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
Subdivision of the Carboniferous Period formerly used in South Wales. This succession of mudstones, conglomerates and coarse sandstones is now referred to as the Marros Group in South Wales. See Carboniferous timechart.
Moor Cliffs Formation / Ffurfiant Moor Cliffs
A thick succession (up to 1100m) of mudstones and siltstones laid down during the Pridoli Stage of the Silurian Period. Formerly known as the Raglan Mudstone Formation.
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.