Abstract In North Wales (fig. 9) the Ordovician, mainly represented by the Gwynedd Supergroup of Woodcock (1990), occupies a broad tract surrounding the Cambrian of the Harlech Dome and extends eastwards to the Bala area and westwards over much of the Llŷn Peninsula (Roberts 1979). The lower parts of the Ordovician are preserved as outliers on Anglesey and the upper parts are well developed in the broadly anticlinal structure of the Berwyn Hills. In outline, the Ordovician of the North Welsh Basin consists of mudstones, siltstones and sandstones of great aggregate thickness deposited in oxygenated waters commonly of no great depth (above wave-base). Basin subsidence roughly balanced deposition except during part of the Caradoc, when deposition was at a maximum (Prigmore et al. 1997). The stratigraphy is less complete than in South Wales, the Whitlandian commonly being absent, and there is little evidence for the Llandeilian and Pusgillian stages. The regional stratigraphy is complicated by a series of volcanic episodes, the deposits of which interfinger with the marine sedimentary units. Rushton & Howells (1998) synthesized a stratigraphical framework for the Tremadoc to Caradoc of Snowdonia, and in Snowdonia and Llŷn alone catalogued nearly 100 named divisions of the Gwynedd Supergroup, with over 20 more that are synonyms or of doubtful value. They distinguished as volcanic groups the products of each volcanic centre. They proposed the term Ogwen Group for the marine sedimentary sequence that envelops the volcanic groups and which commences with the transgressive basal Arenig rocks of North Wales, extending up to the top of the Nod Glas (Fig. 10).

Abstract This chapter considers the Precambrian rocks that lie beneath Lower Palaeozoic strata in England and Wales, but excludes the Monian rocks exposed in Anglesey and the Llŷn Peninsula in northwest Wales (see Chapter 9). It includes the Sam Complex on Llŷn because these rocks lie immediately southeast of the Menai Strait Fault System, and they show close similarities to other basement inliers further south and southeast. Although all these rocks are grouped as late Proterozoic, given the uncertainties associated with defining the chronostratigraphical base of the Cambrian, combined with the paucity of reliable isotopic data, it is possible that some are of early Cambrian age.

Abstract This chapter deals with the oldest rocks that crop out at opposite ends of a narrow belt running southwest from the island of Anglesey in North Wales across the Irish Sea to the southeasternmost corner of Ireland. Between these two outcrop areas, exposures similar to some of those in Anglesey are found on the Welsh mainland along the southwestern margin of the Llŷn Peninsula (Fig. 24). In Wales, since the work of Blake (1888), Greenly (1919) and Matley (1928), these rocks have been referred to as Monian. In Ireland, following work by Baker (1955, 1969, 1970), the exposures have been placed within either the Rosslare Complex or the Cullenstown Formation. Although many previous publications have assumed a Precambrian age for all of these rocks, it is likely that they include some Cambrian rocks. In particular, the sediments that comprise the Cullenstown Formation, correlated with Monian sediments exposed in western Anglesey, are now interpreted as Cambrian (Tietzsch-Tyler & Phillips 1989; Bennett et al. 1989). Nevertheless, given the uncertainty that exists over both the absolute age of many rocks in these areas, and of the Precambrian-Cambrian boundary in general, all Monian and Rosslare/Cullenstown exposures have been included in this rewritten Special Report.