A geological and paleomagnetic study in the Nipigon Strait area, northern Lake Superior, has confirmed previous conclusions drawn from aeromagnetic data, that an unconformity occurs within the Late Precambrian Keweenawan Osler volcanics and separates normally magnetized lavas from older ones with reversed polarity. The new data, together with other paleomagnetic and geological evidence from Keweenawan rocks, suggest that the magnetic reversal occurred when there was a temporary halt or decline in volcanic activity throughout the Lake Superior region. During the quiescent period, coarse clastic sediments were deposited by marginal erosion of the subsiding Keweenawan basin; sinking of the basin with corresponding uplift of the margins may have been greater in the south where the thickest sections of conglomerate and sandstone are preserved.