This paper illustrates the possible relevance of stress measurements to deciphering the evolution of the earth's crust in the region of the North Atlantic Ocean. Particular emphasis is given to the evaluation of the recently proposed St. Lawrence rift system. Active seismicity in the St. Lawrence Valley has been taken as evidence that the area is now one of active rifting, possibly connected to the world-girdling rift system of which the Mid-Atlantic Ridge is a part.

Preliminary analysis of measurements in the vicinity of the St. Lawrence Valley, however, shows a crustal stress system characterized by large horizontal compressive stresses. That type of pattern appears to be typical for eastern North America, the Canadian shield, and Fennoscandia, and is not expected within an active rift system, if rift systems (at least locally) should reflect a state of crustal extension in which lateral stresses are less than the lateral component caused by weight of overburden. The St. Lawrence therefore is probably not an active rift system. It is suggested that recent seismicity and existing stress patterns reflect the result of glacio-isostatic activity superimposed upon mechanisms related to ocean-floor spreading, and that stress measurements may provide a useful tool for the evaluation of the involved mechanisms.