The leading edge of the dune is known as the slip face. This is the surface of the dune that collects the sand grains as they fall from suspension from the wind. The slope of this surface, called the angle of repose, is determined by the diameter of the material of the slope, in this case, the sand grains. The angle of repose of the dunes in the Provincelands is 34°. At angles greater than 34°, the slope becomes unstable and the sand falls forward, advancing the dune. In this picture, sand grains are flying over the crest of the dune and are falling onto the slip face causing the surface to exceed the angle of repose. The form in the middle of the picture has become unstable and is slumping forward or downward to the bottom of the picture.
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