Although there are obvious and close relationships between Paleozoic structure in the upper Mississippi Valley and sedimentary, stratigraphic, geomorphic, and economic problems in this classic area, relatively little attention has been given to structure in the past. The purposes of this paper are to present a new structure contour map based on the Jordan-Oneota (Cambrian-Ordovician) contact and to point out certain implications bearing on other problems. The map is based on data obtained from many sources. Locations and elevations of the datum contact along the Mississippi River and some of the control points, on the Wisconsin, the Black, the Chippewa, the St. Croix, and the Minnesota rivers were taken from measured and plotted longitudinal sections and cross-sections made by the writer from 1928 to 1930 while engaged as consulting geologist to the Nine-foot Channel Board. Many other points on the datum surface were located by the writer, his assistants, . . .