The flat backs of many of the colossal heads represented the flat bases of the monumental thrones from which they were reworked . Only four of the seventeen heartland heads do not have flattened backs , indicating the possibility that the majority were reworked monuments . Alternatively , the backs of many of these massive monuments may have been flattened to ease their transport , providing a stable form for hauling the monuments with ropes . Two heads from San Lorenzo have traces of niches that are characteristic of monumental Olmec thrones and so were definitely reworked from earlier monuments .