The cliff in front of you shows a flood of lava in cross-section, preserving evidence that the forested Yellowstone Plateau was once covered with bubbling, hissing lava. The dark rhyolite cliff is pocked with volcanic bubbles, and waterfalls provide further clues because where the river reaches the edge of a hard lava flow, it erodes softer rock downstream, quarrying a steeper and steeper fall. Cracks in the plateau oozed lava for hundreds of thousands of years, and the most recent magma eruption was only 60,000 years ago, so volcanic activity is still possible here.