The Thomson formation consists of a thick series of slates, graywacke-slates, and graywackes exposed over a large area southwest of Duluth, Minnesota. The formation has been variously correlated but in recent years has usually been considered the equivalent of the Animikie-Virginia slate of the Mesabi range. Detailed work in the past three years suggests that this correlation is untenable and that the formation is pre-Algoman. Detailed evidence is given, and the progressive metamorphism of the formation toward the south is described.
The revised correlation suggests the possibility of the occurrence of upturned Biwabik iron formation in the 50-mile covered interval between Cloquet and the Mesabi range.
The data likewise throw some doubt on the correlations proposed for the Cuyuna range about due west of the southern exposures of the main Thomson area.