Cedar Mesa Sandstone. The slopes at their bases are usually composed of Halgaito Shale. As with other parks in the brightly colored sedimentary layers of the Colorado Plateau, erosion in Monument Valley is controlled by jointing. The vertical cliffs form in thick sandstone layers, and the gentler slopes form in thin-bedded weak shales. Buttes are smaller mesas.
Chimneys are column-shaped.
The two parks occur together in east-central Arizona. The exposed bedrock of the parks consists of shales and siltstones of the Early Triassic Moenkpi and Late Triassic Chinle Formations. The mud and silt, when later buried and compacted, transformed into shale and siltstone. The shales erode into a barren landscape, highly gullied. You can see different color stripes due to the different color beds of strata.