Abstract The Caledonide Orogen of the North Atlantic region, north of the British Isles, outcrops in the young mountainous areas of western Scandinavia, northeastern Greenland and Svalbard, and also comprises the deeper crust beneath the adjacent continental-shelf seas. This mid Paleozoic mountain belt resulted from the closure of an Ediacaran-Cambrian ocean (lapetus) during the Ordovician and the collision of two continents, Baltica and Laurentia, in the Silurian and early Devonian. Long-distance thrusting characterizes the orogen, west-vergent along the Laurentian margin and east-vergent along the Baltoscandian margin. During the Ordovician, subduction along the outer margins of both continents involved continent-arc collision and opiolite obduction. During the subsequent, so-called Scandian continent-continent collision, the mountain belt reached heights comparable to those of the Himalaya-Tibet Orogen today, before collapsing and eroding down to near sea-level by the end of the Paleozoic.