Hopkinton Dolomite, previously divided on the basis of paleontologic units, is here divided into lithologic members to facilitate interregional correlations based on sea-level curves. Rocks formerly called FAVOSITES beds of the Hopkinton are here named the Picture Rock Member of the Hopkinton Dolomite in eastern IA. Consists of tan to brown, very thick bedded or massive, medium crystalline, highly vuggy dolomite that has abundant tabulate coral colonies. Overlies the Farmers Creek Member (new) and underlies the Johns Creek Quarry Member (new) both of the Hopkinton. Thickness at the type section ranges from 10.75 to 11.50 m. Age is early Silurian (Llandoverian).

Source: GNU records (USGS DDS-6; Reston GNULEX).

According to author, one of his primary aims in this report is to strongly encourage adoption of IA nomenclature in adjacent IL. The only major change is the incorporation of the Sweeney and Marcus as members within the Hopkinton and replacement of the Racine with upper members of the Hopkinton, the Scotch Grove Formation, and the Gower Formation. The Hopkinton in the Quad Cities area of IA and IL is divided into (ascending) Sweeney, Marcus, Farmers Creek, and Picture Rock Members. The Picture Rock lithically resembles the Sweeney, but is less cherty. Underlies the Johns Creek Quarry Member of the Scotch Grove Formation. Age is Early Silurian (Llandoverian).

Source: GNU records (USGS DDS-6; Reston GNULEX).