A northwest – southeast trending graben that extends roughly between Sudbury and Ottawa, Ontario. The southern portion of the Ottawa River flows within this structural depression. It formed in the Mesozoic as a consequence of the tensional forces associated with the breakup of the supercontinent Pangea and the initiation of the opening of the Atlantic Ocean. The graben is part of the St-Lawrence valley failed rift system which was itself a reactivation of a rift initiated in the late Precambrian.