Previously divided primarily on the basis of paleontologic units, the approximately 60-80 m thick Hopkinton Dolomite in eastern Iowa also comprises a set of lithologically unique subunits. With the development of a new capability for inter-regional correlation of Lower Silurian strata based on the use of sea-level curves, it is especially appropriate to recognize these subdivisions of the Hopkinton Dolomite as formal member units. Locally, the relationships shown by these units may also contribute to a better understanding of the Plum River Fault Zone and its associated structures in Iowa and Illinois. Willman (1973) named the Sweeney and Marcus Formations for strata in northwestern Illinois that he physically correlated with the Syringopora and Pentamerus Beds of the lower Hopkinton Dolomite. It is recommended that these units receive wider application as the Sweeney and Marcus Members of the Hopkinton Dolomite. The names Farmers Creek, Picture Rock, Johns Creek Quarry, Welton, and Buck Creek Quarry are proposed as members of the remaining middle to upper Hopkinton Dolomite. These names supersede the Cydocrinites, Favosites, Bioherm, Cyrtia, and Pentameroides Beds, respectively (Johnson, 1975; 1980)
