Longmyndian Supergroup

The Longmyndian Supergroup is a 6,000-metre-thick (20,000 ft) sequence of Ediacaran (late Precambrian) rocks that outcrop between the Pontesford–Linley Fault System and the Church Stretton Fault System in the Welsh Borderland Fault System of England and Wales. The supergroup consists of two major geological groups, the Stretton Group and the overlying Wentnor Group. The successions represented by these two groups have also been referred to as the Eastern Longymndian and Western Longmyndian respectively, forming as they do the bedrock of the east and west parts of the Long Mynd in Shropshire. The rocks are a generally regressive sequence from basinal facies to clastic sedimentation. The rocks are thought to be derived from Uriconian mountains that were formed during the southward subduction of an oceanic plate beneath a continental block (ocean closure). The rocks have since been folded due to fault movements and plunge gently to the south. The Longmyndian rocks were deposited in northeast–southwest trending faulted rift basins. These were deposited on top of the Uriconian volcaniclastic deposits. Subsequent ocean closure squeezed these rocks together to provide sub-vertical bedding in the synclinal sequence seen in exposed rocks today. The deposited rocks of the Longmyndian show a variety of depositional facies that tell a story of ocean closure. The Longmyndian deposits rest upon Uriconian rocks but different sections are exposed exclusively between the Church Stretton Fault and the Pontesford-Linley Lineament.