The walk to the Hudson Bay coast is longer than I remember

Here’s something to ponder: Why is land in parts of Ontario rising “quickly,” while other parts of Ontario are sinking? Is it important?
Approximately 20,000 years ago, at the end of the great ice age, Ontario was covered by about two kilometres of ice. The thickness of the ice sheet was greatest over Hudson Bay, where the ice was close to three kilometres thick. As you can imagine, all that ice weighed a lot. It was so heavy that it pressed the land down — just like a pillow when you sit on it. The depression of the land was greatest over Hudson Bay, where the ice was thickest, and least in the Toronto area, where the ice sheet was thin.
Starting 10,000 years ago, the massive glacier began to melt back towards the north. As the ice sheet melted and thinned, the depressed land near Hudson Bay was flooded and the Tyrell Sea, or prehistoric Hudson Bay, was formed. With the weight of the glacier gone, the depressed land across northern Ontario began to rise, the way a pillow rises when you no longer sit on it. This is a geological process called isostatic rebound. About 8,000 years ago, Cape Henrietta Maria, where James Bay and Hudson Bay merge, was more than 100 metres below the surface of the Tyrrell Sea. By 7,000 years ago, it had emerged above sea level.
Geologists calculate that the land at the cape is rising at a rate of 1.2 metres per 1,000 years (almost four feet per 1,000 years or 12 centimetres per 100 years). As that land rises, more land is created towards the North Atlantic Ocean. To better understand, try this experiment:
- Lay a cookie pan with a lip on it flat on a table.
- Cover the entire bottom of the pan with a little water. The bottom of the pan represents the land that was covered by the Tyrrell Sea 8,000 years ago.
- Gently lift up one end of the pan, leaving the other end on the table. The water drains away from the raised end to expose a lot of the now dry bottom of the cookie pan. A small rise in submerged land creates a lot of newly exposed dry land.
This is exactly what is happening to the emerging land along the south shore of Hudson Bay. As the land rises, new shorelines form. In the last 1,000 years, this process has created nearly 30 kilometres of new land along the Ontario shore of Hudson Bay. Elders who live along the coast have witnessed new land being created in their lifetime.
One of the distinctive landforms along the south shore of Hudson Bay is old beaches that mark the former shoreline. In some places, more than 150 old shorelines can be seen due to 300 metres of rebound. The view from the ground or airplane is striking. Parallel, well-drained, raised beaches create a pattern of successive ridges that are covered by conifer trees on higher, drier ridges, which alternate with muskeg, rich in peat.
And the land is still rising. Geologists have measurements that indicate the land in the Hudson Bay region is still depressed. After 10,000 years, Hudson Bay is only half way through its “spring back” and may have another 100 meters to rise. When uplift is complete, Hudson Bay will mostly or entirely disappear.
While the land along Hudson Bay is rising, the land in southern Ontario is static or sinking.
The rebounding land in Ontario is like a teeter-totter. The Great Lakes more or less mark the line where the land is not moving. The line is like the axel of the teeter totter. Along the south shore of the Great Lakes, the land is actually sinking.
While you may find the geological history of Ontario quite interesting, it’s important to remember that geology impacts all of us. Isostatic rebound is a factor that influences the water levels in the Great Lakes, which affects recreational use, Great Lake shipping, and shorelines. It also affects the water levels in Hudson Bay. The land close to Hudson Bay will continue to rise, perhaps as much as another 100 metres over many thousands of years. Hudson Bay will shrink in size, islands may join the Ontario mainland, and the tilting of the land will also affect the flow of water in lakes and rivers in the future. So, in response to the question posed at the beginning of this article: Is it important that the land in Ontario is rising and sinking? Yes, indeed it is.
Understanding the “Ontario beneath our feet” is fundamental to Ontario’s economy and quality of life.
Andy Fyon is the Director of MNDM’s Ontario Geological Survey. For more information about the geology of Ontario: http://www.mndm.gov.on.ca/en/mines-and-minerals/geology.