The Nanda kings appear to have strengthened the Magadha kingdom ruled by their Haryanka and Shaishunaga predecessors, creating the first great empire of northern India in the process. Historians have put forward various theories to explain the political success of these dynasties of Magadha. Pataliputra, the capital of Magadha, was naturally protected because of its location at the junction of the Ganges and the Son rivers. The Ganges and its tributaries connected the kingdom with important trade routes. It had fertile soil and access to lumber and elephants of the adjacent areas. Some historians have suggested that Magadha was relatively free from the Brahmanical orthodoxy, which may have played a role in its political success; however, it is difficult to assess the veracity of this claim. D. D. Kosambi theorised that Magadha's monopoly over iron ore mines played a major role in its imperial expansion, but historian Upinder Singh has disputed this theory, pointing out that Magadha did not have a monopoly over these mines, and the iron mining in the historical Magadha region started much later. Singh, however, notes that the adjoining Chota Nagpur Plateau was rich in many minerals and other raw materials, and access to these would have been an asset for Magadha.