The St. Lawrence Lowlands is an ecoregion of the Mixedwood Plains, a physiographic region of Canada and the United States. It is sometimes named the "Great Lakes/St. Lawrence Lowlands", but it does not properly include the whole of the Great Lakes Basin which, while they may drain to the sea by way of the St.Lawrence, are nonetheless part of the Canadian Shield physiographic region. (For example, Lake Nipigon.)
The Lowlands have deep, arable soils deposited during the last glaciation, when the Canadian Shield was scraped clean of all its soils and these were pushed south. The Great Lakes basin was gouged out and then filled with water which drained to the sea by way of the deep faultline of the St. Lawrence. The primary defining geologic feature of the Lowlands is therefore the presence of deep soils within the watershed and estuary of the St. Lawrence River. This feature occurs in more than one distinct area along the river's course:
a narrow ribbon of land along both shores of the lower St Lawrence Estuary, hemmed in on the north shore by the Canadian Shield and on the south shore by the ranges of the Appalachian Highlands
some lands along that shore of the Island of Newfoundland which faces into the flow of the river and has thus accreted alluvial soils from the Great Lakes basin
The Lowlands are split into these subregions by intrusions from adjacent physiographic regions. Peninsular Ontario lowlands are separated from the lowlands of the lower St Lawrence at the Thousand Islands by the geologic feature called the Frontenac Axis, where ancient granites of the Canadian Shield cross over and become the Adirondacks. The next notable pinching occurs at Quebec City, where again the Shield meets the shore. Anticosti and Newfoundland, both being islands, are separated by stretches of open salt water.
There are some differences in terminology by Canadian and American geographers. American geographers group the lowlands of the Great Lakes basin with the Great Plains as part of the Interior Plains, while for Canadian geographers these are separated by hundreds of miles of harsh Shield. American geographers group the Adirondacks of New York into Appalachia and do not acknowledge their common origins with the Shield intrusion into Minnesota, again because of wide separation.
Economy
Even though the St. Lawrence Lowlands is the smallest landform region in Canada, it is home to approximately 50% of Canadians. The Lowlands are abundant with agriculture, commerce, recreation spots, and transportation links. The St. Lawrence Lowlands are the most heavily industrialized landform in Canada, with about 100% of the country's manufacturing industries.
