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  M-TR11Magnetic Explorationof the 0lmec Civilization Sheldon Breiner, Michael D. Coe  American Scientist , Sep-Oct 1972, Vol. 60, number 5 [courtesy American Heritage Pub.] Sheldon Breiner, Michael D. Coe Magnetic Exploration of the 0lmec Civilization  Magnetic surveys have been highly successful in locating Olmec monuments at the site of the oldest known civilization in  Mesoamerica The dream of all archaeologists has been an instrument that would allow them to see beneath the surface of the ground, even before excavation. Such an instrument has now been developed and successfully tested in the jungle country of southern Mesoamerica-that region of Mexico and Central America that was civilized before its conquest  by the Spaniards. Sheldon Breiner, geophysicist with GeoMetrics, Palo Alto, California, received a B.S., M.S., and Ph.D. in geophysics from Stanford University. From approximately 1960 his work involved numerous field experiments utilizing the then newly developed high sensitivity alkali vapor magnetometers. His research led to such diverse projects as archaeological prospecting, Petroleum and mineral exploration, micropulsation research, avalanche rescue, and earthquake prediction, the last being the subject of his dissertation. He is presently active in developing airborne and marine geophysical survey and interpretation techniques for mineral and petroleum exploration, and is a lecturer and Research  Associate in the Department of Geophysics at Stanford University.  Michael D. Coe, Professor of Anthropology at Yale University, received his B.A. (1950) and Ph.D. (1959) from Harvard University. Before coming to Yale, he taught at the University of Tennessee. He is advisor to the Bliss Collection of Pre-Columbian  Art at Dumbarton Oaks, Washington, D. C. His research interests are Mesoamerica and the srcins of its civilization; recent studies include Maya iconography and cosmology. In connection with his research, he has excavated Olmec sites in southern Mexico and on the Pacific coast of Guatemala. His publications include Mexico (1-962), The Jaguar's Children (1965), The Maya (1966), and America's First Civilization (1,958). The authors wish to thank Froelich Rainey and Miss Elizabeth  Ralph, of the Applied Science Center for Archaeology of the University of Pennsylvania; Ignacio Bernal, of the Instituto  Nacional de Antropologia e Historia, Mexico; D. P. O'Brien and  A. R. Edberg of GeoMetrics; and Varian Associates of Palo Alto, California. Addresses: Dr. Breiner, GeoMetrics, 914 Industrial  Avenue, Palo Alto, CA 94303; Dr. Coe , Department of  Anthropology, Yale University, New Haven, CT 06510. It is now known that the Olmec civilization, dating from approximately 1200 B.C. to 400 B.C., was the earliest of these native cultures (1). Most Olmec sites are concentrated in a relatively small heartland along the humid, fertile coastal lowlands of southern Veracruz and Tabasco, but some colonial Olmec centers have been found in the central Mexican highlands and in the state of Guerrero. The  primary jungle 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. In both the monumental carvings and the finely worked objects of jade and serpentine, the dominant themes seem to be religious symbolization of gods, represented by a combination of the jaguar and the human infant. Of the four largest Olmec sites in the heartland, the oldest (2) now appears to be San Lorenzo, located on a side  branch of the Coatzacoalcos River in southern Veracruz (Fig. 1). This center, discovered in 1945 by Matthew W. Stirling (3), of the Smithsonian Institution, quickly proved to have the finest and largest Olmec monuments of all. During the two years that he explored San Lorenzo, the carvings were typically discovered either at the bottom of deep ravines cutting into the site or on their slopes. None was found in its srcinal position, and Stirling concluded that they had been pushed over the side by non-Olmec invaders at some unknown time. Intrigued by the possibilities of throwing new light upon this ancient civilization, Coe (4) began in 1966 a long-term investigation of San Lorenzo, under the auspices of Yale University and the Instituto Nacional de Antropologia e Historia in Mexico, financed by the National Science Foundation. The first line of inquiry was into the nature of the site itself, which had never been mapped. It was discovered that San Lorenzo was a flat plateau about a kilometer and a quarter long in the north-south direction, with ravines extending from it on the northwest, west, and south sides. Rather than being formed by erosion, the ravines were man-made, resulting from the construction of enclosing ridges, obviously planned. Pairs of them proved to have mirror symmetry, such as a mound on one side  being neatly matched by one on the opposite ridge. Figure 1. (prev page) This greatly reduced map of San Lorenzo in southern Veracruz, Mexico, shows the general outline of the plateau with the ridges and ravines. Note the symmetry of the ridges on the south and west sides of the plateau. The monuments indicated by small circles on the map were found before the magnetometer survey. The colored circles show the   page 2 location of Monuments 51, 52, 53, and 61, which are among the more important objects discovered during the survey. Our second discovery, made through ceramic stratigraphy and radiocarbon dating, was that 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 discovered centuries later in the ravines had simply come to light through the gradual erosion of this fill. Figure 2. View of the Tuxtla Mountains from San Lorenzo, Mexico. Basalt rock from these mountains was transported approximately 70 kilometers to San Lorenzo, where it was carved into monuments. The discovery of this pattern of buried sculptures was purely accidental, a piece of luck that occasionally turns up on every expedition. The  possibility immediately suggested itself that a great number of other Olmec carvings might still lie under the soil of San Lorenzo. How could these be found? Having had experience in the use of magnetometers for archaeological exploration, Froelich Rainey, Director of the University of Pennsylvania's Applied Science Center for Archaeology, suggested using such instruments at San Lorenzo. Several geophysical techniques based on magnetic, electrical, seismic, or gravimetric methods have been used in archaeological prospecting (5). Magnetic surveying has proved to be by far the most practical and useful. Although not a common tool in archaeological kits, magnetometers have been utilized during the past decade at various sites around the Mediterranean (where they helped in finding ancient Sybaris, 6), in England, and in North America. Magnetic anomalies at archaeological sites The magnetic anomalies of significance in archaeological exploration are caused by the contrasting properties of the cultural feature and the soil, water, or rocks covering it (7). The amount of the very common mineral magnetite in the feature as well as its mechanical and thermal history usually determine the size of the disturbance, or anomaly, in the earth's magnetic field, which is actually measured  by the magnetometer. Various rocks, soils, and objects foreign to the site possess different magnetic  properties owing to the widely varying amounts of magnetite and whether or not the magnetic elements of the magnetite grains of the feature are aligned, i.e. the relative proportions of induced and remanent (permanent) magnetism. Buried rocks, walls, artifacts of various types, tombs, trenches, and other such features are all detectable under the right circumstances. The most prominent magnetic anomalies are usually caused by natural materials that have undergone heating. Clay objects that have been subjected to high temperature, such as bricks, tiles, pottery, and firepits, attain a remanent magnetism as a consequence of the alignment of their magnetically susceptible elements with the earth's magnetic field during the process of cooling, Such remanent magnetism is also a property of rocks that have been heated in nature, especially volcanic or igneous rocks. Almost all the San Lorenzo monuments were carved from such rock - basalt - which is not native to the area but which was laboriously brought in from the Tuxtla Mountains, some 70 kilometers to the northwest (Fig. 2). Figure 3. Surveying the San Lorenzo site with the magnetometer. First of all, we had to determine whether or not magnetic surveying would aid the exploration of San Lorenzo, since most sites are, in fact, unsuitable for this technique. In February 1968, Rainey assessed conditions there and obtained samples of the monuments and of the fill in which they are  principally buried. The induced and remanent   page 3 magnetization of the monuments proved to be 2 X 10-1 and 4 X 10-4 emu, respectively, contrasted with a total magnetization of the fill of less than 3 X 10-1 emu. Thus, there was enough difference between the magnetism of the objects and the surrounding fill to make San Lorenzo an ideal site for the effective use of magnetic surveying. Further, there are no deeper-lying rock strata at the site to interfere with the observed anomalies; in fact, because there are no natural rocks, all anomalies would be significant. Finally, San Lorenzo is happily remote from any recent man-made implements, vehicular traffic, other iron and steel interference, and electric power lines. Figure 4. This figure, over one meter high, represents the rain god (Monument 52) and is one of the finest of all Olmec sculptures. It was found as a result of the preliminary magnetometer survey. Magnetic surveying The following month we brought a portable cesium magnetometer to San Lorenzo (Fig. 3). This instrument has a sensitivity of 0.1 gamma (10-6 oersted) and can be operated in either a search or survey mode (7, 8). As a search device, it was used to take occasional readings visually or audibly, noting more the location of the anomaly than its amplitude. This mode is useful for rapid reconnaissance, for obtaining an overview of site conditions, and for tracing long anomalies (such as a wall). We chose initially to traverse San Lorenzo on horseback, particularly in the high grass and some dense forest areas. The local saddles were made of wood, and there were no steel horseshoes to interfere with the instrument. Almost immediately, we located an anomaly and estimated the depth of what turned out on excavation to be one of the finest of all Olmec sculptures-a rain god with typical half-human, half-jaguar features (Fig. 4), lying at the predicted depth of 2'/2 meters at the head of a buried drain system. Several more monuments were found in this manner, and archaeologists were as mystified as the local workmen at the uncanny ability of the magnetometer to see buried objects. Figure 5. Total magnetic intensity contour map (above) and  perspective view of contoured data (center) over a buried Olmec altar (Monument 51), shown ~below) after excavation. The altar caused the prominent anomaly in the center, indicated by the colored block; other anomalies are caused by neighboring small monuments. The sharply defined square depression on the  perspective view is the effect of missing data. The map represents an area approximately 50 meters on a side. It was necessary, however, to conduct a survey systematically in order to obtain complete coverage of the area and to find objects that responded less noticeably to the sensor or that were more deeply
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