Lower Palaeozoic Sites

Careg-Onen Grid reference not disclosed

Careg-Onen is one in a network of two sites. First, Careg Onnen lies on the east side of Red Wharf Bay, a second is in the central, south-west part of Anglesey at Trefdraeth. Together these sites (together with Baron Hill which is currently inaccessible) reveal the nature of the sequence from the Precambrian Mona Complex across an unconformity into beds which are thought to be of Lower Cambrian age, The unconformity is a widespread feature which is thought to be the basal Dyfed unconformity. At Careg-onen, sheets of Mona Complex schist are interlayered with Careg-onen mudstones. Then an unconformity between the two stratigraphic units is inferred from the contrast in deformation between the widely spaced fissility of the sandy-mudstones of the the Careg-onen Beds and the more deformed schists of the Mona Complex. The Mona Complex deformation is generally regarded as Precambrian in age whilst the sediments, post-dating the Mona-Complex deformation, are likely to be Cambrian or possibly Ordovician in age: in either case the Careg-onen Beds are likely to lie above the early Cambrian unconformity and be further evidence of the wide distribution of this unconformity. Evidence for the unconformity was recorded in the cliffs around Careg-onen Cove by Greenly (1919) where folds in the Mona Complex were truncated by a thust and unconformably overlain by the Careg-onen Beds. Critical parts of the Careg-onen Cove are now inaccessible and other parts of the cliffs are obscured. Access to the cove would probably add considerably to the interest of the site.

Trefdraeth SH 406706

Trefdraeth is one in a network of two sites. One is the Carreg Onen site on the east side of Red Wharf Bay, a second is in the central, south-west part of Anglesey at Trefdraeth Together these sites (with Baron Hill which is currently inaccessible) reveal the nature of the sequence from the Mona Complex to rocks through to sediments thought to be Lower Cambrian, separated by the Basal Dyfed unconformity from the Mona Complex basement below. The Trefdraeth conglomerates contain rounded to sub-rounded pebbles, with a 1-10cm long axis. Some pebbles have a pronounced foliation, similar to that of the Mona Complex, suggesting that the Trefdraeth Conglomerate post-dates the late Precambrian deformation of the Mona Complex. The well-rounded pebbles of the Trefdraeth Conglomerate are consistent with the sediments being deposited in a highenergy, shallow-marine environment, probably during the lowermost Cambrian transgression.

Graigwen & Porth Wen SH 40259470 & SH 40009466

This site provides rare exposure of the unconformity between the Precambrian Gwna Group schists and quartzites (Greenly 1919) and the Ordovician (Arenig) conglomerates of the Porth Cynfor Formation and the sandstones of the Torllwyn Formation above (Bates 1972). The Ordovician conglomerates are spectacularly exposed on the west shore of Porth Wen (SH 40259470), whilst the unconformity between the quartzites of the Gwna Group and the Ordovician conglomerates is well-seen on the Graig Wen ridge to the west at SH 40009466. This unconformity is important in understanding the palaeogeography of this area in late Precambrian to early Ordovician times, a time span of around 100 million years. It demonstrates that the sedimentary rocks of the Precambrian must have been involved in uplift and erosion during a tectonic event prior to the area being transgressed by the sea and new sediments being deposited at the onset of the Ordovician Period. Since no intervening Cambrian rocks are to be found in this area, this limits the duration that this tectonic event occurred.

Please note that the inclusion of sites in these listings in no way allows access to the sites described. The information is merely a scientific description. Many of the sites are Sites of Special Scientific Interest (SSSI), and/or on private land where permission to visit could be denied, or the site is unsuitable for safety or other reasons. Access to any site which is not in the public domain is entirely at the discretion of site owners. In no case is the collection and removal of specimens allowed from any site. Bona fide researchers require a permit from CCW to collect any specimens from GCR sites (SSSI) and these are generally allowed only under supervision. Queries regarding any of these sites should be directed to COFNOD, Parc Menai, Bangor. They will be able to answer any queries in the near future when all the relevant information is in their hands.