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Summary Consideration of the arctic configuration of the Caledonides leads to a distinction between eastern and western geosynclinal belts. The western belt , comprising the East Greenland, East Svalbard and southern Barents Sea Caledonides is postulated to continue northwards into the Lomonosov Ridge, whilst the western Spitsbergen Caledonides are thought to have originated as part of the North Greenland geosyncline which is also thought to continue northwards to form the western part of the Lomonosov Ridge. The eastern Caledonian geosynclinal belt comprising the Scandinavian Caledonides appears to swing eastwards to link with the Timan Chain and possibly the Urals. The already postulated (‘Proto-Atlantic’) ocean concept is reviewed in the light of the Arctic Caledonides and named Iapetus . Faunal provincialism suggests that the ocean was in existence up to early Ordovician but had substantially closed by mid Ordovician times. Possible relics of the suture marking the closure of this ocean suggest that it lay to the west of the Arctic Scandinavian Caledonides trending NE to latitude 70° N and thence veered eastwards separating the southern Barents Sea Caledonides from those of Arctic Scandinavia, possibly connecting with the northern Uralian ocean. A previous branch of the ocean may have separated East Svalbard and East Greenland as an ocean-like trough. A further (pre-Arctic) ocean may have existed to the north of the North Greenland–Lomonosov Ridge geosynclines. This is named Pelagus . The closure of these oceanic areas and the deformation of the bordering geosynclines delineates three principal continental plates, namely, Baltic, Greenland and Barents Plates. Their relative dominantly E–W motion up to Silurian times produced compression between the Greenland and both the Baltic and Barents plates but dextral transpression and transcurrence between the latter plates. In Late Silurian to Devonian times an increasing northward component controlled late Caledonian transpression and sinistral transcurrence between the Greenland plate and the combined Baltic and Barents plates.