What are lowlands?

There are three lowland regions surrounding the shield: the Interior Plains, the Great Lakes-St.Lawrence Lowlands, and the Hudson Bay-Artic Lowlands. The bedrock under these lowlands is formed mainly of sediments eroded from the Shield. The sediments were laid down in the seas that existed at various times millions of years ago. As the rock particles collected, the weight of the upper layers compressed the lower layers into sedimentary rocks.