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Glossary of geological terms

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sandstone / tywodfaen

Sedimentary rock composed of sand-sized particles.

Sawdde Fault / Ffawt Sawdde

Major fault running NE-SW through the lower end of the Sawdde gorge and which defines the NW edge of the Myddfai Steep Belt.

Sawdde Sandstone Formation / Ffurfiant Tywodfaen Sawdde

Succession of mudstones, siltstones and sandstones laid down during the Wenlock Epoch of the Silurian Period.

sedimentary rock / craig waddod

Any rock formed by the deposition, particle by particle, of sediments either under water or on land.  They include siltstones, mudstones, sandstones, coal and limestones.  One of the three fundamental types of rock , the other two being igneous and metamorphic.

Senni Beds / Gwelyau Senni

Former name for the Senni Formation

Senni Formation / Ffurfiant Senni

Succession of sandstones laid down during the lower (= early) Devonian Period.

series / cyfres

The rocks formed within the division of geological time known as an epoch. See geological timescale.

shakehole / llyncdwll

A depression in the ground surface due to the solution of a soluble rock such as limestone beneath the surface. Otherwise known as a doline. A shakehole into which a stream flows is often called a swallowhole. They are usually conical in shape and within the Geopark they can vary in size from less than 1m across to more than 100m across and over 20m deep.

shale / sial

Mudstone which readily splits along its bedding planes.  The term has fallen out of favour these days.

silica rock / carreg silica

Particularly pure formation of gritstone ie in excess of 97% silica (= SiO2) once mined and quarried for use, after grinding down, for making refractory bricks.  The ‘silica rock’ is in fact the Basal Grit (now known as the ‘Twrch Sandstone Formation’) of the Millstone Grit series (now known as the ‘Marros Group’) originally laid down in the Carboniferous Period.

silica sand / tywod silica

Readily quarried form of the silica rock (see above) in which natural processes have fragmented the original rock.

siltstone / carreg silt

Sedimentary rock composed of silt-sized grains (very fine).

Silurian / Silwraidd

Geological period which lasted 24 million years from 443 to 419 million years ago. See geological timescale.

St Maughans Formation / Ffurfiant St Maughans

A thick succession (up to 850m) of mudstones and siltstones laid down during the Lower Devonian Period.

stadial / is-gyfnod rhewlifol

A length of time within an ice age when cold (glacial) conditions persisted.  Stadials are separated by ‘interstadials’.

stage / uned stratigraffig

Succession of rocks formed within the minor sub-division of geological time known as an age. See geological timescale.

stratigraphy / stratigraffeg

The study of stratified rocks and their relationship to one another. See geological timescale.

swallowhole / llyncdwll

See ‘shakehole’.

syncline / synclin

A downfold of rocks on any of a variety of scales from metres to tens of kilometres.  The South Wales Coalfield occupies a large syncline on the northern flanks of which sits Fforest Fawr Geopark.  See also anticline.