The majority of the solid rocks of Cheshire are sedimentary rocks laid down during the Permian and Triassic periods. Both the east and west Cheshire Plains are immediately underlain by Triassic sandstones, siltstones and mudstones, although outcrops are restricted to those areas that are not covered by thick expanses of glacial till of glacio-fluvial sands and gravels, such as the Mid Cheshire Ridge and Alderley Edge.