Glossary of geological terms


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.