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Sierra Madera

August 30, 2010

US highway 385 south of Fort Stockton, Texas, is pretty lonely. You can drive for miles without seeing another human being, and just about everything that lives there bites, pokes, or stings. But the drive offers one thing you won't find anywhere else in the country: hills formed by an ancient cosmic collision.

The hills are known as Sierra Madera. They're at the center of an impact crater that formed perhaps a hundred million years ago, when a space rock as big as a football field slammed into Earth. It gouged a crater about eight miles across. The crust at the center of the crater rebounded, creating the hills.

Wind and rain have worn away much of the crater's rim, so today only a few ridges remain. The central hills have eroded, too, but they still rise up to 800 feet above the surrounding landscape.

The hills helped geologists identify the crater. Similar hills are often created by the motions of Earth's crust, which deform layers of rock far below the surface. When scientists drilled around Sierra Madera, though, they found that the underlying rock layers were undisturbed. What's more, some of the rocks at the surface had been subjected to a powerful shock wave. So 50 years ago, geologists proposed that Sierra Madera was created by a cosmic impact. It's one of the few impact craters in the United States, the only one with central mountain peaks -- and the only one you can drive through.

More about cosmic impacts tomorrow.

Script by Damond Benningfield, Copyright 2010

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