The beginning of the Cambrian Period about 570 million years ago is marked by the sudden, widespread appearance of multicellular marine animals having hard parts capable of fossilization. The Cambrian System was named for a succession of rocks in the Harlech Dome area of Wales by Adam Sedgwick in the 1830's. The apparent sudden appearance of fossils in rocks overlying unfossiliferous Precambrian rocks was a dramatic discovery to early geologists in their attempts to divide geologic time. The base of the Cambrian System marked the beginning of the Paleozoic Era, the era of ancient life.
Cambrian rocks underlie Ohio but are nowhere exposed at the surface.
Depth to Cambrian rocks ranges from about a thousand feet in southwestern Ohio to more than 10,000 feet in southeastern Ohio. These rocks therefore must be studied through information obtained from deep oil and gas wells and indirect geophysical techniques such as seismic reflection, gravity, and magnetic intensity. By the beginning of the Cambrian Period the lofty Precambrian mountains, formed a billion years ago by continental collision, had been reduced by erosion to a gentle, rolling landscape. Local relief on the Precambrian surface was not much more than about 300 feet at the time of deposition of Cambrian sediments. Ohio was on the stable, passive margin of the Laurentian continent, which had the ancient Canadian Shield as its nucleus.