How does moving sand create cross stratification inside dunes?

Cross stratification within sand dunes is primarily created by the migration of the dune's leeward slope, known as the slip face. As wind transports sand across the windward side of a dune, the air flow eventually separates at the crest, creating a "dead zone" on the leeward side where the wind velocity drops.12

Sand grains carried over the crest fall into this sheltered area. When the accumulation of sand on this leeward slope reaches the angle of repose—approximately 32° for dry sand—the slope becomes unstable, and the added material slips down the face. This process of continuous deposition and sliding creates inclined layers of sand that are deposited at an angle to the main horizontal bedding planes. As the dune migrates forward over time, these inclined layers are preserved, resulting in the characteristic structure known as cross-bedding.12