Abstract Deformation of the Early Palaeozoic rocks of England and Wales has traditionally been ascribed to a late phase of the Caledonian Orogeny that occurred in end-Silurian time. More recently, it has been recognized that this deformation took place towards the end of the Early Devonian and forms part of the Acadian orogenic belt, which extends from the NE Appalachians through western Europe to Poland ( McKerrow 1988 ) ( Fig. 6.1 ). The Caledonian mountains, as originally defined by Suess in the early 20th century, extended between Scotland and Norway and were thought to result from the deformation of an Early Palaeozoic geosyncline. It was soon recognized that the Caledonian orogen continues into the Appalachians and East Greenland, flanked to the west by a faunally distinctive foreland sequence of Cambro-Ordovician platform deposits, which includes the Durness succession of NW Scotland. In a seminal pre-plate tectonic interpretation Wilson (1966) saw the Caledonides as the result of closure of a ‘Proto-Atlantic’ ocean that existed in late Precambrian and Early Palaeozoic time, subsequently named the Iapetus Ocean by Harland & Gayer (1972). In the first plate tectonic model to interpret Caledonian orogenesis in terms of lithospheric convergence driven by ocean spreading and subduction, Dewey (1969a) envisaged closure between two major continental plates, North America–Greenland and Europe, which produced the N–S-striking North Atlantic Caledonides and NE–SW Appalachians. The British Caledonides, located at the intersection of the two margins,