Ordovician Period, a division of geologic time that began about 500,000,000 years ago and lasted 70,000,000 years. It is the second period of the Paleozoic Era, coming between the Cambrian and Silurian periods. The Ordovician Period is named for the Ordovices, an ancient tribe that lived in Wales, the area where the rocks of this period were first found.

During the Ordovician Period there was a great extension of the seas. Most of the interior of North America was covered by shallow inland seas in which limestone, dolomite, shale, and some sandstone were formed. Some mountain building took place during the period in eastern North America.

The fossil remains of many animals are found in rocks from the Ordovician Period Trilobites and graptolites are particularly common. Crinoids and starfishes make their first appearance in the fossil record in Ordovician rocks.

Valuable ores of zinc, lead, manganese, and gold are found in Ordovician rocks. Petroleum and natural gas are also found.