The obvious natural feature of this monument is the pure gypsum dunes, but perhaps less obvious are the sources of the dunes, Lake Lucero and Alkali Flat. These two areas are the result of the gradual drying of an extensive Pleistocene lake that was rich in the mineral gypsum, with the dunes being the result of weathering and wind transport of these exposed surfaces.
Along the margins of Lake Lucero, translucent golden-yellow crystals of selenite are exposed on the soil surface. These calcium sulfate (gypsum) crystals grow in saturated mud beneath the remains of Lake Otero and may reach lengths of four feet.
When exposed on the surface, these crystals are subject to weathering and erosion and may eventually become gypsum powder and sand grains, which can be carried by winds as dust or sand storms. These wind-transported particles, along with those from Lake Lucero and Alkali Flat, are often deposited to the north as the white sands of White Sands National Monument.
"Fossil" dunes found east and south of the main dunefield offer information about historical wind and climate regimes.