Much of the area was covered with volcanic ash (the Bandelier tuff) from an eruption of the Valles Caldera volcano 1.14 million years ago. The tuff overlays shales and sandstones deposited during the Permian Period and limestone of Pennsylvanian age. The volcanic outflow varied in hardness; the Ancestral Puebloans broke up the firmer materials to use as bricks, while they carved out dwellings from the softer material.