With most sand dunes, the upwind side has a gentle slope, and the downwind side, known as the slip face, has a steep slope. How steep depends on the shape of the sand grains. Each kind of object, from sand to rice to M&Ms, has a specific angle of repose. That is how steep a slope it can hold before the material slides down. It is important for engineers to know the angle of repose of an area's soil, in order to know where the ground will be stable, and where you are likely to have land slides.

In this case, the steep slip face on the left tells us that the wind was blowing from right to left.