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In , the Arenig (or Arenigian) is a time interval during the period and also the suite of rocks which were deposited during this interval.

History
The term was first used by in 1847 with reference to the "Arenig Ashes and Porphyries" in the neighbourhood of , in , . 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:

  1. A lower series of gritty and calcareous sediments, the "Arenig Series" as it is now understood;
  2. A middle series, mainly volcanic, with shale, the " Series"; and
  3. The shale and limestones of the or 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 rocks; this is not the case in South Wales.

The Arenig series is represented in North Wales by the and beds, by the Shelve series of the district, the of the Lake District, the of , and by the of slates and shale in and . It may be mentioned here that the "" Series of H. Hicks was equivalent to the bifidus shale and the Lower Llandeilo Series.

Geochronology
In the geologic timescale, the Arenig or Arenigian refers to an age of the epoch, between 477.7 and 470 million years ago, contemporary with the of the ICS,ICS; see based on a section in Sweden (Diabasbrottet quarry) and with the same boundaries. The Arenig and Floian rocks are the upper part of the Lower Ordovician and follow the (Gasconadian in North America) which is the lower part. Either is followed by the Middle Ordovician ICS or by the Llanvirnian of older chronologies. The Arenig and equivalent Floian are represented in North America by the upper three stages of the which is followed by the Middle Ordovician which is the lower part of the now shortened .

Events
The Arenig rocks were deposited during a sudden worldwide rise in sea level resulting in widespread marine transgression. The early Ordovician surge in marine diversity also began around this time.

Brachiopod fauna
of the Floian

of the Floian

of the Floian

of the Floian

of the Floian

of the Floian

Strophomenida
of the Floian

of the Floian

Cephalopoda

Upper
The following is a list of Actinocerid genera whose fossils are geochronologically found first in upper Arenig strata. These genera may survive into later portions of the Arenig stage, or even into later geological stages. This list should not be thought of in terms of the lifespan of the genera included.

Orthocerida
of the Floian

of the Floian

of the Floian

Lower
The following is a list of Endocerid genera whose fossils are geochronologically found first in lower Arenig strata. These genera may survive into later portions of the Arenig stage, or even into later geological stages. This list should not be thought of in terms of the lifespan of the genera included.

Upper
The following is a list of Endocerid genera whose fossils are geochronologically found first in upper Arenig strata. These genera may survive into later portions of the Arenig stage, or even into later geological stages. This list should not be thought of in terms of the lifespan of the genera included.

of the Floian

of the Floian

of the Floian

Lower

Upper

Trilobite fauna
Trilobites of the Floian

of the Floian'

of the Floian

of the Floian

of the Floian

of the Floian

of the Floian

of the Floian

of the Floian

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