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.
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