West Mitten Butte, a striking geological feature within Monument Valley, is located near the Arizona-Utah border within the Navajo Nation. This massive structure is one of the world’s most photographed landforms. The butte’s impressive height and distinctive shape result from millions of years of geological activity and weathering, rooted in the deep history of ancient sedimentary layers.
Sedimentary Layers and Dominant Rock Types
The bulk of West Mitten Butte is composed of layered sedimentary rock. The most prominent rock type is the Permian-era De Chelly Sandstone, which makes up the sheer, cliff-forming central section of the butte. This sandstone originated from vast, ancient desert sand dunes that covered the region approximately 280 million years ago. The massive cross-bedding visible in the rock layers is a signature of this windblown, or eolian, origin.
Beneath the De Chelly Sandstone lies the Organ Rock Shale, which forms the sloping base and skirts of the butte. This lower formation is a mixture of softer siltstone and shale deposited by ancient river systems. The entire sequence is capped by the Moenkopi Formation and the Shinarump Conglomerate, younger sediments from the Triassic period. These formations show a landscape that transitioned from river floodplains to a vast, arid desert.
The Role of Iron Oxide in Coloring
The vivid red and orange hues of West Mitten Butte result from a chemical staining process, not the inherent color of the sand grains. The coloration is primarily due to the presence of iron minerals within the sedimentary layers. Over geologic time, these iron compounds oxidized, forming hematite.
Hematite, which is iron oxide, acts as a natural pigment that coats the individual sand grains and the cementing material within the rock. This minute iron oxide content, often less than one percent of the rock’s total mass, creates the intense red coloration seen in the De Chelly Sandstone and the Organ Rock Shale. Variations in the concentration and hydration of the iron minerals result in the different shades of red, pink, and orange that stripe the butte’s vertical walls.
Geological Processes of Butte Formation
The shape of West Mitten Butte is the result of two major geological events: regional uplift and differential erosion. The entire region, part of the Colorado Plateau, experienced a gradual upward movement. This uplift elevated the horizontal rock layers to thousands of feet above sea level, exposing the ancient sedimentary rocks to the forces of nature.
Once exposed, differential erosion began to carve the landscape. Wind, water, and ice preferentially wore away the softer rock layers, such as the shales and siltstones of the valley floor. The harder, more resistant Shinarump Conglomerate caprock at the top of the butte acted as a protective shield. This dense layer slowed the weathering of the softer De Chelly Sandstone and Organ Rock Shale directly beneath it. The result is the steep-sided, isolated monolith that rises dramatically from the valley floor, where a strong caprock determines the final shape.