Dating the monuments remains difficult because of the movement of many from their original contexts prior to archaeological investigation . Most have been dated to the Early Preclassic period ( 1500 – 1000 BC ) with some to the Middle Preclassic ( 1000 – 400 BC ) period . The smallest weigh 6 tons , while the largest is variously estimated to weigh 40 to 50 tons , although it was abandoned and left unfinished close to the source of its stone .