 Archaeological investigation of <unk> basalt workshops suggest that the colossal heads were first roughly shaped using direct percussion to chip away both large and small <unk> of stone . The sculpture was then refined by <unk> the surface using <unk> , which were generally rounded <unk> that could be of the same basalt as the monument itself , although this was not always the case . <unk> were found in association with workshops at San Lorenzo , indicating their use in the finishing of fine detail . <unk> 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 <unk> and <unk> . 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 . 
