Recent work by the Minnesota Geological Survey permits the construction of a detailed structural map of the southeastern part of the State with contours drawn on the Jordan-Oneota contact. The major structure is not the “Twin City” syncline as mapped formerly (Trowbridge, 1934) but a series of gently pitching troughs, basins, and domes.
In the region of the Twin Cities approximately 2500 square miles are underlain by a roughly circular basin around whose margin the Jordan-Oneota contact occurs at an elevation of 700 feet above sea level. The bottom of the basin, outlined by the 450-foot contour, is broad and flat. The average dip is about 20 feet per mile. The Hudson-Afton anticline modifies the structure of the east margin. Along the south edge of the basin an east-west-trending anticline extends directly across the area formerly mapped as synclinal. The area south of this structural high is characterized by a downward warping that formed a broad, gently pitching syncline whose major axis extends from southern LeSueur County southeastward into Iowa. There are three minor dome-like structures and smaller folds on the east limb of the syncline. The elevation of the Jordan-Oneota contact rises from 200 feet above sea level in the region of Austin to over 1150 feet on the crest of a local high to the southwest of Winona. Faults occur along the valley of the St. Croix River north of Stillwater and along the Mississippi valley at Hastings and Red Wing.