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 approximately 544 million years ago. Originally formed during several rounds of mountain-building activity, Shield rocks are now among the oldest and most stable on Earth. The Precambrian mountain belts have since eroded away, creating the low, rolling rock plain with innumerable lake-filled hollows that we see today. The best-known examples are the Canadian Shield and the Baltic Shield in Scandinavia.
The Canadian Shield, which is visible in Churchill, covers over 1.8 million square miles. North to south it extends from the Arctic Archipelago to the states of Wisconsin and New York, and 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 Shield can be divided into a number of structural provinces, each representing a mobile area active during a different part of Precambrian time; the three main provinces are the Superior, the Churchill, and the Grenville. The oldest Precambrian rocks, formed more than 3,000 million years ago, occur as small isolated masses within larger areas of younger rocks. The rocks in these belts are volcanics and sediments, and contain thick banded iron formations, one of the main sources of iron ore in North America; other important mineral deposits include gold, silver, nickel, cobalt, zinc, copper, lead, and uranium.
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. But, the bare rock, thin soils, muskeg, and insects of the Shield have presented a barrier to settlement; the agricultural frontier of the prairie provinces and eastern Canada end abruptly at its perimeter.
In Churchill there is the opportunity to search the Hudson Bay coastal beaches for fossils such as trilobites. These ancient life forms existed during glacial periods and are embedded in the shale found in the bay.