sentences:  By the Triassic, the Permian Zechstein Sea had retreated and the climate had become a little wetter giving a gentle transition making the Permian-Triassic boundary uncertain in northern England as there are no fossil horizons or facies changes that make a definitive separation possible as there in continental Europe.The horizon however is characterised by a succession of red marls (calcareous mudstones) deposited on coastal flats, followed by the Sherwood Sandstone (formerly Bunter Sandstone).The 'British Isles' were not islands, but had an intra-continental position within Pangea.