Nature
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The Upper Falls of the Yellowstone River formed where geologic conditions created a waterfall in a river channel at the meeting of harder rock and softer rock that erodes more easily and quickly. Here, volcanic and hydrothermal activity created the 109-foot (33 m) falls. About 480,000 years ago, lava formed a layer of rock that resists erosion and naturally cracks in a zig-zag pattern. Over time, hydrothermal springs rose through some of these cracks, altering and weakening the lava. The Yellowstone River then flowed through the zig-zag cracks and eroded its channel, and once it reached the softer, hydrothermally altered rock, erosion increased and created the Upper Falls.
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