Arches National Park, located in eastern Utah, preserves over 2,000 documented natural sandstone arches, pinnacles, and balanced rocks. This extraordinary landscape is the result of millions of years of complex interactions between deep, subterranean forces and relentless surface erosion. The park’s iconic scenery, characterized by its deep red and orange hues, is a visual record of this long geological history. Understanding the park’s formation requires looking beneath the surface to the unstable foundation upon which the visible rock structures rest.
The Foundation: Salt, Sand, and Sedimentation
The story of Arches National Park begins approximately 300 million years ago with the deposition of the Paradox Formation. This deep, unstable layer consists of massive salt beds created when a shallow sea repeatedly flooded the region and evaporated in the arid climate. The Paradox Formation, which is thousands of feet thick in places, acts like a weak, mobile layer buried beneath the younger rocks visible today.
Over millions of years, this salt layer was buried by younger, horizontally deposited layers of sand, mud, and silt, accumulating to a thickness of over a mile. Most of the park’s visible arches are carved from the Entrada Sandstone, deposited about 150 million years ago from vast coastal desert dunes. This sandstone is highly porous and held together by a natural cement, making it strong enough to stand but vulnerable to chemical weathering.
The Entrada Sandstone rests upon the less permeable Carmel Formation, a layer made of finer sand and clay. This contrast between the porous Entrada and the tighter Carmel layer dictates how water moves through the rock. Water is trapped at the base of the Entrada layer, accelerating the process of chemical dissolution.
Uplift and Fracturing: Creating the Fins
The buried Paradox salt layer became a driver in shaping the park’s surface features because salt, when subjected to immense pressure from the overlying rock, behaves like a viscous fluid. This pressure caused the salt to flow and bulge upward into elongated domes called salt anticlines. This upward movement began to warp and fold the thick blanket of sedimentary rock, including the arch-forming Entrada Sandstone.
A major episode of tectonic activity, known as the Laramide Orogeny, occurred between 75 and 50 million years ago, increasing the compressional forces on the region. This tectonic stress, combined with the continued movement of the underlying salt, caused the brittle overlying sandstone to fracture into a series of deep, parallel cracks and joints. These fractures were concentrated along the flanks of the salt anticlines, such as the Salt Valley and Moab anticlines.
The salt layer also underwent dissolution when slightly acidic groundwater reached it through these vertical fractures. As the underlying salt dissolved and was carried away by water, the overlying rock structure partially collapsed, forming long, parallel grabens (down-dropped blocks). The remaining vertical blocks of sandstone, separated by these parallel fractures, were then exposed by erosion, creating the towering, thin walls of rock known as fins.
Erosion’s Role: From Fins to Arches
Once the fractured Entrada Sandstone was exposed, the process of differential weathering began to carve the fins into arches. The arid climate of the region, which receives only 8 to 10 inches of precipitation annually, provides the ideal conditions for this process, as too much moisture would rapidly dissolve the sandstone cement, leading to collapse rather than arch formation.
The mechanism of arch creation is the constant attack on the fractured fins by water, ice, and gravity. Water seeping into the pores and cracks of the sandstone slowly dissolves the calcium carbonate cement that binds the sand grains together. During colder periods, water trapped in these fractures freezes and expands, exerting pressure on the rock walls in a process called frost wedging.
This persistent weathering enlarges the cracks and forms alcoves or openings at the base of the fins. As the opening is widened and deepened, often by water pooling on the less porous Carmel layer beneath the Entrada, a hole eventually erodes completely through the fin. The more resistant caprock and the pillars on either side remain, forming a stable arch structure. This process is continuous, and the forces that create the arches will eventually cause them to collapse, as exemplified by the fall of Wall Arch in 2008.