Glossary of geological terms


Basal Grit / Grut Gwaelodol

Coarse-grained sandstone laid down during the Namurian Stage of the Carboniferous Period. The modern name for this rock unit is the Twrch Sandstone Formation. The ‘silica rock’ is a more informal term for this particular unit in connection with fire-brick manufacture. However not all of the Basal Grit is sufficiently pure silica for it to have been sought after by miners for this end-use.

Bishopston Mudstone (Formation) / (Ffurfiant) Carreg Laid Bishopston

Thick sequence of mudstones between the underlying Twrch Sandstone (Basal Grit) and the overlying Farewell Rock. Formerly known as the ‘Middle Shales’. The modern name derives from the locality in Gower.

boulder clay / clog-glai

An unsorted mix of clay and stones left behind when a glacier melts.  Also known as till.

b.p. / c.p.

Abbreviation used by geologists to denote ‘before present’ – for example an event may be described as having happened 28,000 years b.p.

brachiopod / braciopod

A group of sea-dwelling shellfish which thrived during the Carboniferous Period.  The fossilised remains of various species of Anthracoceratites, Gastrioceras and Reticuloceras are useful as marker horizons in the Namurian rocks of the Carboniferous Period.

Breconian or Brecon Series / Cyfres Brycheiniog

Local stage name of the Lower Devonian during which time the rocks comprising the Senni and Brownstones Formations were laid down. See the Devonian timechart.

Brownstones Formation / Ffurfiant y Cerrig Cochion

Thick succession of sandstones (up to 350m) laid down during the Lower Devonian Period