The Saint Lawrence rift system is a seismically active zone paralleling the Saint Lawrence River. The rift system trends northeast and southwest and forms a half-graben that links the Ottawa-Bonnechere and the Saguenay grabens. The rift system extends more than 1,000 km (620 mi) along the Saint Lawrence valley from the Ottawa – Montreal area. Within the system, fault reactivation is believed to occur along late Proterozoic to early Paleozoic normal faults related to the opening of the Iapetus Ocean.