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• Farewell Rock / Y Garreg Ddiffaith

Thick sandstone near the base of the Westphalian stage of the upper Carboniferous period.  Prominent within the valleys of the Mellte, Hepste and Nedd Fechan rivers.

• Farlovian or Farlow Series / Cyfres Farlow

Local stage name of the upper Devonian during which the Plateau Beds were deposited. See Devonian timechart.

• Ffairfach Grit / Grut Ffair-fach

Coarse sandstone laid down during the Llanvirn epoch of the Ordovician period.

• Ffinnant Sandstone Formation / Ffurfiant Tywodfaen Ffinnant

A succession of sandstones and mudstones deposited at the end of the Wenlock epoch of the Silurian period.

• Fibua Formation / Ffurfiant Ffibiwa

Mudstones and siltstones laid down during the Ludlow epoch of the Silurian period.

• Flandrian (stage) / Fflandriaidd

The last 10,000 years of geological time, since the end of the last Ice Age.  Also known as the 'post-glacial'.  See Quaternary timechart.