Upper Falls of the Yellowstone
Yellowstone National Park, United States
A waterfall is a clue that you are standing on a geologic crossroads.<br><br> A waterfall forms in a river channel where harder rocks meet softer rocks that erode more easily and quickly. Here, volcanic and hydrothermal activity have created the 109-foot (33 m) Upper falls.<br><br> <b>Why here?</b><br><br> 1 About 480,000 years ago, lava formed a layer of rock that resists erosion. The lava naturally cracks in a zig-zag pattern.<br><br> 2 Over time, hydrothermal springs rose through some of these cracks, altering and weakening the lava.<br><br> 3 The Yellowstone River flowed through the zig-zag cracks and eroded its river channel. Once the river reached the softer, hydrothermally-altered rock, erosion increased and created the Upper Falls.