The Canadian Shield refers to the exposed portion of the continental crust underlying North America. The crust, also known as the North American Craton, consists of rock from the Archean and Proterozoic eons and extends from Mexico to Greenland. The Canadian portion runs from central Ontario north to the Arctic Archipelago, and fromLabrador west to the Northwest Territories. The Shield includes some of the oldest rocks on Earth (possibly more than 4 billion years old). While at times a barrier to settlement, the Shield has also yielded great resources, including minerals, coniferous forests and the capacity forhydroelectric developments.