How does a dune slip face form?

A dune slip face forms as a result of wind patterns and the physical properties of sand. As a dune grows, its leeward slope becomes increasingly steep until the wind can no longer follow the surface contour, causing the airflow to separate and create a "dead zone" in the lee of the dune.1

Sand transported up the windward slope falls into this sheltered area. When this accumulation of sand reaches the angle of repose—typically about 32 to 33 degrees—the slope becomes unstable. At this point, the added sand slips down the incline, maintaining this characteristic angle. This process allows the dune to remain in equilibrium, effectively migrating forward as sand is eroded from the windward side and deposited on the lee.21