Fins, Arches National Park, Utah, Postcard

"Sandstone Fins Courtyard, Arches National Park, Utah" by Catherine Sherman. A narrow sandy passageway among the red sandstone fins of Arches National Park in Utah leads to this secret courtyard, sheltering shrubs and a small tree losing its leaves in early Autumn. A fin is a geological formation that is a narrow, residual wall of hard sedimentary rock that remains standing after surrounding rock has been eroded away along parallel joints or fractures. Fins are formed when a narrow butte or plateau develops many vertical, parallel cracks. There are two main modes of following erosion. The first is when water flows along joints and fractures and opens them wider and wider, eventually causing erosion. The second is where the rock type (stratum) is harder and more erosion resistant than neighboring rocks, causing the weaker rock to fall away. Fins are considered an intermediary stage of many other erosional geologic features like windows, arches, and hoodoos. The formation of such features is a simplified four-step process. The first step is uplift that results in deep parallel, vertical fractures within the plateau. The second step is weathering and erosion that enlarges the fractures, producing fins. The third step is erosion attacking fins from below, causing windows and arches. The fourth step is continuing weathering that enlarges holes, causing arches and holes to collapse, creating spires or hoodoos. Arches National Park contains more than 2,000 natural sandstone arches, including the well-known Delicate Arch, which constitute the highest density of natural arches in the world. It also contains a variety of other unique geological resources and formations. The national park lies above an underground evaporite layer or salt bed, which is the main cause of the formation of the arches, spires, balanced rocks, sandstone fins, and eroded monoliths in the area.