Upper Ordovician Maquoketa Formation in the Graf Area, eastern Iowa
James Hall (1858) was the first to recognize that a shaledominatedinterval, the Hudson River Group, separates the Galena Limestone (Middle-Upper Ordovician) from the overlying Niagara Limestone (Silurian) in eastern Iowa. Hall’s primary exposure of these rocks was located in the Little Maquoketa River Valley near present-day Graf, Iowa. There he observed“orthoceratite beds, ” graptolitic shales, and an abundance of “extremelysmall” fossils. Charles White (1870) named this sequenceof bluish and brownish shales the Maquoketa Shales after “typicallocalities on the Little Maquoketa River about twelve mileswestward from Dubuque” (i.e., near Graf). Joseph James (1890) described a railroad cut exposed in 1886 near the Graf station, which he termed the “typical locality of the shales in Iowa.” Thissame section is accessible today, but has been expanded to thenorthwest by subsequent road construction. Additional descriptions of the Graf section are givn by Calvin and Bain (1900), Thomas (1914), and Tasch (1955).
The Graf section (S½NW¼SW¼Sec.29, T.89N., R.lE., DubuqueCounty) is recognized as the type locality of the Upper Ordovician Maquoketa Formation, although the bulk of the Maquoketa Shale sequence in the area is represented by covered slopes. Access to this section is most convenient from the southon gravel roads that begin 0.5 mi (0, 8 km) west of the intersection of Old U.S. 20 and County Road Y21 (Figs. 1, 2). The Maquoketa Formation throughout the outcrop belt of northeast Iowa is expressed geomorphically as covered slopes to rollingsurfaces
Figures & Tables
North-Central Section of the Geological Society of America
One hundred field guides, with area maps, to locations in Illinois, Indiana, Iowa, Michigan, Minnesota, Missouri, Nebraska, Ohio, and Wisconsin.