Arenig

thumb|right|Arenig Fawr, the mountain which lends its name to the geological series In geology, the Arenig (or Arenigian) is a time interval during the Ordovician period and also the suite of rocks which were deposited during this interval.

== History == 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.