Gravel being added to a cone, maintains a constant steepness.
A granular material, when piled on top of itself, will develop an angle of repose, a constant angle at which it can maintain itself without sliding --when more material is added to the top, it steepens the slope, thereby causing it to slide and return to that angle. The angle of repose varies for different materials... those with more cohesion naturally have steeper angles.
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