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.