Anglesey’s Cambrian, Ordovician and Silurian rocks….

Anglesey’s Cambrian, Ordovician and Silurian rocks together record the destructive plate margin sediments of the Iapetus Ocean. …

Anglesey’s Cambrian Rocks (542 – 488 Ma)

The Cambrian geological period covers the time span 542-488 Ma. Adam Sedgwick and Roderick Murchison coined the term ‘Cambrian’ in 1835 after their pioneering geological studies in Wales. During the Cambrian Period, England and Wales lay deep in the southern hemisphere, roughly near the margins of present-day Antarctica. Until recently, it was thought that there were few, if any, rocks of Cambrian age on Anglesey. However, rocks of the New Harbour and South Stack groups, once thought to be Precambrian in age, are now assigned to the Cambrian.

Warming seas after late Precambrian glaciation saw an explosion of life in the Cambrian and many marine animals, including hard-bodied, shelled creatures such as trilobites and brachiopods, first appeared and evolved. Cambrian rocks also preserve a diverse range of trace fossils such as animal tracks, trails and burrows. Cambrian-age worm burrows occur in the rocks of the South Stack group but are very sparse and hard to find. The National Museum in Cardiff has a good collection of fossils from this period from mainland Wales, described in this article

South Stack Group: 

this group consists of bedded alternating sandstones and siltstones, including some massive quartzites. These are clearly turbidites – clastic sedimentary rocks deposited by turbulent underwater currents laden with debris in a marine sedimentary basin.

New Harbour Group:

these, too, are mostly turbidites, deposited about 500 million years ago, but their grainsize is typically much finer, with abundant, greenish silty or muddy layers (pelites) alternating with thin sandy layers (psammites). The sediment is chiefly the reworked product of island-arc volcanoes. The group also includes some bedded cherts, tuffs and spilites – undersea lavas of ocean-ridge affinity – metamorphosed into mafic greenschists. Less competent (rigid) than the South Stack Group, the strata usually show spectacular, intense folding. 

Gabbros and serpentinites: apparently restricted to intrusion into the New Harbour Group, these basic to ultrabasic intrusive rocks are highly altered in places (with conversion of olivine to serpentine). Locally, they have been quarried for ornamental serpentine, and at one locality, for chromite.

Most of Anglesey’s Cambrian rocks were deposited as sands and muds in the Iapetus Ocean and then plastered onto the underside of an opposing tectonic plate in an accretionary prism.

Holyhead Mountain

Quartzite on Holyhead Mountain (photo: John Conway)

New Harbour Schist

New Harbour Schist (photo : John Conway)


Anglesey’s Ordovician Rocks (488 – 443 Ma)

Anglesey contains a large area of Ordovician rocks but only smaller tracts of Silurian strata. The Ordovician were not defined until 1879 when Charles Lapworth  recognised the geological overlap between the Cambrian and Silurian rocks and named it after another Iron Age tribe, the Ordovices from northwestern Wales. The Ordovician ‘appropriated’ rocks from the upper part of Sedgwick’s Cambrian and the lower part of Murchison’s Silurian.  Anglesey’s Ordovician rocks were deposited on the margins of a marine basin, which escaped the intense volcanic activity prominent elsewhere in Wales, particularly in neighbouring Snowdonia.

Ordovician slates near Carmel Head

Ordovician slates near Carmel Head (photo : John Conway)

Anglesey’s Silurian Rocks (443 – 416 Ma)

The Silurian System was defined by Murchison in 1835, initially based on rocks preserved in the Welsh Borderland, and named after an Iron Age tribe, the Silures, whose homelands lay in Glamorgan and towards the Wye Valley.  The Silurian rocks at Parys Mountain are of world significance for their ‘Kuroko-style’ mineral deposits formed in a sub-marine hydrothermal vent……

The Black-Smokers of Amlwch…..

Silurian strata are poorly represented on Anglesey, and comprise a relatively small area of graptolitic shales outcropping at Parys Mountain. However, the chief interest of that locality is its world-famous complex mineral deposits. Parys Mountain was a historically famous source of copper and in the late 18th Century it was the world’s biggest producer of that metal. (see page specialising on Parys mountain)

In the early 1960s, the first of several major exploratory drilling programmes commenced at Parys Mountain. Successive companies were attracted to the area with the most recently active being Anglesey Mining Plc, and as time has gone by, a much better geological picture of the stratigraphic sequence and the nature of the mineralisation has emerged. The drilling has also resulted in the discovery of stratiform lenses of massive sulphide mineralization, containing percentage levels of copper, lead and zinc, with noteworthy concentrations of silver and gold, both to the west and the north of the old mine. Several million tonnes of ore reserves have now been proved to be present in these areas.  

The mineralisation is accompanied by intense silicification and pyritisation of the surrounding sedimentary and igneous rocks. It is dominated by quartz and pyrite, with important quantities of chalcopyrite and, in some sulphide lenses (the “bluestone”), galena and sphalerite. Numerous uncommon compounds of lead, bismuth, arsenic, antimony, silver and gold are also present. The ore deposit is thought to belong to the Volcanogenic Massive Sulphide (VMS) class, deposited on the late Ordovician or early Silurian sea-bed from heated solutions percolating up through the rocks and exiting into the water above as “black-smokers”.

Parys mountain: lead-pyrite-chalcopyrite mineralisation, panoramic view of the Great Opencast and the viewing platform. Photos : John Conway

The destruction of the Iapetus Ocean

Meanwhile, by the end of Silurian times, ‘proto’ Anglesey had reached 20 degrees south of the equator. Virtually all of Iapetus had disappeared, and in the final collision, the conjoined Avalonia and Baltica (England and Wales) were at last welded to Scotland, as Avalonia collided with Laurentia. The stitch-marks, if any were visible in Scotland, would follow a line running ENE from the Solway Firth.  These events left their mark in the form of folding of Anglesey’s Cambrian, Ordovician and Silurian rocks and the already deformed Precambrian strata. In the case of the latter, complicating the structural picture even more! This deformation is part of the Caledonian Orogeny.  Part of a massive belt stretching from the Appalachians in the US through the British Isles and on through Scandinavia.

The resulting Ordovician slates have been quarried on both sides of Anglesey but are not of great quality.

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