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: • A lower series of gritty and calcareous sediments, the "Arenig Series" as it is now understood; • A middle series, mainly volcanic, with shale, the "
Llandeilo Series"; and • The shale and limestones of the
Bala or
Caradoc Stage. It was to the middle series (2) that Sedgwick first applied the term "Arenig". In the typical region and in North Wales generally the Arenig series appears to be unconformable upon the
Cambrian rocks; this is not the case in South Wales. The Arenig series is represented in North Wales by the
Garth Grit and
Ty Obry beds, by the Shelve series of the
Corndon district, the
Skiddaw Slates of the Lake District, the
Ballantrae Group of
Ayrshire, and by the
Ribband Series of slates and shale in
Wicklow and
Wexford. It may be mentioned here that the "
Llanvirn" Series of
H. Hicks was equivalent to the bifidus shale and the Lower Llandeilo Series. == Geochronology ==