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Glossary of geological terms

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Tawe Valley Disturbance / Cylchfa Ffawtio-plygu Cwm Tawe

Linear zone comprising geological faults and folds stretching from Swansea up the Tawe valley to the Abercraf area where it becomes known as the Cribarth Disturbance.

Telpyn Point Sandstone Formation / Ffurfiant Tywodfaen Telpyn Point

This formation comprises the ‘Cumbriense Quartzite’ and overlying mudstones which were laid down during the Namurian Stage of the Carboniferous Period.  It is underlain by the Bishopston Mudstone and overlain by the Farewell Rock at the base of the South Wales Coal Measures.

Telychian / Telychaidd

Stage of the Lower Silurian Period during which the mudstones of the Cerig Formation were laid down. See Silurian timechart.

Temeside Mudstone Formation / Ffurfiant Cerrig Llaid Temeside

Mudstones and siltstones laid down at the start of the Pridoli Epoch of the upper Silurian Period

terrane / tiriogaeth grammenol

A section of the Earth’s crust, the rocks of which share a common geological history. The British Isles are composed of numerous terranes, several of which are now locked together to form modern day Wales. Most of the Geopark sits on the Wrekin Terrane. This and neighbouring terranes comprise the ‘Avalon Composite Terrane’.

Tertiary / Tertaidd

Name of a former geological period which lasted nearly 63 million years from 65 to 2.6 million years ago  (or to 1.8 million years ago according to older definitions). It has now been replaced by the Palaeogene and Neogene periods.  See geological timescale.

Tilestones  (Formation) / (Ffurfiant y) Teilfeini

Thin mica-rich sandstone laid down during the start of the Pridoli Epoch of the upper Silurian Period.  This formation also defines the lowermost part of the ‘Old Red Sandstone’.  As the name suggests, it was formerly quarried along much of its outcrop for roofing stone.

till / til

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

Tirabad Formation / Ffurfiant Tirabad

Succession of mudstones laid down during the Wenlock Epoch of the Silurian Period.

Towy (or Tywi) Glacier / Rhewlif Tywi

A glacier or river of ice which flowed down the broad valley of the present-day River Towy during one or more of the ice ages.  The furthest point downstream it ever reached was the Carmarthen area.

Tresglen Beds / Gwelyau Tresglen

Succession of rocks laid down during the Ludlow Epoch of the Silurian Period.

Triassic / Triasig

Geological period which lasted 53 million years from 252 to 199 million years ago. See geological timescale.

Trichrug Beds or Formation / Gwelyau or Ffurfiant Trichrug

Succession of sandstones and mudstones laid down during the Ludlow Epoch of the Silurian Period.

Twelve Foot Sandstone / Tywodfaen Deuddeg Troedfedd

A sandstone bed laid down during the Namurian division of the Carboniferous Period.  Prominent in the valleys of the Mellte and Hepste rivers.

Twrch Sandstone (Formation) / (Ffurfiant) Tywodfaen Twrch

New name for the ‘Basal Grit’ sandstone bed laid down during the Namurian division of the Carboniferous Period.