Geology and Hydrography in lower St Lawrence


The landscape and ecosystems in Lower St Lawrence region are influenced by the geology of the Appalachian Mountains. The geology and hydrography in this area have been defined by the history and the evolution of Quebec’s landscaping: the past movements of the tectonic plates, the uprising of the Appalachian Mountains and the erosion of the soil by water and ice.


This region is dominated by the presence of sediments composed of disintegrated rocks. In fact, the rocky surface of the Earth is eroded by wind, water and glaciers. The fragments that are produced are transported, dropped, compacted and cemented to form stratums. These sedimentary rocks are mostly, made of calcite: they are formed by particles that have a high rate of calcium and that can be found on the river’s bed. The accumulation of dead corals also forms a calcite rock.  Sands stone, mudrocks, calcites, conglomerates and shales are among the existing sedimentary rocks in this region.  Volcanic rocks such as basalt can also be found locally. 

Between the Cambrian (550 million years ago) and the Carbonaceous (350 million years ago) geological periods, these rocks were raised and pleated creating the Appalachian Mountains. This mother-rock is covered by deposits such as sand, clay and other sediments caused by the erosion of the Appalachian Mountains. 

Lower St Lawrence’s landscape is inclined toward the river.  The scenery falls down from the heights of the southern mountains toward the marine terraces formed by the last glaciation. The northern coast of the landscape is bordered by the St Lawrence River’s estuary.

The sedimentary mother-rock is shaped by a glacial action that has left traces such as rivers, lakes and valleys. In some cases, the rivers are following the faults of the mother-rock caused by its uprising.

The rain and snow that fall on Lower St-Lawrence’s region are divided among the watersheds of rivers such as Mitis, Temiscouata, Matane, Saint-François and Rimouski. The water in most of these basins drains into the St. Lawrence River. However, the water from Cascapedia’s and Matapedia’s rivers drains toward the Chaleur Bay. These rivers are characterized by their periods of high and low water levels. It affects the species living in their waters, some as plants and creatures living on their shores. It also affects the coastal organisms of the salt waters of St. Lawrence’s Gulf. These cycles are tempered by dams, used to regulate river’s water levels or to harness their energy.


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Vaillancourt, Marie-Andrée, Lafontaine Caroline, Caractérisation de la Baie Mitis, Jardins de Métis et Pêches et Océans Canada, Grand-Métis, 1999, 185 p.