Sweden was covered by a thick ice cap until 14,000 years ago, when the ice began to retreat. The first human traces, in southern Sweden, date from 10,000 BC. Between 8000 and 6000 BC the country was populated by hunters and fishermen, using simple stone tools. Artefacts found in graves show that the Bronze Age was marked by a relatively advanced culture. From 500 BC to AD 800 agriculture became the basis for society and the economy. The Viking Age (800–1050) took expansion eastwards. Swedish Vikings reached into today’s Russia, where they set up trading stations and principalities, such as Novgorod and Rurik. The Vikings also travelled to the Black and Caspian Seas and developed trading links with the Byzantine Empire and the Arabs.