The second main phase of occupation on the islands is what is referred to as the Bronze Age civilisation. This dates from about 1500 BC to 500BC. It so called because of the bronze objects made by these people. Cnoc Raithní in Inis Oírr is an example of a bronze age burial mound in which various bronze objects were found( cf. Monuments of Inis Oírr). Bronze age objects were also found at Dún Aonghasa one of the great stone forts of the islands. The Bronze Age was followed by the Iron Age and the arrival of the Celts. The Iron Age is so called because the people used Iron. The Celts were especially famous in Europe for their smithcraft. Some of the forts are also thought to be iron age.