View northward across the Grand Canyon of the Yellowstone from near Point Sublime. Dark cliffs on the right are little-altered rhyolites erupted early in the postcollapse history of the Yellowstone caldera. In contrast, the brightly colored areas to the left reveal the effects of intense hydrothermal alteration of the same rhyolites. The deeper parts of the canyon, including most of the cliffs visible in this view, expose a rhyolite lava flow, the Canyon flow; the cliffs forming a ledge near the top of the cliffs are fused rhyolitic bedded ash-fall tuffs that were so hot as they fell that they welded into dense lava-like rock. The uppermost slopes, largely tree-covered but forming a white rim above the brown and yellow cliffs in the altered part of the canyon, are lacustrine sedimentary rocks, laid down in and near a lake that once filled the caldera before it was drained by downcutting of the Yellowstone River to form the Grand Canyon. Mountains on the skyline beyond the canyon rim are older volcanic rocks of the Absaroka volcanic field that lie outside the Yellowstone caldera.