Archaeological records for the period between Holocene and Iron Age remain equally limited. A pivotal event in Cambodian prehistory was the slow penetration of the first rice farmers from the north, which began in the late 3rd millennium BC. The most curious prehistoric evidence in Cambodia are the various "circular earthworks" discovered in the red soils near Memot and in the adjacent region of Vietnam in the latter 1950s. Their function and age are still debated, but some of them possibly date from 2nd millennium BC.