 The flat backs of many of the colossal heads represented the flat bases of the monumental <unk> from which they were reworked . Only four of the seventeen <unk> 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 <unk> the monuments with ropes . Two heads from San Lorenzo have traces of niches that are characteristic of monumental <unk> <unk> and so were definitely reworked from earlier monuments . 
