Glossary of geological terms
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M.a. / Ma
Abbreviation used by geologists to denote ‘millions of years’ (actually ‘Mega annum’) – for example an event might be described as having happened 65 M.a. ago. ‘m.y.a.’ or ‘mya’ is also sometimes used for ‘millions of years ago’
Main Limestone / Prif Galchfaen Carbonifferraidd
Set of limestones deposited during the Visean stage of the Carboniferous period.
Malvern Lineament / Llinelliad Moelfryn
A north-south aligned zone of faults and folds which give rise to a series of landscape features including the Malvern Hills but extending from near Bristol to near Stoke-on-Trent and which defines the eastern edge of the Wrekin Terrane.
Marine band / Haenlin forol
A layer of sediment, particularly during the Carboniferous period, within which are found numerous fossils of sea-dwelling creatures. They mark definite points in geological time and so can be used to correlate rocks in different places. See also brachiopods.
Marros Group / Grwp Marros
The new official name for the Namurian succession of Millstone Grit rocks in South Wales.
Mesozoic / Mesosoig
Literally ‘middle life’ – the era of geological time comprising the Triassic, Jurassic and Cretaceous periods. Preceded by the Palaeozoic era and succeeded by the Cainozoic era.
Middle Coal Measures / Cystradau Glo Canol
Subdivision of the Carboniferous period. See geological timescale.
Millstone Grit / Grut Melinfaen
Coarse sandstones deposited during the Namurian stage of the Carboniferous period. These rocks are highly variable in nature, being very pure quartzite (>97% silica =SiO2) in some places.
Millstone Grit series / Cyfres Grut Melinfaen
Former subdivision of the Carboniferous period. This succession of mudstones, conglomerates and coarse sandstones is now referred to as the Marros Group. See Carboniferous timechart.
Mudstone / Carreg laid
Very fine grained sedimentary rock. The term ‘shale’ is sometimes used instead although its use is now out of favour and more properly refers only to well-layered mudstones which easily split along their bedding planes.
Mwmfri Sandstone Member / Aelod Tywodfaen Mwmfri
A thin bed of sandstone within the mudstones of the Cerig Formation laid down during the Silurian period.
Mynydd Myddfai Sandstone Formation / Ffurfiant Tywodfaen Mynydd Myddfai
Succession of sandstones, some of which are conglomeratic, laid down within the Ludlow epoch of the Silurian period.