment size to steepness of natural slopes of loose material.