Abstract Even before 1800, geological resources such as chert, iron, limestone, and coal were being utilized from the Pennsylvanian rocks of eastern Ohio and western Pennsylvania. These materials were of great interest to the early geologists of the region. This field trip discusses these products in the context of early grain milling, iron furnaces, and allied industries of Ohio and Pennsylvania in the late eighteenth and early nineteenth century, with a focus on two publicly accessible sites: McConnells Mill Park in western Pennsylvania, and Mill Creek Park in eastern Ohio. These parks contain publicly accessible gristmills and iron furnaces, and outcrops. We also provide new observations on cultural materials related to these industries, especially iron-furnace slag and millstones.