Ancient Parts of Continents: Cratons and Shields
A craton is a part of a continent that is stable and forms the central mass of the continent. The craton region of North America includes the region between the Rocky Mountains (to the west) and the Appalachian Mountains (to the east) and include the Canadian Shield.
A shield is a large area of exposed Precambrian-age crystalline igneous and high-grade metamorphic rocks that form tectonically stable areas. In all cases, the age of these rocks is greater than 570 million years and sometimes dates back 2 to over 4 billion years. For instance: the Canadian Shield is part of the North American craton region. Shallow inland seas have flooded over and retreated from North America's craton/shield region in the past billion years.