The term was first used by Adam Sedgwick in 1847 with reference to the Arenig Ashes and Porphyries in the neighbourhood of Arenig Fawr, in Merioneth, North Wales. The rock-succession in the Arenig district has been recognized by W. G. Fearnsides (On the Geology of Arenig Fawr and Moel Llanfnant, Q.J.G.S. vol. lxi., 1905, pp. 608-640, with maps). The above succession is divisible into: