PHOENIX (AP) — Spanish explorers named them mesas, meaning tables, when they came upon them. But in geological terms, a mesa is simply a flat-topped mountain or hill. Millions of years of constant erosion — from the eternal forces of gravity and wind — have created the iconic mesas of the American Southwest.

For Native Americans, the original inhabitants of these lands, the elevation of the mesas formed natural fortifications that provided vantage points and tall sides that were supposed to protect them from outside threats.

They remain sacred landmarks with a deep spiritual connection. Old Oraibi, a Hopi village in Northern Arizona located on a mesa, is the oldest continuously inhabited community in the United States dating to 1150.

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LIKE SO MUCH American imagery, mesas lurk in our collective American unconscious from frequent appearances in films and television — many of them filmed in Monument Valley, Arizona.

Director John Ford used the impressive red mesas and buttes of Monument Valley as backdrops for “Stagecoach” and many more of his Western movies. Stanley Kubrick used Monument Valley to depict a distorted alien landscape at the end of the movie “2001: A Space Odyssey,” when astronaut Dave Bowman passes through the Stargate.

Real-life history has been made there, too. Robert J. Oppenheimer chose the Los Alamos mesa in New Mexico as the secret location to bring together a group of elite scientists to think on how to split the atom and create the deadliest weapon in human history. To this day, nuclear research continues at the Los Alamos Scientific Laboratory.

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I WAS YOUNG when I saw my first mesa.

I still recall watching it from a distance as we drove towards Acoma Pueblo, also known as Sky City, in New Mexico. That mesa stands 365 feet (111 meters) high. Acoma Pueblo is also one of the oldest continuously inhabited villages in the United States. It had been there for centuries before Hernando de Alvarado, of the Coronado expedition, came upon it in 1540.

One of the strongest memories I have of that place is the cemetery atop the mesa. It was an overcast day when I visited the graves of tribal veterans who fought and died in U.S wars. On this same mesa stands the San Esteban del Rey Mission, built between 1629 and 1640. All the construction material for the church, including the earth for the cemetery, had to be carried by foot up the steep mesa.

The strange rock formations that formed the mesas evoke the millions of years of this Earth’s age. But to me, they also summon the pain of a people being torn from their land — and the truth that nothing is permanent, that the world is forever changing.

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Part of a recurring series, “American Objects,” marking the 250th anniversary of the United States. For more American objects, click here. For more stories on the anniversary, click here.