Contour map (lines of equal elevation) of the present altitude of the Pre-Cretaceous unconformity (erosional surface cut into older rocks) in Minnesota, scale 1 inch = about 32 miles.
Sloan, Robert E. (Robert Evan), 1929- (1964). Cretaceous System in Minnesota, RI-5, Plate 2. Minnesota Geological Survey. https://reflections.mndigital.org/catalog/mgs:910 (map)