The colorful and fantastic shapes along these canyon walls are part of an ever-changing landscape. Horizontal layers of multicolored sandstone, clay, and shale are complemented by scattered beds of lignite coal and patches of pastel pink scoria. Scoria, or clinker, forms when soft lignite burns and bakes the surrounding clay to this bright color. Other layers contain concentrations of petrified logs and stumps of redwood, cypress, and cedar. Because the rock layers are easily eroded, the scene is constantly changing.