A saucer-shaped low plateau in northeast Thailand occupying 60,000 square miles and drained by the Chi and Mun rivers. Rice agriculture began before the 4th millennium BC. The plateau's development is divided into General Period A, c 3600-2000 BC; General Period B, c 2000-800/400 BC; General Period C, c 800/400 BC-300/500 AD; and General Period D, c 300/500-1300 AD. The initial settlement had polished stone adzes and stone and shell jewelry indicating some trade. The second period was the transition to the use of tin-bronze and production of bronze using imported metals. In the third period, iron replaced bronze and wet rice cultivation was established. The fourth period saw an expansion of settlements, formation of small states, long-distance trade, and some Indianization.