Québec is situated at the junction of three separate major geological domains or provinces: the Grenville Province, the St.Lawrence Platform, and the Appalachian Orogen. These rocky assemblages, ranging from 450 million to one billion years in age, are partially overlain by unconsolidated sediment (clay, sand, and gravel) that developed during the Quaternary, the current period of geological history. Today's landscape was generated by the collision and break-up of tectonic plates and refined by erosional processes.

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The Grenville Province is a subdivision of the Canadian Shield, and it is here that the oldest rocks in the region are found. These rocks are the deep roots, uncovered by erosion, of a chain of mountains that formed between 1500 and 900 million years ago. At that time, Laurentia, the ancestor of the Canadian Shield, collided with other continents now dispersed across the surface of the Earth. An imposing, Himalaya-type mountain range grew out of this collision. The rocks of the Laurentians contain deformation and metamorphic structures that tell the story of those tumultuous times.

Nine hundred million years ago, Mount Sainte-Anne was part of a mountain range that resembled the present-day Himalayas. (Courtesy of J.L. Lethiec)

At the end of the Precambrian, about 600 million years ago, the region around Québec was located near the Earth's equator. Part of the Grenville mountain range subsided in a long trench known as a "rift". Over time, the trench gradually widened, giving birth to an ocean. From 600 to approximately 460 million years ago, the Earth's crust remained relatively stable in the Québec region, as sediments accumulated on the platform along the edge of the continent. The landscape at the time probably resembled that of the present-day Atlantic coast. The fault at Montmorency Falls, where the rocks of the Grenville Province come into contact with those of the St. Lawrence Platform, is thought to be a remnant of this time.

Five hundred years ago, the rocks of the St. Lawrence Platform formed in a tropical sea. At that time, the region was located at a much milder latitude.

From 460 to about 380 million years ago, a major tectonic upheaval produced another chain of mountains: the Appalachians. Laid down along major breaks in the Earth's crust, thick sequences of rocks were carried over several tens of kilometres and overrode the St. Lawrence Platform. Then, once again, erosion resumed the inexorably slow work of demolition. The Québec promontory, most of île d'Orléans, and the entire south shore of the St. Lawrence are part of the Appalachian Orogen, although the first hint of mountainous terrain only appears about 40 km to the south.

About 450 million years ago, the Upper Town of Québec and the areas of Sainte-Foy, Sillery, and Cap-Rouge resembled a present-day alpine landscape.

Between 1 800 000 and 10 000 years ago, several periods of glaciation took place, each burying almost all of the northern part of the continent under an impressive buildup of ice. The slow movement of ice sheets (continental glaciers) helped flatten and erode the mountains and plateaus of the Grenville Province and the Appalachian Orogen. The most recent of these glaciations ended about 10 000 years ago. Many of the valleys and rivers of the Québec region are the legacy of the advance, retreat, and melting of these continental glaciers. About 12 000 years ago, at the height of the deglaciation process, the continent was still isostatically depressed owing to the weight of the ice sheet. As a result, an arm of the Atlantic was able to invade the St. Lawrence lowlands, and the Champlain Sea was born. The sediments left behind by this postglacial sea led to the development of the best farmland in the St. Lawrence valley.

Eighteen thousand years ago, the entire region lay under a thick layer of ice similar to the Antarctic ice sheet. (Courtesy of M. Morency)