You pass Wilson Arch on the way into Moab from the south along US 191. It was named for Joe Wilson, an early pioneer who lived nearby. It is Entrada sandstone like most of the material in the area, and the primary building material within Arches NP.
The action of water seeping into cracks and fissures, freezing and melting repeatedly over eons, slowly and selectively destroys the integrity of the sandstone. It first forms a series of 'fins', blades of rock standing vertically. In some of these fins, seemingly at random, the centers decay under the relentless torment of wind and water. When that happens, an arch is formed.