Canada's Arctic Lands are generally thought to lie north of the tree-line and cover 2.6 million km2 (26 per cent of the country). They include the Arctic Coastal Plains and Arctic Lowlands, the Inanition Region of the High Arctic, and parts of the Canadian Shield in Nunavut, northern Québec and Labrador. However, extensive areas of Subarctic Lands must also be recognized. Taken together, Canada's Arctic and Subarctic Lands comprise nearly 40–45 per cent of Canada's land surface. Geological structure and lithology (i.e., physical characteristics of rock) largely shape the landscape. For example, the mainland east of Great Bear and Great Slave lakes, the Ungava Peninsula and most of Baffin Island are part of the Canadian Shield, and are composed of resistant igneous, metamorphic and sedimentary rock. Higher elevations consist of bedrock outcrops, while upland surfaces and upper valley-side slopes are covered by angular rock-rubble accumulations. Bedrock is disrupted by joint and fissure widening, and by its separation into angular blocks. By contrast, areas of unconsolidated sediments from the Paleocene to Quaternary periods (65 million to 10,000 years ago) form more undulating, poorly drained lowland terrain. Several distinctive landforms occur in the Arctic Lands, most often associated with the growth of permafrost and ground ice. Tundra polygons, a tortoise-shell pattern of cracks up to 30 m apart with ice wedges below the cracks, cover many thousands of square kilometres. Other distinctive periglacial landforms are pingos, over 1,500 of which have been counted near the Mackenzie Delta. Glaciation over much of northern Canada formed a landscape similar to an upright saucer with its centre flooded by Hudson Bay. The eastern rim, extending from Labrador north along Baffin Island and into Ellesmere Island, is a mountainous zone with elevations of 1,500 m and higher in the north, and a heavily fjorded coast. Glaciers cover about five per cent of the Arctic land surface. The zone between the Shield and the Western Cordillera is a Palaeozoic plain (542–253 million years old) gently sloping from 500 m in elevation downwards to the Arctic Ocean. The islands are mostly sedimentary rocks forming plains, uplands and hills. The rock layers in the south are mainly flat lying, but in the Arctic Archipelago they have been folded and then eroded. Surface elevations rise from near sea level in the northwest to approach the high mountain rim in the east. Faulting followed by further deepening during glaciation may have caused the many channels among the islands. Canada's Arctic lands are regarded as being either Arctic or subarctic in nature. The boundary between the two approximates the northern limit of trees. This is a zone, 30–150 km in extent, north of which trees are no longer able to survive. Ecologists refer to the barren, treeless Arctic as tundra. The tundra progressively changes into polar desert at extreme high latitude as climate becomes increasingly colder and drier. The tree-line also approximates the southern boundary of the zone of continuous permafrost; that is, north of the tree-line, the terrain is perennially frozen and the surface thaws for a period of only two-to-three months each year. Lakes and rivers are ice-free from June–October in the south and from July–August in the north; they are ice-covered for the rest of the year. The largest river in Canada, the Mackenzie, flows north from Great Slave Lake and empties into the Beaufort Sea.