The Canadian Shield refers to the exposed portion of the continental crust underlying the majority of North America. The crust, also known as the North American Craton, extends from northern Mexico to Greenland and consists of hard rocks at least 1 billion years old. With the exception of the Canadian Shield, the rocks of the North American Craton are buried deep within the continent and covered by soil and other material. At 5 million km2, the Shield makes up roughly 50 per cent of Canada’s land mass. Shaped like a horseshoe — or the shields carried during hand-to-hand combat — the Canadian Shield extends from Labrador in the east to include nearly all of Québec, much of Ontario and Manitoba, the northern portion of Saskatchewan, the northeast corner of Alberta, much of the Northwest Territories and Nunavut and into the Arctic Archipelago. (It also reaches into parts of the United States, in New York, Wisconsin and Minnesota.) While at times a barrier to settlement, the Shield has also yielded great resources, including minerals, coniferous forests and the capacity for hydroelectric developments.