During the late Miocene, initiation of motion on the West Salton detachment fault caused rapid subsidence of the Salton Trough region, resulting in a marine incursion between 6.3 and 4.3 million years ago. This resulted in the Imperial Formation, composed of locally derived fossiliferous shallow marine sandstones and conglomerates that were deposited in a shallow inland sea across much of the Salton Trough. In the Coyote Mountains, these rocks were deposited directly on the older metamorphic and volcanic rocks. The Imperial Formation preserves many corals as well as shallow marine invertebrates and vertebrates.