 All seventeen of the confirmed heads in the Olmec heartland were sculpted from basalt mined in the Sierra de los <unk> mountains of Veracruz . Most were formed from coarse grained dark grey basalt known as Cerro <unk> basalt after a volcano in the range . Investigators have proposed that large Cerro <unk> 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 <unk> head was found on El Vigia hill in the Sierra de los <unk> and the stone from Tres <unk> Colossal Head 1 and <unk> Colossal Head 1 ( also known as Tres <unk> Monuments A and Q ) came from the same hill . 
