Eroded sedimentary rock. Flat-topped plateaux formed during the Permian period (between 286 and 245 million years ago). The cliffs have horizontal layers of soft shale, capped by harder, more resistant sandstone. Over time, erosional forces such as water, ice and wind have acted on the rocks, producing these distinctive flat-topped cliffs known as mesas or buttes. Photographed in Monument Valley, Arizona, USA.