All of these formations are of Arenig age with the exception of the lowermost which is Tremadocian. The Group is the equivalent of north-west England's Skiddaw Group and south-east Ireland's Ribband Group. The Manx Group is poorly fossiliferous but acritarchs and graptolite provide some insight into the succession's biostratigraphy. The succession has been subject to low-grade metamorphism. A microgabbro was intruded into rocks of the Creggan Mooar Formation to the east of Peel during this period. Manx Group sandstones were used in the construction of Peel Castle. Snaefell, the high point of the Isle of Man, together with the ridge extending north-eastwards to the island's second highest summit, North Barrule are formed from the mudstones of the Barrule Formation. The same rocks also form the summit of South Barrule, highest summit in the south of the island. A celebrated site in the far south is 'The Chasms' where large fissures have opened up in the coastal cliffs of the Mull Hill Formation's quartz-arenites. Nearby Spanish Head and the Calf of Man are formed from Lonan Formation rocks.