{{Unreferenced, date=December 2009
The Ontarian River is the term used for the pre-glacial river that began the creation of the valley in Silurian age shales and
limestone
Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms whe ...
s now occupied by
Lake Ontario
Lake Ontario is one of the five Great Lakes of North America. It is bounded on the north, west, and southwest by the Canadian province of Ontario, and on the south and east by the U.S. state of New York. The Canada–United States border ...
. The valley was greatly deepened by glacial action during the
Ice age
An ice age is a long period of reduction in the temperature of Earth's surface and atmosphere, resulting in the presence or expansion of continental and polar ice sheets and alpine glaciers. Earth's climate alternates between ice ages and gre ...
.
The original flow is thought to have been westward beginning from the
Laurentian shield
The Canadian Shield (french: Bouclier canadien ), also called the Laurentian Plateau, is a geologic shield, a large area of exposed Precambrian igneous and high-grade metamorphic rocks. It forms the North American Craton (or Laurentia), the an ...
in the present area of the
St. Lawrence River
The St. Lawrence River (french: Fleuve Saint-Laurent, ) is a large river in the middle latitudes of North America. Its headwaters begin flowing from Lake Ontario in a (roughly) northeasterly direction, into the Gulf of St. Lawrence, connecting ...
to eventually join the
Mississippi River
The Mississippi River is the second-longest river and chief river of the second-largest drainage system in North America, second only to the Hudson Bay drainage system. From its traditional source of Lake Itasca in northern Minnesota, it fl ...
drainage system.
During continental glaciation, the tremendous weight of the glacier depressed the area of the St. Lawrence so much that a lower point of drainage was established eastward through the present river system. The area is still rebounding, however, and the rise could further rearrange drainage within geologic time by raising the outlet of Lake Ontario sufficient for it to drain at some other, lower point.