In the Canadian Shield, four orogenies, each followed by a profound unconformity, serve to divide the succession into five natural time-stratigraphic units. These are named for convenience in mapping, description, and discussion. Rock units in type areas, rather than abstract time, form the basis for the classification. Isotopic dates place the geological units in a scale of years and are extremely valuable for the purpose of long range correlation.

You do not currently have access to this article.