Over time the GTA's climate has changed dramatically, encompassing several ice ages or glaciations. Glaciers form when colder climate allows snow to accumulate into thick sheets of ice that flow under their own weight.

The last advance of the Laurentide Ice Sheet started about 100 000 years ago and extended south into Ohio. In the GTA, the ice sheet melted only about 10 000 years ago.

The ice sheet deposited compact layers of sediments called tills that form broad plains across the region. Sediment from melting glaciers was deposited in lakes and in ridges creating eskers, drumlins, and moraines. The landforms that we see today are therefore mainly a result of glaciers and their meltwater.