Main Page | See live article | Alphabetical index The Entrada Sandstone was deposited in the Colorado Plateau, Colorado, USA, during the Jurassic Period somewhere between 180 and 140 million years ago. The sand grains and siltstones of tidal mudflats, beaches and sand dunes make up the 3 members of this rock formation: the Dewey Bridge, Slickrock and Moab members. The lower unit, comprised of siltstones, sandstones and mudstones, is the Dewey Bridge member, named after the type locality at Dewey Bridge, Utah. This brick-red layer has a blocky look to it. The middle layer is the Slickrock member, named for the type locality at Slickrock, Colorado; Rounded beach sands were cemented together to create this uniform layer. The Moab Tongue member is the uppermost deposit, named after the type locality of Moab, Utah. The whitish sands from inland dunes make up this "cap rock" layer, as seen atop Delicate or Broken Arch in Arches National Park. This article is from Wikipedia. All text is available under the terms of the GNU Free Documentation License.