In fact, archeological records of height of the Khmer empire for the period between Holocene and Iron Age remain equally limited. Other prehistoric sites of somewhat uncertain date are Samrong Sen (not far from the ancient capital of Oudong), where the first investigations began in 1877, and Phum Snay, in the northern province of Banteay Meanchey. Prehistoric artifacts are often found during mining activities in Ratanakiri.
And most outstanding prehistoric evidence in Cambodia however are probably various "circular earthworks", discovered in the red soils near Memot and in the adjacent region of Vietnam as of the end of the 1950s. Their function and age are still debated, but some of them possibly date from 2nd millennium BC at least.
For a pivotal event in Cambodian prehistory was the slow penetration of the first rice farmers from the North, which begun in the late 3rd millennium BC. They probably spoke ancestral Mon-Khmer.
Iron was worked by about 500 BC, with supporting evidence coming from the Khorat Plateau, which is now in modern day Thailand. In Cambodia, some Iron Age settlements were found beneath Angkorian temples, like Baksei Chamkrong. Others were circular earthworks, like Lovea, a few kilometers north-west of Angkor. Burials, much richer, testify to improvement of food availability and trade (even on long distances: in the 4th century BC trade relations with India were already opened) and the existence of a social structure and labor organization.