The controversy involved the boundary between the Cambrian and Silurian Systems. The name comes from an ancient Welsh tribe, the Ordovices. The continents were moving together and as a result, mountain building occurred. This was apparently caused by the collision of North America with Greenland and Europe, resulting in the first step in the building of the Appalachian Mountains on the east coast of North America. This lasted throughout the Ordoviian and is called the Taconic Orogeny.
Most Ordovician rocks are rich in fossils.
Brachiopods were the dominant invertebrate. Few of the Cambrian trilobites survived into the Ordovician, but the Ordovician seas were populated with new forms. Gastropods and bivalves became more plentiful and larger, and the other major class of mollusks, the cephalopods, also increased in size and diversity. Corals appeared for the first time along with graptolites. Graptolites are useful for correlation of rocks on different continents.