The geological, geochemical and palaeomagnetic characteristics of the Late Precambrian-Early Cambrian rocks of S Britain are interpreted in terms of formation by plate-tectonic processes within the Iapetus Ocean during the time interval approx 700-400 m.y. The model envisaged is based on the inferred occurrence of SE-directed subduction of oceanic lithosphere below England and Wales during this time. The earliest products of this subduction were the intrusive Stanner-Hanter, Malvernian and Johnston igneous complexes with volcanic arc characteristics which formed approx 700-650 m.y. The crust in N Britain developed as an integral part of the Laurentian Shield during the Mid-Proterozoic (1600-1000 m.y.); in contrast, the basement of S Britain, S of the Iapetus suture (Solway-Shannon line), is younger than approx 900 m.y. and formed initially entirely within the Iapetus Ocean. It is concluded that whereas the basement to the north of the suture formed an essentially continental unit from 2900 to 400 m.y., the basement of England and Wales is a result of crustal growth from approx 900 to 400 m.y. and formed by accretion of island arcs, associated accretionary prisms and fore-arc basin sediments within the Iapetus Ocean.-R.A.H.Dept Earth Sciences, Open Univ, Milton Keynes M67 6AA, UK.