Apatite fission-track ages of basement from the Superior craton in eastern Manitoba, western Ontario, and eastern North Dakota record two periods of Late Proterozoic epeirogeny and erosion. The latter period, which involves basement beneath and adjacent to the eastern flank of the Williston Basin, precedes basin subsidence and roughly coincides in time with breakup of the Proterozoic supercontinent. We speculate that these events are genetically related. ¿ American Geophysical Union 1988