The landGeologyThe lands have developed geologically around four nuclei of ancient crystalline . The largest of these, the Canadian Shield, underlies all the Canadian Arctic except for part of the . It is separated by Baffin from a similar shield area that underlies most of Greenland. The Baltic (or Scandinavian) Shield, centred on , includes all of northern Scandinavia (except the Norwegian coast) and the northwestern corner of Russia. The two other blocks are smaller. The Angaran Shield is exposed between the Khatanga and Lena rivers in north-central Siberia and the Aldan Shield is exposed in eastern Siberia. In the sectors between the shields, there have been long periods of marine sedimentation, and consequently the shields are partly buried. In some areas thick sediments were subsequently folded, thus producing mountains, many of which have since been destroyed by erosion. Two main orogenies (mountain-building periods) have been recognized in the Arctic. In Paleozoic times (about 542 million to 251 million years ago) there developed a complex mountain system that includes both Caledonian and Hercynian elements. It extends from the Queen Elizabeth Islands through Peary and along the east coast of Greenland. Mountain building occurred during the same period in Svalbard, Novaya Zemlya, the northern Urals, the , and Severnaya Zemlya. There is considerable speculation as to how these mountains are linked beneath the . The second orogeny occurred during the Mesozoic (251 million to 65.5 million years ago) and Cenozoic (the past 65.5 million years) eras. These mountains survive in northeastern Siberia and . Horizontal or lightly warped sedimentary rocks cover part of the shield in northern Canada, where they are preserved in basins and troughs. Sedimentary rocks are even more extensive in northern Russia and in western and central Siberia, where they range in age from early Paleozoic to Quaternary (the past 2.6 million years). It is evident that the polar landmasses have been transported on lithospheric plates through geologic time and that their positions relative to each other and to the have changed, with significant modification to ocean circulation and to climate. Motion of plates in the Paleogene and Neogene periods (about 65.5 million to 2.6 million years ago) led to igneous activity in two regions. One was associated with mountain building around the North Pacific, and active volcanoes are still found in Kamchatka, the Aleutian Islands, and Alaska. The other area of igneous activity extended across the North Atlantic and included the whole of Iceland, Jan Mayen Island, and east Greenland south of Scoresby Sound; it was probably connected to west Greenland north of Disko Bay and to east Baffin Island. Volcanism continues in Iceland and on Jan Mayen, and hot springs are found in Greenland. Continental ice sheets of the pastLittle is known about the climate of the northern lands in early Cenozoic times; it is possible that the tree line was at least 1,000 miles farther north than at present. During the Cenozoic, however, the polar lands became…

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