Abstract The base of the Ordovician has in the past been taken at the base of the Arenig Series in the British Isles, not least because Lapworth (1879, p.14) defined it that way – he recognized the System as comprising ‘Strata included between the base of the Lower Llandovery Formation and that of the Lower Arenig’. The base of the Arenig Series was taken as the lower boundary of the Ordovician System by Williams et al . (1972); hence Tremadoc rocks were excluded from the Ordovician correlation charts, and included in the Cambrian correlation charts by Cowie et al. (1972).In other parts of the world, for example in Scandinavia and the United States, an horizon at or close to the base of the Tremadoc Series had usually been considered to mark the base of the Ordovician. Henningsmoen (1973) discussed the history of these different usages, and noted that Lapworth himself entertained several views on where the base of the Ordovician should be taken, and indeed by 1902 (in Groom 1902, p.148) was evidently prepared to accept at least some part of the Tremadoc within his system.

Abstract >The Early Palaeozoic history of England and Wales was substantially influenced by the separation of Avalonia from Gondwana and its subsequent migration towards Laurentia. At the start of the Early Palaeozoic, the vast palaeocontinent of Gondwana straddled the South Pole and extended northwards into low latitudes. On the margin that hosted North Africa and North and South America there were areas of crust that were later to become detached terranes. The largest of these was Avalonia, the remnants of which now extend from NE USA, through the Atlantic Provinces of Canada and through England and Wales to Belgium and North Germany ( Cocks 2000 ). Elsewhere, at lower latitudes on the Gondwana margin, there were crustal segments that were later to become the terranes of Armorica (Britanny, Normandy and the Massif Central regions of France), Perunica (much of central Europe, but mainly preserved in the Bohemian part of the Czech Republic) and Iberia (Spain). In addition there are some smaller continental fragments whose history is difficult to establish. The area of England and Wales lay within Eastern Avalonia, which consisted of an initial crustal fragment that separated from Gondwana ( Fig. 3.1a ) and then accreted smaller terranes as it moved towards Laurentia. The core Avalon Terrane was probably assembled by accretion of crustal fragments on the Gondwana margin in the Late Precambrian or early Cambrian. At about the same time, this terrane accreted both the basement of the Welsh Basin ( Woodcock & Gibbons 1988 ) and an amalgamation