The entire province of Ontario is underlain by very old (about 2.5-1.0 billion year old) Precambrian rocks.  During the Paleozoic Era, sagging of the Earth's crust produced basins that served as catchment areas for sediments. Over time, these sediments were lithified to form sedimentary rocks. 

In Ontario, three main sediment catchment areas are recognized: 1. the Michigan Basin (underlying the regions of soutwestern Ontario, the Lakelands and the Georgian Bay sector of Near North),  2. the Appalachian Basin (underlying Festival Country, Metropolitan Toronto, and Getaway Country), and 3. the Ottawa Embayment (part of the Quebec Basin that underlies the easternmost part of Ontario East).  The characteristics of the sedimentary rocks in these basins provide a great deal of information on the geological history of Ontario between about 500 and 350 million years ago. The stacking pattern of Paleozoic sedimentary rocks in southern Ontariocan be examined by clicking on the map below.