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 drained toward the Atlantic Ocean to the east, and as the weight of the glaciers left this area, the land slowly began to rise. The soil on which trees and other vegetation grow in this part of the continent are the result of gradual sediment build-up since the last ice age.
The Ottawa River drains such a vast area, including much of the interior north-west of Quebec below James Bay that it actually has two flood seasons, one in early May and again in mid June. Today, the Quebec side of the River has a Boreal Shield (Canadian Shield with trees) in the Gatineau Hills, with the rest of the region, on both sides of the river being Mixed Wood Plain Ecozone. This zone extends from Mattawa in the north to just east of the Thousand Islands near Kingston, and southeast toward Montreal and the St. Lawrence Valley.
More history of Ottawa