How did such a small river make such a big valley?
The deep wide lower Don valley, so noticeable from the Prince Edward (Bloor Street) Viaduct and The Leaside Viaduct, was formed about 10,000 years ago after the draining of Lake Iroquois, when the nearest ice front was somewhere around North Bay. At that time the level of water in the Lake Ontario basin fell to (or near) sea level. As a result of this and the lack of stabilizing vegetation, many streams, including The Don, cut deep ravines through the soft alluvial plain of sand, gravel and clay tills to this lower lake called Admiralty Lake. The lower level came about because the St. Lawrence valley had been under as much as 2 km of ice for over 100,000 years and thus had been pushed hundreds of metres down into the mantle. The western end of the lake was freed from the ice several thousand years earlier than the eastern end, and probably had only been under the ice sheet for a small part of the Wisconsinan ice age, and so was higher. The sea, which came into this valley and is called
