During the last ice age, 24,000 to 12,000 years 

ago, the climate in this region was considerably 

colder and wetter. A large cap of glacial ice 

was perched atop present-day Sierra Blanca. 

As the earth warmed approximately 11,000 

years ago, ice melt and rain 

flowed down the mountains 

into Lake Otero. Water 

flowing down the mountains 

dissolved gypsum in the 

surrounding rocks and 

carried it to the bottom of 

the basin. 

Gradually, the climate 

became more arid and Lake 

Otero receded, leaving 

behind large deposits of 

gypsum on the floor of the 

Tularosa Basin. Lake Otero 

disappeared by about 10,000 

years ago. Today, all that 

remains of Lake Otero is a 

small remnant called Lake 

Lucero at the foot of the 

San Andres Mountains in 

the southwest corner of the 

monument. The gypsum 

dunefield began to form 

 shortly after Lake Otero 

 dried up.

A Source of Gypsum Forming the Tularosa Basin An Ancient Lake

A curious white line on the horizon, White Sands has long sparked wonder 

in people passing through the Tularosa Basin. 

Standing on top of a dune overlooking this 

sea of sand, it can be difficult to imagine 

where all the brilliant white sand came from. 

To understand the origins of the world’s 

largest gypsum dunefield, we must look 

back over millions of years during which 

just the right geologic and climatic processes 

took place in just the right order. Many of these 

processes continue today, allowing 

us to witness the formation of these 

unusual sands. 

Geology of a 

Gypsum Dunefield

Beginning about 70 million years ago, pressure 

from colliding tectonic plates pushed up the 

land from Colorado to Mexico. These forces 

eventually formed the Rocky Mountain range.

The plates began pulling 

in the opposite direction 

about 30 million years 

ago, pulling the crust 

apart and forming 

mountains and lowlying basins along fault 

lines. The Tularosa Basin 

and its surrounding 

Sacramento and San 

Andres mountains 

formed at this time.

Two to three million 

years ago the ancient Rio 

Grande flowed along 

the southern edge of the 

Tularosa Basin. Sand 

deposits from the river 

blocked off the southern 

edge of the basin, 

preventing snowmelt 

and rainfall from 

flowing out of it. This 

allowed for the formation 

of the 1,600-square-mile 

Lake Otero

The geologic story of White Sands began 250 

million years ago when earth’s continents were 

still joined in the megacontinent Pangaea. At 

this time much of the Southwest was covered 

by the shallow Permian Sea, including parts of 

present-day Arizona and New Mexico. 

Over millions of years, sea levels rose and 

fell many times. In times of evaporation, 

concentrations of calcium and sulfate in the 

water increased enough to combine and 

form the mineral gypsum (CaSO4

•2H2O). 

Thick layers of gypsum were deposited in 

sedimentary rocks on the bottom of this sea.

White Sands National Park Service

U.S. Department of the Interior

White Sands National Monument

Lake Otero once covered much of the Tularosa Basin. 

When it dried, gypsum deposits were left behind on 

the basin floor.

250 million years ago

Gypsum was formed and deposited in the Permian Sea.

70 million years ago

Tectonic forces pushed 

up the land and 

the ancient seabed 

containing gypsum 

deposits

30 million years 

ago

The crust began 

pulling apart, 

creating Tularosa 

Basin.

24,000-12,000 years ago

Ice age conditions formed a 

glacier on Sierra Blanca. Rain 

and snowmelt fill Lake Otero.

2-3 million 

years ago

The Ancient Rio 

Grande River 

blocked off 

southern part of 

basin, allowing 

formation of 

Lake Otero.

10,000 years ago

Ice age ended. Lake 

Otero dried up. 

Dunes began to 

form.

New Mexico

Colorado

Arizona

Utah

Extent of the Permian Sea about 280 million years ago 

© Ron Blakey, from Ancient Landscapes of the Colorado Plateau
Water with dissolved gypsum in it flows to 

Alkali Flat and Lake Lucero during wet parts 

of the year, usually during July and August 

monsoons. There, the heat of the sun causes 

the water to evaporate, leaving the gypsum 

behind. Gypsum precipitates out of the water 

in the form of crystals. Smaller crystals form 

on the surface of these playas. Larger crystals 

form under the muddy surface of Lake 

Lucero and are shaped like long jagged spars 

and round segmented disks the size of bicycle 

tires. The crystal form of gypsum is known 

as selenite.

Although crystal growth continues in 

Alkali Flat and Lake Lucero, most of the sand 

in the dunefield came from crystals formed in 

ancient Lake Otero, which was much larger.

Lake Lucero is a remnant of ancient Lake Otero. The same 

processes that formed crystals in Lake Otero continue today 

in this playa.

Each year some of the crystals emerge from 

the protective mud surface of Lake Lucero as 

wind and rain remove sediments from around 

them. Once exposed, crystals begin the gradual 

process of erosion into smaller and smaller 

pieces. 

Gypsum is a soft mineral—softer than a 

human fingernail—so it breaks down easily. 

Water seeps into cracks, freezes, and breaks 

fragile sheets of selenite apart. The process 

continues over time until large crystals become 

small shards. 

When pieces become small enough that they 

can be picked up by the wind they are known 

as “cornflakes,” and this is when things really 

speed up. Grains of gypsum picked up by the 

wind bounce along the surface and leap-frog 

over each other, breaking down into smaller 

and smaller pieces. 

As the soft grains collide with the ground and 

with one another their surfaces get scuffed. 

These scuffs reflect light rather than letting 

it pass through and give the gypsum sand its 

distinctive snowy appearance.

Today, rainfall in the mountains still flows 

down the sides of the Tularosa Basin. Water 

dissolves gypsum in rocks as it travels and 

settles in low areas of the basin. Much of it 

settles in Alkali Flat and Lake Lucero. These 

areas are ephemeral lakes, or playas, that 

fill with water only parts of the year. On the 

surface of these playas, the final steps in the 

formation of rare gypsum sand can still be 

seen today.

The dunes at White Sands probably began 

forming as late as 7,000 to 10,000 years ago, 

when Lake Otero started to dry up. The dunes 

are shaped by prevailing southwesterly winds, 

which pile the sand up and push dunes into 

various shapes and sizes. The different types of 

dunes depend on the amount of sand available, 

the power of the wind in a particular spot, and 

the vegatation anchoring sand in place. 

Because of the consistent wind, dunes are 

evershifting. Some dunes in the heart of the 

dunefield can travel twelve to thirteen feet per 

year, keeping any thing but the occassional 

grasses from taking root. Others, on the edges of 

the dunefield, move only inches to a few feet per 

year and are firmly held in place by a variety of 

desert plants.

Gypsum is a common mineral in rocks all over 

the world. Basins are common too, even here 

in New Mexico. So, why is White Sands the 

world’s largest gypsum dunefield? Water!

Water is the secret ingredient necessary 

for gypsum sand to form. Although it is 

not obvious, White Sands is actually a very 

wet environment. These dunes are at one 

hundred percent humidity. Only a few feet 

under interdunal areas lies what is called a 

perched water table—water that is held above 

the normal water table found in the rest of 

the basin by a barrier, in our case a layer of 

impermeable clay. This shallow water table 

acts like a glue holding the dunes in place, 

preventing them from blowing away with 

the wind.

Lake Lucero From Crystals to Fine Grains A Sea of White Sand

Heavy rain in 2006 flooded interdunal areas throughout the monument. Normally, White Sands has a perched 

water table that lies one to two feet below the surface of interdunal flats. During this storm, the unusually heavy 

rainfall raised the water table to the surface, which resulted in extensive flooding in the dunefield for six months. 

Selenite crystals on the surface of Lake Lucero

A Secret Key Ingredient