 Later, much of England was submerged in shallow waters as the polar ice sheets melted and the Tethys Ocean and Zechstein Sea formed, depositing shale, limestone, gravel, and marl, before finally receding to leave a flat desert with salt pans.The Mesozoic comprises the Triassic, Jurassic and Cretaceous periods (251–66 Ma).As Pangaea drifted during the Triassic period, England moved away from the equator until it was between 20° and 30° north.