Abstract In the southern islands of the Canadian arctic archipelago and on the adjacent mainland three types of terrain predominate-uplands of crystalline rocks, plateaux of flat-lying strata, and lowlands of younger, relatively undisturbed strata. Forming the eastern boundary of the region lie the Baffin-Ellesmere mountains adjacent to Baffin Bay and Davis Strait, and to the north-west are the arctic ocean lowlands. Each of these major areas is profoundly different from the others in its geology, and these differences reflect crustal behaviour since Pre-Cambrian time. In the southern arctic islands region, several major northerly projecting basement features, the Minto, Wellington, Boothia and Melville-Southampton Arches separate the shallow Palaeozoic accumulations of the Wollaston, Victoria Strait, JonesLancaster and Foxe Basins. The Minto and Wellington Arches contain Pre-Cambrian sedimentary strata with some lavas and sills which are relatively little disturbed. In contrast, the Boothia and Melville-Southampton Arches are largely composed of granitic gneisses and greenschists much folded along north-south and north-eastsouth-west lines respectively. Early ‘post-Archaean’ sediments and numerous basic intrusives of uncertain age also occur. The rocks of these arches have a complex Pre-Cambrian history and the Boothia Arch was elevated dramatically in late Silurian –early Devonian and again possibly in Tertiary times. Facies variation within the strata of the basins gives little indication of positive movement of the arches in early Palaeozoic time. The influence of the Canadian Shield to the south and the developing Franklinian Geosyncline to the north were more important in facies control. Much of the deposition within the basins was of shallow water carbonates with some fine clastics and, at least locally, evaporites. Many kinds of lime-secreting and other organisms influenced sedimentation and biogenic rocks are common. This region seems also to have been occupied by some of the earliest-known vertebrate faunas, and it may have been a centre of Silurian agnathan radiation. In late Silurian–early Devonian time an orogeny sharply elevated the Boothia Arch and may have established the Cornwallis Fold Belt. The Siluro-Devonian Peel Sound Formation, spreading from the Boothia Arch into the Victoria Strait, ?Melville and Jones-Lancaster Basins, exhibits lateral facies changes from alluvial fans through fluvial to marine strata. Little is known of the geological history of the area from Devonian until Quaternary times. It is thought to have been a broadly positive area undergoing denudation and shedding sediment northward to the Sverdrup Basin. Nevertheless, CretaceousTertiary sediments occur in a few small outcrops far north of what has been regarded as the shoreline of those times. On Somerset Island the presence of Lingula in such strata may indicate a connexion with the Sverdrup Basin sea or with open water to the south. Sediment transport southward from the northern side of Barrow Strait is suggested near the axis of the Boothia Arch in southern Somerset Island. Fault activity at a later date is perhaps a rejuvenation of earlier trends of movement. The recently suggested isostatic origin of these arches or uplifts seems to be in accord with the facts, and appears to indicate that a wide area of relative isostatic equilibrium now exists between the Franklinian Geosyncline and the (Canadian Shield) craton, but that this was not the case in Palaeozoic time.