In a geology, a shield is 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.