The Sundance Sea did not occur at a single time; evidence suggests that it was a series of five successive marine transgressions. Each was separated by a period of erosion.The sea advanced and receded from the Middle Jurassic onwards. The terrestrial sediments of the Morrison Formation eroded from rising highlands to the west were deposited on top of the calcareous Sundance Formation as the sea retreated for the last time, late in the Jurassic.