Seashells in the Desert

Fossilized Shells

by Pamela Dimmick

Imagine that we're hiking in California’s blazing desert, 100 miles from the Pacific coastline.  Imagine the huge mountains and the winding canyons that would surround us.  Cacti dot the sandy landscape.  What's the last thing you would expect us to find?  Seashells!  Yet find them we will: in dried out washes, on an ancient shell reef, in canyon walls. 

Shell reef area

Shell Reef Area

We could also search America’s Great Plains, more than 1,000 miles east of where we’re hiking.  Any chance we might find seashells there, in the middle of the continent?  You bet.  The shells provide evidence of an ancient sea – the Western Interior Seaway – that existed in what's now America’s Heartland. Referring to an exploration of the California desert in 1776, Father Pedro Font wrote:

“I have come to surmise that in olden times the sea spread over all the land, and in some of the great recessions which histories tell us about, it left these salty and sandy wastes uncovered.  Indeed one finds on the way many piles of oyster shells, mixed with earth and half buried, and other shells and maritime signs.  It is not possible that people should have made such mountains of shells by carrying them from the sea so far a distance merely to bury them in piles.”

Shell Reef Sign

Font was describing what he saw in Yuha Basin, located 20 to 25 miles west of today’s El Centro, California.  The basin offers a vast shell bed of 6-million-year-old oyster fossils.  Erosion has left the dry wash streambeds littered with the fossils.  Strata in the streambed walls clearly show where oysters once covered the basin floor. 

California’s Shell Reef, located in the middle of an active off-road vehicle area in Anza Borrego State Park, is another treasure trove of fossils.  Dune buggies, ATVs, dirt bikes and other off-road vehicles zip past the reef, the riders oblivious to the ancient riches it hides.  Although a small fence protects the reef, you can still enter the area and examine the fossilized oyster shells.  Shell Reef formed about four million years ago. 

Seashells desert

Geological forces thrust the reef upward out of an ancient sea while the surrounding mountain ranges were forming.  Another great spot in Anza Borrego State Park to search for ancient seashells is Split Mountain, where you'll discover huge canyon walls, anticlinal folds (strata that dip in opposite directions from a common crest), and fossilized oyster shell reefs.  You'll also find a spectacular seasonal wash called Fish Creek Wash.  The north fork of the wash has several tributaries with names such as Oyster Shell and Mollusk Wash.  The clay hills surrounding these washes are loaded with fossilized shells. 

Shell fragments from the Fish Creek area

Shell Fragments from the Fish Creek Area

In the desert areas south of Anza Borrego State Park lie still other geologic formations rich in fossils, for instance the Imperial Formation in the Fish Creek and Coyote Mountains. Just outside the village of Ocotillo, Fossil Canyon’s sandstone and mudstone walls reveal veins of white shell fossils and pieces of coral.  What’s unique about this formation is the coral – it's of a variety found in the Atlantic, rather than the Pacific, Ocean. 

How did these fossils end up 3,000 miles away from the Atlantic?  At some time between 24 to 54 million years ago, a passageway across South America connected the Atlantic and Pacific oceans, affording the environment for the growth of Atlantic coral.  When geologic forces and changing conditions closed the passageway over time, the Atlantic coral in the Pacific died off, leaving the fossils as evidence of the connection.  Okay, so we found seashells in California, but seashells in South Dakota?  Kansas?  On mountaintops in western Texas?  Fossilized seashells found in these states reveal clues to the ancient Western Interior Seaway, which once split North America.  It covered much of the western interior of the continent between 69 and 80 million years ago.  It stretched from the Arctic southward to the Gulf of Mexico and from central Utah eastward to the western Appalachians. 

Over time, the Western Interior Seaway receded, leaving its floor exposed.  Vast rock and sediment layers remain, along with fossils from creatures that once lived in the sea.  The Pierre Shale is a formation deposited by the Western Interior Seaway.  Exposures occur in Canada and in many U. S. states, including South Dakota, Colorado, Nebraska, Minnesota, New Mexico and Wyoming.  The shale has fossilized shells, including ammonites, which are extinct ancestors of the famous spiral-shaped nautilus seashell.  Strangely, ammonites are often found in rocks with no fossils of bottom-dwellers.  The ammonites look like tightly coiled rams horns. In medieval times, many people mistook ammonites for petrified, coiled snakes. 

A different type of sea bottom remnant is Smoky Hill Chalk, a deposit more than 600 feet thick, located in western Kansas.  This sediment layer is near the center of the Western Interior Sea.  Over millions of years, it would be covered by thousands of feet of shale.  The chalk preserved many fossils, including giant clamshells, which reached sizes of four feet or more in diameter.  The clam’s giant size suggests a large gill area, an adaptation for life in murky, oxygen-poor bottom waters.  Schools of small fish took shelter within the shells of the giant clams.  Some clamshell fossils contain up to a hundred fish fossils inside.  Other treasures found in the chalk include pearls of the great clam. 


The Capitan Reef, a 400-mile long north-south trending reef that crosses the border between southern New Mexico and western Texas, is one of the premier fossil reefs of the world.  Many exposures of this massive reef appear on mountaintops, thousands of feet above today’s sea level.  Other sections remain underground.  A spectacular piece of the reef is a formation called El Capitan, which is located in the Guadalupe Mountains National Park, near the eastern edge of the Chihuahuan Desert.  This 8,000-foot-high peak towers above the surrounding desert landscape. 

Guadalupe Mountains National Park

El Capitan in the Guadaloupe Mountains

El Capitan is a chunk of the limestone reef that rose during the mountain building of the Guadalupe Mountains 10 to 12 million years ago.  Weather has eroded much of the surrounding sandstone.  Because of the limestone’s greater resistance to erosion, the towering cliff remains, revealing its trove of seashells. 

We’ve found seashells from shore to shore, on mountaintops and places in between.  Chances are, you have seashell fossils in geologic formations in your home state.  Be sure to check local regulations about fossil collecting, which is often prohibited.  Take a camera and shoot plenty of photos but leave the fossils behind for future generations.

Fossil Hunting in the Yuha Basin
A Visit to Fossil Canyon and Painted Gorge
Guadalupe Mountains National Park


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