Geology
Great sandstone layers once covered this region, but erosion has left the valley a wide flat plain, interrupted by formations that rise high into the air.
The area is a part of the Colorado Plateau, a region that covers 130,000 square miles within northwestern New Mexico, southeastern Utah, and northern Arizona. The valley floor is Cutler Red siltsone, or river-deposited sand.
The iron oxide in the weathered siltstone gives the valley its red color. The blue-gray rocks in the valley contain manganese oxide.
The sandstone layers can be clearly seen in the buttes. The lowest layer is Organ Rock shale, the middle de Chelly sandstone and the top layer is Moenkopi shale capped by Shinarump siltstone. Erosion of the soft shales of the Cutler Formation revealed the buttes as vertically jointed slabs of sandstone.
East Mitten Butte. Source: Wikimedia Commons
Organ rock shales are Permian deposits found in the Cutler red beds, layers of siltstones and shales found extensively on the Colorado plateau.These layers were deposited by streams carrying sediments from the ancestral Rockies.
The deChelly sandstone is crossbedded, a type of deposition that results in many different surfaces on an inclined rather than horizontal plane. Windblown dune deposits produce this type of deposition; this process can be seen to be occurring today at Great Sand Dunes National Monument.
Spread across the states of New Mexico, Arizona, Nevado, California, Utah and Colorado, the Moenkopi is a Lower Triassic formation, red in color and typically sandstone. In Monument Valley, the formation is represented by shale deposits.
Parts of the valley have been mined for uranium, which is found in some areas of the Shinarump siltstone.