The Canadian Shield was formed by a combination of plate tectonics, volcanic eruptions, sedimentary deposits and erosion. It was the first part of North America to be permanently above sea level, and the rest of the North American continent formed around it.
The Canadian Shield's oldest rocks formed two billion years ago. Its bacterial and algal fossils, among the oldest on Earth, were also formed at this time. Formerly composed of mountainous terrain, the Canadian Shield was flattened by billions of years of erosion and glacial incursion. The only vestige of the early mountains to persist into the modern era was the Adirondack range of northern New York. None of the Canadian Shield's volcanoes remained active into the modern era; they went dormant and were eventually eroded down to flat plain by glacial activity.
Parts of the Canadian Shield were submerged in later sea level changes, including Hudson Bay and the sea between Canada and Greenland. The Canadian Shield's former extent persisted underground as well, with its furthest extent to the southeast under the Appalachian Mountains and its westernmost extent under the Rocky Mountains. Those mountain ranges were formed through later geological processes and were not part of the original Canadian Shield, unlike the earlier Adirondack Mountains.