<P> Although all the colossal heads are broadly similar, there are distinct stylistic differences in their execution . One of the heads from San Lorenzo bears traces of plaster and red paint, suggesting that the heads were originally brightly decorated . Heads did not just represent individual Olmec rulers; they also incorporated the very concept of rulership itself . </P> <P> The production of each colossal head must have been carefully planned, given the effort required to ensure the necessary resources were available; it seems likely that only the more powerful Olmec rulers were able to mobilise such resources . The workforce would have included sculptors, labourers, overseers, boatmen, woodworkers and other artesans producing the tools to make and move the monument, in addition to the support needed to feed and otherwise attend to these workers . The seasonal and agricultural cycles and river levels needed to have been taken into account to plan the production of the monument and the whole project may well have taken years from beginning to end . </P> <P> Archaeological investigation of Olmec basalt workshops suggest that the colossal heads were first roughly shaped using direct percussion to chip away both large and small flakes of stone . The sculpture was then refined by retouching the surface using hammerstones, which were generally rounded cobbles that could be of the same basalt as the monument itself, although this was not always the case . Abrasives were found in association with workshops at San Lorenzo, indicating their use in the finishing of fine detail . Olmec colossal heads were fashioned as in - the - round monuments with varying levels of relief on the same work; they tended to feature higher relief on the face and lower relief on the earspools and headdresses . Monument 20 at San Lorenzo is an extensively damaged throne with a figure emerging from a niche . Its sides were broken away and it was dragged to another location before being abandoned . It is possible that this damage was caused by the initial stages of re-carving the monument into a colossal head but that the work was never completed . </P> <P> All seventeen of the confirmed heads in the Olmec heartland were sculpted from basalt mined in the Sierra de los Tuxtlas mountains of Veracruz . Most were formed from coarse grained dark grey basalt known as Cerro Cintepec basalt after a volcano in the range . Investigators have proposed that large Cerro Cintepec basalt boulders found on the southeastern slopes of the mountains are the source of the stone for the monuments . These boulders are found in an area affected by large lahars (volcanic mudslides) that carried substantial blocks of stone down the mountain slopes, which suggests that the Olmecs did not need to quarry the raw material for sculpting the heads . Roughly spherical boulders were carefully selected to mimic the shape of a human head . The stone for the San Lorenzo and La Venta heads was transported a considerable distance from the source . The La Cobata head was found on El Vigia hill in the Sierra de los Tuxtlas and the stone from Tres Zapotes Colossal Head 1 and Nestepe Colossal Head 1 (also known as Tres Zapotes Monuments A and Q) came from the same hill . </P>

Olmec colossal heads were created using this method of sculpting