the larger groups of West African agriculturalists, akin to the migratory Bantu agriculturalists and their encounters with Central African hunter-gatherers. Iron Age The iron industry, in both smelting and forging for tools and weapons, appeared in West Africa by about 2000-1200 BC. Iron smelting facilities in Niger and Nigeria have been radiocarbon dated to 500 to 1000 BC, and more recently in Nigeria from 2000 BC. Though there is some uncertainty, some archaeologists believe that iron metallurgy was developed independently in West Africa. Archaeological sites containing iron smelting furnaces and slag have been excavated at sites in the Nsukka region of southeast Nigeria in what is now Igboland: dating to 2000 BC at the site of Lejja (Eze-Uzomaka 2009) and to 750 BC and at the site of Opi (Holl 2009). Smelting furnaces appear in the Nok culture of central Nigeria by about 550 BC and possibly a few centuries earlier. The increased use of iron and the spread of ironworking technology led to improved weaponry and enabled farmers to expand agricultural productivity and produce surplus crops, which together supported the growth of urban city-states into empires. By 400 BC, contact had been made with the Mediterranean civilisations, including that of Carthage, and a regular trade in gold being conducted with the Saharan Berbers, as noted by Herodotus. The trade was fairly small until the camel was introduced, with Mediterranean goods being found in pits as far south as Northern Nigeria. A profitable trade had developed by which West Africans exported gold, cotton cloth, metal ornaments, and leather goods north across the trans-Saharan trade routes, in exchange for copper, horses, salt, textiles, and beads. Later, ivory, slaves, and kola nuts were also traded. Tichitt culture In 4000 BCE, the start of sophisticated social structure (e.g., trade of cattle as valued assets) developed among herders amid the Pastoral Period of the south and central Sahara. Saharan pastoral culture was intricate, as evidenced by fields of tumuli, lustrous stone rings, axes, and other remnants. By 1800 BCE, Saharan pastoral culture expanded throughout the Saharan and Sahelian regions. The initial stages of sophisticated social structure among Saharan herders served as the segue for the development of sophisticated hierarchies found in African settlements, such as Dhar Tichitt. After migrating from the Central Sahara, Mande peoples established their civilization in the Tichitt region of the Western Sahara. The Tichitt Tradition of southeastern Mauritania dates from