The Canadian Shield is the ancient core of the North American Continent. It is composed mainly of highly metamorphosed granite, with smaller areas of metamorphosed sedimentary and igneous rocks and some areas of relatively horizontal but still quite ancient sedimentary rocks. These rocks are generally quite resistant to weathering and erosion, but have been subjected to intense and repeated glaciation. The topography of the landscape reflects the interplay between rock resistance and glacial action, with areas of relatively weaker rocks having been eroded to lower elevations while more resistant rocks form topographic highs. The distribution of lakes strongly reflects patterns of rock resistance. Faults and bedding layers in rocks tend to cause many lakes in the shield to be long and narrow. Nowhere is this more evident in maps of the Shield than in Lake Manicoua-gan, a reservoir constructed in the 1970s and forms a circle roughly 70 km in diameter. The circular structure is the result of a meteor collision 214 million years ago. The deepest lake in Quebec, Pingualuit Crater (252 m deep) is also the result of a meteor impact. Lake Mistassini is another example of a structurally-controlled lake. It is more than 2000 km2 in area, consisting of a series of northwest-southeast trending arcs paralleling the underlying bedrock structures.
The number of lakes in the Canadian Shield is very large. Ontario and Quebec alone list over 12 000 lakes greater than 3 km2 in area; the number of smaller lakes is much larger (Refer to 'see also' section). They occur throughout the Shield region, but are especially numerous in the highlands of Quebec and Labrador, the Ungava peninsula, northern Manitoba and Saskatchewan, Nunavut, and the eastern portions of the Northwest Territories. They are less abundant in the Hudson Bay lowlands of northern Ontario and eastern Saskatchewan.
The age and mineralogical composition of Shield rocks means that they are relatively deficient in calcium. In addition, the cool temperatures of the region and widespread coniferous forest lead to development of highly acidic soils. Thus, the lakes of the region tend to be acidic and highly oligotrophic. In addition, of course, they are ice-covered for much of the year.
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