North American Database of Archaeological Geophysics
North American Database of Archaeological Geophysics
Abstract/Summary:
Project Name: San Lorenzo, Veracruz, Mexico;
Reference: Breiner, S. and M.D. Coe. (1972). Magnetic Exploration of the Olmec Civilization. American Scientist. 60:566-575 (used with permission).
San Lorenzo is the largest Olmec site located along the humid, fertile coastal lowlands of southern Veracruz and Tabasco. It is located on a side branch of the Coatzacoalcos River in southern Veracruz. Olmec sites are best known for their magnificently carved monuments, usually made from basalt and weighing up to forty tons. The most striking are the colossal heads—gigantic stone portraits of rulers who are depicted as thick-lipped, flat-faced personages wearing what appear to be helmets. San Lorenzo, discovered in 1945 by Matthew W. Stirling of the Smithsonian Institution, proved to have the finest and largest Olmec monuments of all. The carvings were typically discovered either at the bottom of deep ravines cutting into the site or on their slopes. In 1966, it was discovered, through ceramic stratigraphy and radiocarbon dating, that in about 900 B.C. a major act of destruction took place at San Lorenzo. Every single piece of carved stone had been mutilated and then dragged onto specially prepared floors built on the ridges, which were completely covered by a fill composed of soil, gravel, and other debris. The monuments that Stirling had discovered in the ravines in 1945 had simply come to light through the gradual erosion of this fill.
A magnetic survey was undertaken in order to locate other Olmec carvings that were still buried. Because they were carved out of igneous rock (basalt), the carvings possessed a remnant magnetism that was different from the magnetism of the surrounding fill. This made San Lorenzo an ideal site for the effective use of magnetic surveying. The survey occupied three field seasons, during which archaeologists from Mexico's Instituo Nacional de Antropología e Historia and from Yale University conducted the digging to test whether there actually were monuments under the mapped magnetic anomalies. Seventeen Olmec monuments were discovered that would otherwise have completely eluded even the shrewdest and most patient archaeologist. The majority were not in the ridges (where the magnetic survey was incomplete) but on the central part of the San Lorenzo plateau, where we had not expected to find them.