In geology, the Arenigian (or `Arenig`) refers both to a time interval during the Lower Ordovician period and also to 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, N...