It is unknown what triggered the Vikings' expansion and conquests. This era was at the same time as the Medieval Warm Period (800 – 1300) and stopped with the start of the Little Ice Age (about 1250 – 1850). The lack of pack-ice during their time may have allowed the Norsemen to go "a-viking" or "raiding". It is believed that the heathen Norsemen suffered from unequal trade practices by Christian merchants who were given preference through a Christian network of traders. A two-tiered system of pricing existed among merchants who secretly traded with the Norse heathens. Viking raids occurred both separately and together with regular trading expeditions.