Fig. 4--Isopach map of the Sauk II 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 are essentially of early Late Cambrian (Dresbachian) age. The map shows two depocenters, the one in southwest Michigan being a continuation of the Illinois Basin and the central Michigan thick area being the perennial Paleozoic depocenter of the Michigan Basin.