Deep beneath the surface lies a thick layer of salts. Squeezed by the tons of rock above it, the salt flowed and bulged upward, creating long domes. The rock layers covering these domes were forced to crack into a series of more-or-less parallel lines.
The fin that Turret Arch is carving away is over 30 m (100 feet) wide, but the arch itself is relatively young and small. There is a secondary, smaller arch directly to the left of the main arch, and maybe within a few hundred thousand years or so the two of them will join to create a larger arch.