The line of the Scarborough Bluffs follows the old shoreline of Lake Iroquois, which was formed in the last ice age. The ice sheet blocked the St. Lawrence River (which currently drains the Great Lakes), and Lake Iroquois emptied into a channel to the southeast. As the glaciers retreated, the ice dam melted. The water level dropped sharply to the level of the present-day Lake Ontario. Over time, eroded alluvial deposits from the Bluffs have settled westward to form the Toronto Islands.