The Precambrian Shield is an extensive structural unit of the Earth's crust composed of exposed basement rocks formed during the Archean or Proterozoic eons which together comprise the Precambrian Era ending 544 million years ago. Originally formed during several rounds of mountain-building activity, Shield rocks are now among the most stable on Earth. The Precambrian mountain belts have since eroded away, creating the low, rolling rock plain we see today. The best-known examples are the Canadian Shield and the Baltic Shield in Scandinavia.
The Canadian Shield covers about 4.8 million km2. North to south it extends from the ARCTIC ARCHIPELAGO to the states of Wisconsin and New York; east to west from Labrador to the western Northwest Territories. Repeated advances of glacial ice have scoured its surface and left it strewn with countless lakes, rivers, streams and ponds. Along its edge lie many of the great lakes and waterways of Canada: the eastern shores of GREAT BEAR LAKE, GREAT SLAVE LAKE, LAKE ATHABASCA and LAKE WINNIPEG; the northern shores of LAKE OF THE WOODS, LAKE SUPERIOR and LAKE HURON; and the north shore of the ST LAWRENCE RIVER.
The origin and age of the Shield were among the great mysteries of Canadian GEOLOGY. Canada's oldest rocks (3.96 billion years) were found east of Great Bear Lake. The Shield's southern limits were traced by Alexander Murray, who, in 1851-52 examined the country below Gananoque, Bytown [Ottawa], the St Lawrence and Ottawa rivers and the perimeter from Kingston to Lake Superior. A.C. Lawson made an important contribution by working out the Precambrian succession in the 1880s, but a more current time scale was not developed until the 1950s, when geologists such as C.H. Stockwell had seismic and gravity measures at their disposal (see GEOLOGICAL HISTORY). Stockwell divided the Shield into 3 great provinces - Superior, Churchill and Grenville - and 23 subprovinces. It is now considered to fall into 5 provinces: Bear, Churchill, Labrador, Southern and Grenville (see GEOLOGICAL REGIONS).
The Shield has had a profound effect on Canadian history, settlement and economic development. In pre-European times it was the home of Algonquian nomadic hunters, who developed the birchbark CANOE to travel its myriad waterways. Similar canoes were used by the COUREURS DE BOIS, VOYAGEURS and explorers to penetrate the continent.
Abundant fur-bearing animals in frontier regions provided the basis of a colonial economy until the early 1900s. The hegemony of MONTRÉAL was thus extended far into the wilderness, via the Ottawa River and connecting waterways into the North-West, creating the precedent for future Canadian sovereignty over these lands.