__ HOW A SAND HEAP IS BUILT ? __

A sand particule slides towards the nearest edge.

A ridge separates the two sides.

A ridge is therefore at equal distance of the edges of the two sides.

Top side view of a sand heap

The ridges are thick marked.

The arrows indicate the path of the sand when it flows.

Two corresponding arrows are equidistant.

__ POSITION OF THE RIDGES __

formed by the edges of the two sides is obtained.

Starting from a quadrilateral, and by moving one side, two possible

orientations for the the ridges are obtained.