Glossary of geological terms



Palaeogene / Palaeogen

Name given to the first (older) of two new periods which replace the Tertiary Period.  It lasted 43 million years from 66 to 23 million years ago and precedes the Neogene.

Palaeozoic  / Palaeosoig

Literally ‘ancient life’ – the era of geological time which comprises the Cambrian, Ordovician, Silurian, Devonian and Carboniferous periods.  Succeeded by the Mesozoic Era.

Pangaea or Pangea / Pangaea

A super-continent which came into being during the Carboniferous Period before gradually breaking up to give the modern assemblage of continents.  It was surrounded by Panthalassa – the global sea.

Pembroke Limestone Group / Grŵp Calchfaen Pembroke (Penfro)

Modern name for the main Carboniferous Limestone of South Wales. Overlies the Avon Group.

Pennant Measures / Cystradau Pennant

Subdivision of the Carboniferous Period. See Carboniferous timechart.

Pennant Sandstone / Tywodfaen Pennant

Collective name for various thick sandstones laid down during the Carboniferous Period and forming the plateaux into which the Welsh Coalfield valleys to the south of Fforest Fawr have been deeply cut.

Pentre Bach Syncline / Synclin Pentre Bach

A major NE-SW trending downfold of rocks of Silurian age seen to the SE of Llandovery.

period / cyfnod

A major division of geological time.  The rocks formed within a period are termed a ‘system’.  See geological timescale.

Permian / Permaidd

Geological period which lasted 47 million years from 299 to 252 million years ago. See geological timescale.

Plateau Beds / Haenau’r Llwyfandir

A succession of hard-wearing sandstones laid down during the upper Devonian Period which form the summits of many of the higher hills of Fforest Fawr Geopark.

Pontar-Llechau Formation / Ffurfiant Pontarllechau

Thin succession of sandstones near the base of the Old Red Sandstone and of late Silurian age.  The (officially mis-spelt) name comes from Pont ar Llechau over the Afon Sawdde.

Precambrian / Cyn-Gambriaidd

All of geological time before the start of the Cambrian Period 539 million years ago.  Since the Earth is believed to have come into being nearly 4540 million years ago, it represents nearly 90% of all geological history.

Pridoli / Pridoli

Latest epoch of the Silurian Period during which the rocks of the Raglan and Temeside Mudstone Formations were laid down.  The Pridoli is also the earliest (ie lowermost) part of the Old Red Sandstone. See Silurian timechart.

protalus (or pronival) rampart / cefnen eirdreulio

Accumulation of rocky debris at the foot of a semi-permanent snow-patch.  Once the snow has melted, it remains as a stony ridge.

Psammosteus Limestone / Calchfaen Psammosteus

Distinctive thin limestone between the Raglan Mudstone Formation and the St Maughan’s Formation. Later termed the Bishop’s Frome Limestone but now known as the Chapel Point Limestone.


Quaternary / Cwaternaidd

Current geological period which has so far lasted 2.6 million years. See Quaternary timechart.