After visiting Walnut Canyon, we drove east to Petrified Forest National Monument, where the snow caught up with us. Here, at the north edge of the park, students check out the colorful badlands of the Painted Desert, formed by the erosion of the Late Triassic Chinle Formation. The Chinle is rich in bentonite, a clay mineral formed from volcanic ash with a propensity to swell dramatically when wet. Our group was, thus, treated to a rare opportunity to watch the bentonites swell. (Wow.) The mesa on which we are standing is actually capped by the dark Tertiary Period volcanic Bidahochi basalts, a rare exception in this park dominated by the Chinle formation.