text:  The colossal heads cannot be precisely dated . However , the San Lorenzo heads were buried by 900 BC , indicating that their period of manufacture and use was earlier still . The heads from Tres <unk> had been moved from their original context before they were investigated by archaeologists and the heads from La Venta were found partially exposed on the modern ground surface . The period of production of the colossal heads is therefore unknown , as is whether it spanned a century or a millennium . Estimates of the time span during which colossal heads were produced vary from 50 to 200 years . The San Lorenzo heads are believed to be the oldest , and are the most skillfully executed . All of the stone heads have been assigned to the Preclassic period of Mesoamerican chronology , generally to the Early Preclassic ( 1500 – 1000 BC ) , although the two Tres <unk> heads and the La <unk> Head are attributed to the Middle Preclassic ( 1000 – 400 BC ) . 
  