By Melissa Cole, Curator
Did you know that Lake Ontario started as a small stream that gradually opened up through the erosion of soft Silurian rocks over thousands of years?
Lake Ontario is one of the five Great Lakes of North America. Surrounded on the north, west, and southwest by the province of Ontario, and on the south and east by the state of New York, its water boundaries, along the international border, meet in the middle of the lake.
Oshawa lies on a glacial geological feature called the Lake Iroquois shoreline. We know it today as a fairly level band of land ringing Lake Ontario, bordered by the ridge of the prehistoric Lake Iroquois shoreline.
Lake Iroquois was a proglacial lake, meaning that the lake was situated between rock deposits and an ice sheet. The northern shore of this lake was the southern edge of the retreating glacier. Lake Iroquois was formed by melting glacial ice in the Lake Ontario basin.
At that time, the St. Lawrence River Valley was blocked with ice, and the lake level rose 30 m (~100 ft) above present day Lake Ontario. The lake drained to the southeast, through a channel passing near present day Rome, New York. The lake was fed by Early Lake Erie, as well as Lake Algonquin, an early partial manifestation of Lake Huron and Georgian Bay, that drained directly to Lake Iroquois across southern Ontario.
The stream turned into a river that was widened and sculpted by the powerful movement of the continental glaciers. The current level, shape, and direction of flow of Lake Ontario was established over 12,000 years ago.
As it retreated, the glacier left behind Lake Iroquois, a larger version of present-day Lake Ontario.
The old shoreline runs west-east, running roughly parallel to today’s King Street in Oshawa. The shoreline is typified by washed sand and gravel bluffs. It is located well away from the present shore of Lake Ontario. Remnants of this shoreline can still be seen in various communities today along the north shore of Lake Ontario. The ridges of the old shoreline are evident in Oshawa, where the banks of the old Lake Iroquois shoreline can be seen looking north of Highway 401. Iroquois Shoreline Park, located on the hills of Grandview Street North and the appropriately named Ridgemount Blvd., is the approximate location of the original shoreline of Lake Iroquois. Further west, the Scarborough Bluffs also formed part of the shoreline of the original lake. Further east, remnants of the shoreline are visible at Stephen’s Gulch in Clarington and Highway 401, near Cobourg.
This land was valued by Indigenous communities and later by settlers for farming and settlement. Archaeological reports show that from 1400 – 1450CE, ancestral Wendat communities were utilizing the land around the area of Grandview Street and Taunton Road.
The next time you are taking a drive, I would recommend a drive to the top of the hill of Grandview Street North near Ridgemount Blvd. in Oshawa. If you stand looking south from that ridge, all that land would have been water.
Richard Foster Flint, Glacial Geology and the Pleistocene Epoch. 2008
Grahame Larson and Randall Schaetzl, Review: Origin and Evolution of the Great Lakes. 2001