Central steppe is an informal term for the middle part of the Eurasian steppe. It is grassland with some semi-desert and becomes dryer toward the south. On the east it is separated from Dzungaria and the eastern steppe by the low mountains along the current Chinese border. On the west it merges into the western steppe along the narrowing between the Ural Mountains and the Caspian Sea. On the north it is bounded by the forests of Siberia. The southern boundary has three sections. In the east the Tian Shan mountains of Kyrgyzstan extend about 650 kilometres west and give the steppe a sharp southern boundary. The center is approximately the line of the Syr Darya which runs from the eastern mountains northwest to the Aral Sea. South of the Syr Darya the steppe grades into semi-desert but there are cities based on irrigation agriculture which give the area a different history. The western part between the Aral and Caspian Seas is dry and thinly populated. The Syr Darya and the area between the Urals and Caspian were not significant barriers and the low mountains of Dzungaria were fairly easy to cross. The other boundaries were significant barriers to movement.