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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 Caradoc Dicranograptus—shales.
Defrel or Orthis
Llandeilo Group Rhyolitic ashes   = Upper
Massive ashes   = Middle
Acid andesitic ashes = Lower
Upper Ashes
of Arenig.
Daerfawr Shales. Zone of Didymograptus Murchisoni.
Platy ashes
Great Agglomerate
Lower Ashes of Arenig
 (Hypersthene Andesites).
Arenig Group Olchfa or Bifidus—shales (Didymograptus bifidus).
Filltirgerig or Hirundo Beds
Erewnt or Ogygia—limestone
Didymograptus Hirundo.
Henllan or Calymene—ashes
Llyfnant or Extensus flags
Basal Grit
Didymograptus extensus.

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.

References.—Adam Sedgwick, Synopsis of the Classification of the British Palaeozoic Rocks (1885); Sir A. Ramsay, “North Wales,” Geol. Survey Memoir, vol. iii.; C. Lapworth, Ann. Mag. Nat, Hist. vol. vi., 1880; G.A.J. Cole and C.V. Jennings, Q.J.G.S. vol. xlv., 1889; C.V. Jennings and G.J. Williams, ibid. vol. xlvii., 1891; Messrs Crosfield and Skeat, ibid. vol. lii., 1896; G.L. Elles, Geol. Mag., 1904; J.E. Marr and T. Roberts, Q.J.G.S., 1885; H. Hicks, ibid. vol. xxxi., 1875. See also Ordovician.

(J. A. H.)
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