Land and Resources, Geology
Paleozoic era, Tertiary Period, Pleistocene Epoch, tectonic forces, North American plate
The Canadian Shield, which occupies the eastern half of Canada’s landmass, is an ancient craton (stable continental platform). It is made of rocks that formed billions of years ago during the Precambrian Era of Earth history and includes granites, gneisses, and schists 2 to 4 billion years old. It became the nucleus of the North American crustal plate when Earth’s crust first experienced the tectonic forces that drive continental drift.
In the Paleozoic era (about 570 million to 240 million years ago), large parts of Canada were covered by shallow seas. Sediments deposited in these seas formed the sandstone, shale, and limestone that now surround the shield. During the Cambrian and Silurian periods of the Paleozoic Era, layers of rocks were formed that appear as outcroppings in Nova Scotia, New Brunswick, and Newfoundland and Labrador, along the St. Lawrence valley, and on the shores of Lake Ontario. Flat beds of Paleozoic and younger rocks extend westward across the Great Plains through the provinces of Manitoba, Saskatchewan, and Alberta. The rocks in these areas contain valuable deposits of oil and gas. In the Canadian Cordillera, the rocks were subjected to tectonic forces generated by the collision of the North American plate with the Pacific plate. In the ensuing upheavals, which began during the Cretaceous Period (about 138 million to 65 million years ago), mountain ranges rose throughout the Canadian Cordillera. The easternmost of these ranges, the Rocky Mountains, run from Canada south through Montana, Wyoming, Colorado, and New Mexico. They were built by uplifting and folding of sedimentary rocks and, to a lesser degree, by volcanic activity. The strata composing them range in age from the Paleozoic Era to the Tertiary Period (about 65 million to 1.6 million years ago) and contain valuable deposits of metals as well as fossil fuels.
During the Pleistocene Epoch (about 1.6 million to 10,000 years ago), nearly all of Canada was covered by vast ice sheets that extended into the northern United States. As these ice sheets moved, they profoundly modified Canada’s landscapes, creating many thousands of lakes and extensive deposits of sand, clay, and gravel.
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