The midcontinent province between the Appalachian-Ouachita orogen and the Rocky Mountains is that portion of the United States craton which has been relatively stable tectonically for roughly the past billion years. Diastrophism has been restricted largely to broad, slow, vertical movements and highly attenuated, passive response to orogenic activity, which has occurred at plate margins around the North American craton. Thus, except for very localized structures associated with rare anoro-genic intrusions and meteorite impacts, the Phanerozoic sedimentary rocks are only disturbed by minor faulting and gentle warping. As a result, regional geophysical investigations of the midcontinent have primarily focused on the study of the more laterally variable physical properties of the crystalline rocks of the basement and deeper portions of the crust. However, a great number of detailed geophysical investigations have been conducted over cratonic basins in the search for structural and stratigraphic variations within the Phanerozoic sedimentary rocks. With a few notable exceptions, these surveys, largely seismic reflection and gravity, have been conducted to locate hydrocarbon accumulations and consequently are represented by proprietary data. Thus, although geophysical studies, particularly those employing modern, common-depth-point, seismic-reflection techniques, have great utility in mapping the minor structural and stratigraphic variations that occur within the Phanerozoic sedimentary rocks of the midcontinent, our principal source of information regarding the subsurface nature of these rocks is drillhole data. In contrast, our knowledge of the basement igneous and metamorphic rocks of the craton is based on regional geophysical studies working in concert with the irregularly distributed basement