Abstract The late Proterozoic rocks of England and Wales comprise part of eastern Avalonia. Characterization of the basement rocks in southern Britain allows the recognition of five distinct terranes in this part of eastern Avalonia known as the Monian Composite Terrane, the Cymru Terrane, the Wrekin Terrane, the Charnwood Terrane and the Fenland Terrane (Gibbons & Horák 1996; Pharaoh & Carney 2000; Fig. 2.1 ). During the Neoproterozoic, eastern Avalonia was situated on the NE margin of Gondwana on the southern margin of the Ran Sea (Nance & Murphy 1996; Hartz & Torvik 2002; Fig. 2.2a–d ). The Ran Sea itself was formed as a result of rifting of the older Rodinian continental landmass ( Fig. 2.2 ). Avalonian tectonics during this late Proterozoic period were driven by subduction on the NE margin of Gondwana, resulting in associated magmatism and arc basin development. With progressive obliquity of subduction, arc magmatism was replaced by a regime dominated by large-scale transform faulting that progressively dissected and dispersed the arc. The switch from arc mag- matism to intra-continental wrench-related volcanism and magmatism was diachronous, and is first seen in western Avalonia (Murphy et al. 2000). Neoproterozoic sediments of the Avalon Terrane are almost exclusively siliciclastic or volcaniclastic and were deposited within numerous geographically restricted strike-slip basins (Pharaoh et al. 1987a; Nance et al. 1991; McΙlroy et al. 1998; Hartz & Torvik 2002; Fig. 2.3 ).