Angle of repose is the steepest angle which the surface of a mass of loose particulate matter makes with the ground. Angle of repose is affected by the size, mass, angularity, and dampness of the particles, and by the force of downward acceleration, but it is consistent for a given material. Knowing the angle of repose of a material in various circumstances is vital for engineers and landscapers, for example.

The way in which most people are familiar with angle of repose is sand castles. Dry sand generally has an angle of repose of about 30°, so piled dry sand forms shallow inverted cones. When sand is dampened, the surface tension of the water binds the grains together, and the angle of repose is increased to more than vertical, so that walls and towers and sea caves can be made. When sand is too wet, however, the water lubricates the grains instead of sticking them together, and the angle of repose is decreased to nearly 0. This should be required knowledge for anyone who is thinking of buying a house built on a sandbar.

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Dry sand   Damp sand  Saturated sand

Angle of Repose is also the title of the Wallace Stegner novel that won the 1972 Pulitzer Prize for fiction. I'd never heard of it before researching this node, but it's the story, as told by a disabled 20th century man, of a strong-willed Eastern woman who travels through 19th century America.

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