Stacks of layered sediments and ice on Mars which are at least 4-6km thick in the north and 1-2km thick in the south. Cratered plains surround the ice caps and seem to extend beneath them, indicating that the ice covers the older craters and is, therefore, one of the youngest features on the planet. During the Martian winter, the ice caps grow in extent and are covered with CO₂ frost and ice. The summertime brings with it receding ice caps and sublimation of the CO₂ ice.