The Canadian Shield is surrounded by three major lowlands - namely, the Interior plains, the Great Lakes-St. Lawrence Lowlands, and the Hudson Bay-Artic Lowlands. These lowlands were formed due to the eroded sediments of the sheild, which then settled into the seas that existed millions of years ago. However all these lowlands were not formed at the same time. So the sedimentry rocks found in the different regions are of different ages.
