Monument Valley, located on the Navajo Nation near the border of Arizona and Utah, is an iconic landscape defined by its towering, isolated rock formations. Known to the Navajo as Tsé Biiʼ Ndzisgaii, or “valley of the rocks,” the region features vast, sandy desert terrain punctuated by massive, vertical sculptures of red rock. The distinctive red color comes from iron oxide, a form of rust, that stains the sand and sandstone. The dramatic buttes and mesas are the final, enduring remnants of a much larger, layered plateau, reflecting hundreds of millions of years of geological history.
Laying the Foundation: Sediment Deposition
The initial phase of Monument Valley’s formation began approximately 300 million years ago, during the late Paleozoic era, when the area was a low-lying basin. This basin collected massive amounts of sediment from ancient mountain ranges, rivers, and shallow seas. This material compacted and cemented over millions of years, forming the layers of sedimentary rock visible today.
The most prominent formation is the De Chelly Sandstone, the remnant of wind-blown sand dunes from the Permian period. Below this hard sandstone lies the Organ Rock Shale, a softer, dark-red layer of mudstone and siltstone formed by streams in a tidal flat environment.
The Shinarump Conglomerate forms a resistant caprock on the highest formations. This upper layer consists of coarser-grained river deposits, including sandstone and pebbles. The stacking of these layers—soft shale at the base, hard sandstone in the middle, and tough conglomerate at the top—set the stage for the sculpting that followed.
Raising the Plateau: Uplift and Fracturing
Starting around 70 million years ago, the region became part of the Colorado Plateau and underwent a massive elevation event. This tectonic process, known as the Colorado Plateau uplift, raised the land thousands of feet above sea level.
The pressure from below caused the hard, brittle rock layers to fracture. These stresses created numerous vertical cracks and joints, establishing lines of weakness. This fracturing was a necessary precursor to landscape carving, as it provided pathways for water to penetrate the rock mass.
The Monument Valley region sits on the Monument Upwarp, a specific geological feature within the larger plateau. This localized uplift contributed to the fracturing and exposure of the rock layers, transforming the solid plateau into the isolated monuments seen today.
Sculpting the Icons: Differential Erosion
The final phase that created the valley’s buttes and mesas is differential erosion, which began after the plateau was elevated. This involves the unequal wearing away of rock, where softer materials erode much faster than harder materials. Water exploited the existing fractures and joints in the uplifted rock.
Water and wind widened these cracks, isolating massive blocks of the plateau from the main landmass. These large, flat-topped blocks are known as mesas, defined as being wider than they are tall. The sides, composed largely of the softer Organ Rock Shale, eroded quickly, forming steep aprons of debris at the base of the cliffs.
The harder Shinarump Conglomerate layer acted as a protective caprock on the top of these formations. This resistant cap slowed the vertical erosion of the underlying, softer layers, preserving the rock column beneath it. As the sides eroded faster than the top, the mesas gradually shrank in width.
When a mesa’s top becomes smaller than its height, it is classified as a butte, like the famous Mittens or Merrick Butte. This process whittles the buttes down into more slender forms. The ultimate stage results in tall, thin spires or needles, such as the Totem Pole, before the formation eventually collapses, leaving only the valley floor.