The landscape of the river basins is a very wide, flat, well-watered plain continuously refreshed with soil and sediment brought down by the rivers. The Lower Central plain from the delta north to Ang Thong Province is a flat, low area with an average of 2 m above sea level. Further north and into the plains of the Ping and the Nan the elevation is over 20 m. Then the mountains that are the natural boundary of the Chao Praya watershed form a divide, which has, to some degree, historically isolated Thailand from other Southeast Asian civilisations.