Bouguer gravity anomaly map (anomaly related to different densities of rocks in the upper crust, Bouguer anomaly is a corrected difference between an observed gravity measurement and value predicted from a generalized earth model), shown as contour lines (isolines) of equal value, map of Minnesota, scale 1:500,000.
Chandler, Val W., Schaap, Brian D. (1992). Bouguer gravity anomaly map of Minnesota, S-16. Minnesota Geological Survey. https://reflections.mndigital.org/catalog/mgs:1120 (map)