Triassic 250-205 Ma ( Arran at 32-39 degrees North)

The initiation of the breakup of the supercontinent Pangea caused a rise in sea level and change of climate during the Triassic.

On Arran, this is represented by extensive lacustrine and shallow marine shales and sandstones of the Triassic. The climate was still hot however, and the shallow lakes often dried out to leave features such as dessication cracks and pseudomorphs of halite crystal in the representative mudstones. Broadly associated with the Murcia Mudstone Group of the Upper Triassic in England, the basal beds of the Triassic on Arran are named the Auchenhew Beds and outcrop just south of the King’s Cave on the west coast.

These parallel bedded siltsones and mudstones are interbedded with fine sandstones and are interpreted as being coastal and intertidal deposits. The Triassic on Arran forms the country rocks over a large area in the south, and excellent coastal sections can be seen from Largybeg Point to Drumadoon. Often dipping gently to the south, the well bedded rocks of the Trias have permitted the intrusion of many Tertiary igneous sills and sheets. The quartz-phorphyry felsite sill forming the Doon cliff if a fine example, which sits on a mudstone of the Auchenhew Beds. Xenoliths of Triassic siltones have been found in the Central Ring Complex as blocks foundered during the caldera collapse, these belong to the Dereneneach and Westbury formations.