Neolithic agricultural settlements (c. 5500–3500 BC), such as those at Norovlin, Tamsagbulag, Bayanzag, and Rashaan Khad, predated the introduction of horse-riding nomadism, a pivotal event in the history of Mongolia which became the dominant culture. Horse-riding nomadism has been documented by archeological evidence in Mongolia during the Copper and Bronze Age Afanasevo culture (3500–2500 BC); this culture was active to the Khangai Mountains in Central Mongolia. The wheeled vehicles found in the burials of the Afanasevans have been dated to before 2200 BC. Pastoral nomadism and metalworking became more developed with the later Okunev culture (2nd millennium BC), Andronovo culture (2300–1000 BC) and Karasuk culture (1500–300 BC), culminating with the Iron Age Xiongnu Empire in 209 BC. Monuments of the pre-Xiongnu Bronze Age include deer stones, keregsur kurgans, square slab tombs, and rock paintings.