The Precambrian is the longest segment of geologic time, ranging from the formation of the earth 4.6 billion years ago to the beginning of the Cambrian Period about 542 million years ago. It is divided into the Archean Eon (4.6-2.5 billion years ago) and the Proterozoic (2.5 billion to 542 million years ago). Precambrian rocks underlie the state but are nowhere exposed at the surface because they are buried beneath much younger Paleozoic rocks. In western Ohio, Precambrian rocks lie at a depth of about 2,500 feet and dip eastward to a depth of about 13,000 feet in eastern Ohio. These rocks have been studied from samples taken from deep oil and gas wells and by remote geophysical methods.
Precambrian rocks beneath Ohio consist of igneous, metamorphic and sedimentary rocks. In western Ohio, Precambrian rocks are primarily granite and its fine-grained equivalent, rhyolite. These rocks have been radiometrically dated to about 1.5 billion years old. About a billion years ago, the continent began to split apart, forming the East Continent Rift Basin, a portion of which is present in western Ohio. As rifting progressed, the basin was filled with up to 20,000 feet of sediment. Rifting eventually stopped, resulting in a failed rift.
In central and eastern Ohio, Precambrian rocks are primarily igneous and metamorphic rocks about 800 to 900 million years old and represent the eroded roots of a mountain range, known as the Grenville Mountains, that stretched from Canada to Mexico. At this time, Ohio was part of the eastern edge of the North American continent (called Laurentia) and the Grenville Mountains were the result of the collision of what is now northern South America with the Laurentian continent.
The boundary between these two regions, known respectively as the Granite-Rhyolite or Central Province on the west, and the Grenville Province on the east, is a north-south zone in west-central Ohio known as the Grenville Front. This 30-mile-wide zone consists of westward-thrust rocks that mark the western edge of a mountain-building event known as the Grenville Orogeny.
By the beginning of Cambrian time, the Grenville Mountains had been eroded to a gently rolling topography with only about 300 feet of relief. These rocks would be flooded by Paleozoic seas in Late Cambrian time and would form the foundation of Ohio.