Silurian, suh LOOR ee uhn, Period was a time in Earth's history that lasted from around 440 million to 410 million years ago. Geologists first identified rocks of this age in the border region between England and Wales. The period takes its name from the Silures, an ancient Celtic tribe that lived in the area. Geologists have also found Silurian Period rocks in many other places, including notable formations in North America and China.