Continental rifting in the area now known as the Michigan Basin occurred some 1.1 b.y. ago (Van Schmus and Hinze, 1985), along with similar tectonism in other portions of the mid-continental United States. Although little is known of the subsequent 500 m.y., it appears that a change from a continental to a marine depositional regime took place during the Late Cambrian (Dresbachian) when northerly transgressing epeiric seas advanced into a slowly developing ancestral Michigan Basin. The record of those seas is documented by Late Cambrian to Middle Ordovician formations. These are, in ascending order, the Mt. Simon, Eau Claire, Galesville, Franconia, Trempealeau–Prairie du Chien (T-PDC), St. Peter, and Glenwood. On the margins of the basin, the basal Mt. Simon Sandstone rests disconformably on older Precambrian basement. There, also, T-PDC rocks (Late Cambrian–Early Ordovician age) were eroded, producing a major interregional unconformity (the post-Sauk unconformity) on which the St. Peter Sandstone lies and which marks the top of the Sauk sequence. In the central Michigan Basin, however, deposition of the Sauk sequence was continuous and the post-Sauk unconformity was not developed. Again, on the margins of the basin, a younger (Middle Ordovician) post–St. Peter unconformity was developed between the St. Peter and Glenwood Formations, but again is not present in the central basin where essentially continuous deposition of the entire section took place. The configuration of the present-day Michigan Basin was established during the Early Ordovician. Since that initial configuration, however, significant structural elements have been added during subsequent Paleozoic time.