The Entrada Sandstone is a formation in the San Rafael Group found in the U.S. states of Wyoming, Colorado, northwest New Mexico, northeast Arizona, and southeast Utah. Part of the Colorado Plateau, this formation was deposited during the Jurassic Period sometime between 180 and 140 million years ago in various environments, including tidal mudflats, beaches, and sand dunes. The Middle Jurassic San Rafael Group was dominantly deposited as ergs (sand seas) in a desert environment around the shallow Sundance Sea.