The Sea to the West
In Middle Jurassic time the land now called central Utah, which then was the eroded surface of the Navajo Sandstone, sank beneath an arm of a shallow sea that came in from the north, and most of the area remained beneath this sea until Late Jurassic time. Sediment carried into this sea and into bordering lagoons and estuaries later hardened into the sedimentary rocks of the Carmel Formation, Entrada Sandstone, and Curtis and Summerville Formations. The Carmel and Curtis contain abundant marine fossils of Middle Jurassic age, and in central Utah the intervening unfossiliferous Entrada also is believed to have been deposited in or near the sea, and the unfossiliferous Summerville Formation probably was deposited upon a tidal flat that was submerged part of the time.