The dunes at White Sands are derived from gypsum-bearing marine deposits laid down in a shallow sea over two hundred million years ago. With the subsequent rise and fall of the surrounding region, the present day Tularosa Basin was created, nestled between the San Andres and Sacramento mountain ranges. Since no rivers drain the basin, runoff from the surrounding mountains, laden with gypsum and other sediments, is trapped in the basin. This water then pools in low spots, often forming huge shallow lakes. When that water evaporates in the windy, dry environment, it deposits gypsum in the form of selenite crystals, which eventually break down from weathering into sand-sized particles. The white sands can now be moved by the winds and shaped into the dunes that cover 275 miles of desert, creating the largest gypsum dune field in the world.