Devils Tower was the first declared United States National Monument, established on September 24, 1906, by President Theodore Roosevelt. The Monument's boundary encloses an area of 1,347 acres (5.45 km2).
In recent years about 1% of the Monument's 400,000 annual visitors climb Devils Tower, mostly through traditional climbing techniques.
The landscape surrounding Devils Tower is composed mostly of sedimentary rocks.
The oldest rocks visible in Devils Tower National Monument were laid down in a shallow sea during the Triassic period, 225 to 195 million years ago. This dark red sandstone and maroon siltstone, interbedded with shale, can be seen along the Belle Fourche River. Oxidation of iron minerals causes the redness of the rocks. This rock layer is known as the Spearfish Formation.
Above the Spearfish formation is a thin band of white gypsum, called the Gypsum Springs Formation. This layer of gypsum was deposited during the Jurassic period, 195 to 136 million years ago.
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