Fig. 5--Isopach map Sauk III Sequence (after Fisher et al., 1988, modified with permission of the publisher, the Geological Society of America, Boulder, Colorado USA, Copyright©1988 Geological Society of America). These strata include the Franconian and Trempealeauan stages of the Upper Cambrian and the Lower Ordovician Prairie du Chien Group (Foster Formation of Fisher) and show clearly the great thickness preserved by downwarp during and at the end of the Sauk III Sequence. The depocenter is located in the usual central Michigan position.