In a geology, a large, low-relief, exposed mass of Precambrian
rock, commonly having a gently convex surface and surrounded by belts of
younger rock. Shields contain rock that is more than 2,500 million years
old, now changed metamorphism (see metamorphic)
but originally composed of basaltic lava. Shields
form the nucleus of continents; for example, the Canadian Shield, located
between Hudson Bay and the Great Lakes, occupies two-thirds of Canada.
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