White Sands National Monument

The formation of the gypsum dunefield in White Sands National Monument started 10,000 years ago when the ice age ended and Lake Otero began to dry up. Lake Otero covered much of the Tularosa Basin and once completely dried up, gypsum precipitated from the water that was evaporating. The dunefields are ever-changing due to the amount of gypsum sand available, the power of the wind, vegetation, and soil moisture which can influence how far the sand drifts. One of the visible marks for wind direction that is prevalent throughout the national monument are ripple marks (Fig. 1).

Figure 1. above shows ripple marks and in the center of the image is a layer gypsum crust that formed over the sand as a result of overnight dew crystallizing the gypsum in the sand. Arrow shows wind direction according to ripple marks.

Gypsum can form to have a variety of different structures while maintaining the same chemical formula (CaSo4 * 2H2O), also termed a polymorph. Gypsum varieties include selenite, alabaster, and satin spar, two of which were seen in the field (Fig.2).

Figure 2. (A) large selenite crystal identified by its clear appearance and perfect cleavage, pencil is 18cm (B) alabaster identified by its lack of shape. Both samples were located at a selenite/alabaster mine.

References

White Sands National Monument (2018). Geology of a Gypsum Dunefield. [Brochure]