​ARENIG GROUP, in geology, the name now applied by British geologists to the lowest stage of the Ordovician System in Britain. 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) as follows:—

Ordovician	${\displaystyle \left\{{\begin{matrix}{\mbox{ }}\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\{\mbox{ }}\end{matrix}}\right.}$	Caradoc	${\displaystyle \left\{{\begin{matrix}{\mbox{ }}\\{\mbox{ }}\end{matrix}}\right.}$	Dicranograptus—shales.
				Derfel or Orthis—limestone.
		Llandeilo Group	${\displaystyle \left\{{\begin{matrix}{\mbox{ }}\\\\\\\\\\\\{\mbox{ }}\end{matrix}}\right.}$	Rhyolitic ashes   = Upper	${\displaystyle \left.{\begin{matrix}{\mbox{ }}\\\\{\mbox{ }}\end{matrix}}\right\}}$
				Massive ashes   = Middle		Upper Ashes of Arenig.
				Acid andesitic ashes = Lower
				Daerfawr Shales.⁠Zone of Didymograptus Murchisoni.
				Platy ashes	${\displaystyle \left.{\begin{matrix}{\mbox{ }}\\{\mbox{ }}\end{matrix}}\right\}}$	Lower Ashes of Arenig
				Great Agglomerate		(Hypersthene Andesites).
		Arenig Group	${\displaystyle \left\{{\begin{matrix}{\mbox{ }}\\\\\\\\\\\\{\mbox{ }}\end{matrix}}\right.}$	Olchfa or Bifidus—shales (Didymograptus bifidus).
				Filltirgerig or Hirundo Beds	${\displaystyle \left.{\begin{matrix}{\mbox{ }}\\{\mbox{ }}\end{matrix}}\right\}}$	Didymograptus hirundo.
				Erewnt or Ogygia—limestone
				Henllan or Calymene—ashes	${\displaystyle \left.{\begin{matrix}{\mbox{ }}\\\\{\mbox{ }}\end{matrix}}\right\}}$
				Llyfnant or Extensus—flags		Didymograptus extensus.
				Basal Grit
(unconformity)⁠

The above succession is divisible into: (1) a lower series of gritty and calcareous sediments, the “Arenig Series,” as it is now understood; (2) a middle series, mainly volcanic, with shales, the “Llandeilo Series”; and (3) the shales 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 shales in Wicklow and Wexford. It may be mentioned here that the “Llanvirn” Series of H. Hicks was equivalent to the bifidus-shales and the Lower Llandeilo Series.