At the end of the Rigvedic period, the Aryan society began to expand from the northwestern region of the Indian subcontinent, into the western Ganges plain. It became increasingly agricultural and was socially organised around the hierarchy of the four varnas, or social classes. This social structure was characterized both by syncretising with the native cultures of northern India, but also eventually by the excluding of indigenous peoples by labelling their occupations impure. During this period, many of the previous small tribal units and chiefdoms began to coalesce into monarchical, state-level polities.