Not Exactly a Day at the Beach

White Sands National Monument
By Jan Bannan
White Sands
White Sands National Monument

It is quickly evident that the sand is different at White Sands National Monument. This stark white landscape is the world's largest gypsum dunefield, an ecological island of some 300 square miles surrounded by open prairies, ebony lava flows called Malpais, and jagged buttes of the Tularosa Basin. The dunes form a dazzling wave of moving hills undulating across the sun-drenched northern edge of the vast Chihuahuan Desert. The park was established on January 18, 1933.

Approximately ten million years ago, a huge block of the Earth's crust began to settle in this area when fault lines, running roughly north to south, gave way to create the Tularosa Basin. Left exposed were the layered sedimentary cliffs of the Sacramento Mountains to the east and the San Andres Mountains to the west. Called a "graben" because of the way it was formed, the basin has no outlet for its water.

For eons, rain and melting snow washed eroded sediments from the surrounding mountains into the basin, and a large body of water, Lake Otero, formed. The lake became saturated by a gypsum (hydrous calcium sulfate), which water had dissolved from certain layers of the cliffs.

What was once cold and wet terrain changed gradually as the climate became warm and dry. About 30,000 years ago, the lake slowly evaporated, leaving a remnant body of water now called Lake Lucero—the lowest part of the basin, where gravity is strongest—and a surrounding valley floor of exposed gypsum. Around 20,000 years ago, the stage was set for the subsequent gusting winds to play their role in the formation of the dunes.

When Lake Otero evaporated, gypsum crystallized into a form called selenite, a three-layered structure similar to that of mica. The selenite today lies around crusty Lake Lucero, a dry lake bed most of the year. After weathering flakes the crystals of selenite, the wind picks up the fine chips and tumbles them into fine grains of sand (remember, sand here is a generic terms that refers to size).

At White Sands, the southwest winds are the driving force that causes the gypsum sand to skip and hop, displacing other grains as they land while the entire dune surface flows or creeps to the northeast. Sandstorms in early spring, with gusts of more than 45 miles per hour, play a major role in moving sand; the rest of the year the breezes blow at a mild five miles per hour. Except for some late-winter northerly storm winds, when short-lived counter dunes form atop larger dunes, it is primarily unidirectional dunes from the southwest that shape the dunes.

The dunefield is 97 percent gypsum. Simple mounds near Lake Lucero modulate further on to form transverse dunes, which can break apart and give rise to barchan formations (crescent shaped with pointed tips or horns). Vegetation can invert these formations to produce parabolic shapes. The transverse formations are the largest ones here and can be 400 feet thick, with heights of 40 feet and crests that run 800 feet long.

© Article copyright Fulcrum Books. All rights reserved.

Published: 29 Apr 2002 | Last Updated: 15 Sep 2010
Details mentioned in this article were accurate at the time of publication


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