The Escarpment
At the beginning of the last great Ice Age, 120,000 years ago, Toronto lay beneath an ice-sheet more than 2 kilometres high. As the glacier retreated the meltwater created an inland sea, twice as large as present day Lake Ontario, called Lake Iroquois. The lake created a shore bluff between the Don and Humber Rivers, where the coastal waters of Lake Iroquois had a sharper drop-off. Silt and mud washed back into the lake, and sand and gravel formed a beach and spits.
As Lake Iroquois drained, first to the south and then through the St. Lawrence river, its water levels fell below those of the present time. This smaller body of water was called Lake Admirality.
As Lake Admirality rose to become today's Lake Ontario, the land north of the escarpment was open and rough, while to the south, the beginnings of Toronto merited the name "Muddy York".
Visit the 5 staircases along the escarpment for great views of Lake Ontario and the city.
Plaque via Alan L. Brown's site Toronto Plaques. Full page here.