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. The Winnipeg area is located on the bottom of what was glacial Lake Agassiz, with deep clay bottom deposits from that lake, which today ha become the flat prairie landscape. At the end of the last ice age, all the waters drained south, toward the Mississippi River. After the glaciers left this area, the waters were able to flow northwards into Hudson's Bay.
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. Winnipeg sits just west of where the Canadian Shield meets the great western plains, with its thousands of miles of relatively flat grasslands.
More history of Winnipeg