Cross sections based upon seismic surveys and extrapolations from deep well data show that Paleozoic structures are underlain by basement ridges and scarps. Thickening and thinning of Lower Ordovician and Cambrian strata over basement scarps and ridges demonstrate basement influence on early Paleozoic sedimentation. A study of basement lithologies and isotopic age determinations help in the recognition of extensions of structural provinces of the Canadian shield into this region. Linear gravity and magnetic anomalies in Michigan, Indiana, Ohio, and Kentucky may be caused by Keweenawan-type basalts superposed on the predominantly granitic basement. Sparse data suggest a southwestern extension of the Grenville front through Michigan and Indiana.