Triassic (245 - 208 million years ago)
During the Triassic period the Zechstein sea had retreated and the climate had become a little wetter. A Lack of fossils makes the Permian-Triassic boundary uncertain in Northern England. The horizon is characterised by a succession of red marls (calcareous mudstones) deposited on coastal flats, followed by the Sherwood Sandstone. The sandstones were fed by rivers mainly from the south.
Towards the end of the Triassic, high rates of evaporation returned and low lying areas and North East Yorkshire became sabkha environments (a wide area of coastal flats). Periodic flooding caused by spring tides and strong on-shore winds followed by intense evaporation results in the precipitation of carbonate-sulphate and halite deposits. It was in this type of environment that the Mercia Mudstone Group was deposited.