The rocks that form the Canadian Shield were formed about four billion years ago during the Archeon Eon of the Precambrian Era. Erosion of this extremely rugged, mountainous landscape deposited enormous quantities of clays, silts, sands and gravels into the surrounding waters. Compressed by their sheer cumulative weight and the heat of the shifting Earth's crust, sedimentary and metamorphic rocks formed during the Proterozoic Eon of the Precambrian Era.
More recent rocks that were formed above these ancient layers have since been largely removed by the scouring action of glaciers that covered northern North America in the several ice ages in the past 100,000 years.
The last ice age scraped the rocks in a NNE (north-north-east) to SSE (south-south-east) direction. At the end of the last ice age, all the waters in this part of North America flowed toward the Mississippi. As the weight of the ice lifted off the rocks, they rose, and now water flows eastward toward the St Lawrence. The soil on which trees and other vegetation grow in this part of the continent are the result of gradual sediment buildup since the last ice age.