From Ohio History Central
The Canadian Shield — also called the Laurentian Plateau is a large geological shield covered by a thin layer of soil that forms the nucleus of the North American or Laurentia craton.
Precambrian igneous and metamorphic rocks are exposed in eastern and central Canada and form the core of the North American continent. Continental collisions and mountain-building events added crust to the shield during Precambrian time. Some of these rocks underlie Paleozoic sedimentary rocks in Ohio.