Archeologists digging here in the remote mountains southwest of Cuernavaca, where villagers speak an ancient Indian tongue, have unearthed the earliest stone buildings found so far on the North American continent, along with a monumental carved stone head.

Confirmed dates for the site, built by the ancient Olmec civilization, are 1200 B.C. to 600 B.C., with preliminary laboratory testing of organic remains indicating dates going back as far as 1400 B.C. (roughly the time of King Tutankhamen in Egypt), and two layers beneath that material yet to be excavated. The oldest stone structures previously discovered in North America date from about 200 B.C.

Gillett Griffin, director of the Princeton University Art Gallery and a leading Olmec scholar, said, "Copalillo is the most important site being worked on in Mexico. The find means high civilization existed much earlier in Mesoamerica than we thought." Carlo Gay, a scholar who has published extensively on ancient art in the Guerrero region, called the discoveries "exciting and important."

The Olmecs, who are thought to have founded the "mother civilization" of Pre-Columbian America, left behind their typical calling card -- a large stone human head, which is a smaller version of those found on ancient sites in the Gulf Coast swamps of Veracruz and Tabasco. The 3-by-3 foot head was found set into a wall surrounding a ceremonial center consisting of highly finished stone architecture, with huge blocks of travertine smoothly worked and fitted together without mortar. Adjacent to the building remains are stone drains and an aqueduct, and the earliest ball court in America, where the Olmec played a version of the rubber-ball game that spread over the entire continent centuries before Columbus set foot in the New World.

Also found at the site are four slablike stone carvings of jaguar faces that were once mounted in a crenelated pattern atop the walls of the square-shaped ceremonial center. The back of one of the inverted-T-shaped monoliths has what might be evidence of the earliest use of written numbers in the New World: two bars and four dots are carved in a cartouche or frame, forming the number 14 in the Pre-Columbian calendrical system. (Previously, the earliest written numbers in North America were found on a carving from Tres Zapotes on the Gulf Coast, expressing the date 31 B.C. in bar-and-dot notation.)

In the back of another of the jaguar monoliths, excavators found a hollowed-out pocket in the stone containing the decayed remains of an ancient offering, now awaiting testing in a Mexico City laboratory. One of the jaguar stones, taken into Mexico City for inspection and then returned to the site, weighed almost 3 tons. Jaguars had religious significance to the Olmec, who may have traced their lineage to the cats.

The excavation, conducted by archeologist Guadalupe Martinez Donjuan for the Instituto National Antropologia y Historia (INAH), the government agency in charge of archeological sites in Mexico, is in its third season. Preliminary results of the dig -- including the discovery of the stone head -- are reported this month in a special publication issued by the INAH office of the west Mexico state of Guerrero.

Copalillo is located in a range of dry, rugged mountains near the juncture of the Mezcala and Amacuzac Rivers, in the Balsas River drainage system in Guerrero. The archeological excavation is nestled up against a cone-shaped hill that adjoins the river. Long before the Spanish arrived, the Indians called it Teopantecuanitlan, which translates from the native Nahuatl language as Place of the Palace of the Jaguars, according to Martinez Donjuan. Although the Indian population long believed the area to be sacred, Copalillo came to the attention of archeologists after villagers complained to government officials of looters digging in their fields. INAH then launched a rescue operation, uncovering the ancient carved jaguars.

A visit to the site reveals fresh remnants of small fires near the excavated stone jaguar carvings, where villagers and farmers still burn copal incense in homage to the ancient gods.

"They come to make offerings to the monuments when they think we are not looking," said the archeologist, who grew up in Guerrero.

A recent group of visiting scholars was asked by Martinez Donjuan not to step on flowers planted by local farmers at the entrance to an excavation level dating before 1000 B.C. Although the Spanish named the area El Rincon, or The Nook, INAH now lists the site under its Indian name.

"So far, we have three phases of construction spread over nearly 100 square kilometers," said Martinez Donjuan. The earliest phase, from 1400 B.C. TO 1200 B.C., saw the rise of massive yellow clay walls in the ceremonial center, with two staircases decorated with carved clay masks resembling beasts with flame-shaped eyebrows, a typical Olmec motif.

The second phase stretches from 1200 B.C. to 1000 B.C. and includes the skillfully fitted stone walls lining the previous clay architecture, the aqueduct, stone drains, and an interior court with two sunken rectangular areas. This phase corresponds to the earliest Olmec dates at San Lorenzo on the Gulf Coast, which contains carved stone artifacts but no stone architecture.

The third phase has cruder construction with chinked stone walls in V-shaped patterns, dating from 800 B.C. to 600 B.C. There are six structures in a semicircle at the northern approach to the site from this period, which also includes the outer wall where the stone head was found, said Martinez Donjuan. "The head is cut off at the forehead, and may have been reworked and set into the wall."

Because of the almost complete symmetry of the site, which is only millimeters off a perfect north-south axis, the archeologist said she expects to find another monumental head in the unexcavated portion of the wall.

Dating of the site has been done with carbon-14 testing, which measures the rate of decay of organic matter such as wood, bone, or remains of plants. Results are then compared to the sequence of pottery types found at Copalillo going back to 1200 B.C. Much material from the sites remains to be tested, pending the reopening of the INAH laboratory, damaged in last year's Mexico City earthquake.

A habitation survey was conducted by archeologist Christine Niederberger. Anthropologist Reyna Robles is working with Martinez Donjuan to compile all tests, dates and ceramic evidence for publication this summer.

The importance of the new site lies in its location, early time horizon and extensive waterworks including canals and a dam, according to Joaquin Garcia Barcena, head of INAH's central office for Prehispanic archeology. "It has early stone architecture, an early use of a calendar date, and the earliest well-defined ball court," he noted.

The symbols on the carvings are agricultural motifs, Martinez Donjuan said. Still unanswered is whether the desiccated, almost lunar, landscape of Guerrero could support extensive agriculture in ancient times. The earliest strains of maize, or corn, are attributed to the Olmec, who are thought to have developed extensive farming methods.

The excavations at Copalillo once more raise the question of where the Olmec originated. A people evidently in love with stone, they left beautifully carved jade artifacts, concave magnetite mirrors and carved stone masks wherever they went. Olmec remains have been found beneath Mexico City, near Oaxaca and at sites as far south as Copan, Honduras -- nearly the extent of the Pre-Columbian culture region known as Mesoamerica, the area of high civilization north of the Panama isthmus.

This stone tradition originally led the late Miguel Covarrubias, an artist who was one of the first to proclaim the importance of the Olmec in the 1940s, to predict that they originated in the stony mountains of Guerrero. He believed that the Balsas River contained the source of Olmec jade in ancient mines that are still unknown, and that a major Olmec site would someday be found in these remote parts.

"The important work done by Guadalupe Martinez Donjuan cerainly tends to confirm a highland origin," said Griffin of the Princeton University Art Gallery, who has long promoted Covarrubias' theory.

Griffin was among the only group of U.S. scholars allowed to visit the Copalillo site so far, last December. The group, led by architect E. Logan Wagner, a specialist in Pre-Columbian and Colonial architectural restoration, included Jane and D.J. Sibley and F. Kent Reilly of the University of Texas at Austin.

Other scholars also hailed the new findings. Michael Coe, a leading Yale University anthropologist and author who excavated the lowland site of San Lorenzo in the 1960s, said, "Guadalupe Martinez Donjuan is to be congratulated. Her finds sound most interesting." Coe, who favors a lowland origin for the Olmec civilization, said he could not comment further since he has not seen any of the material or publications about it.

Security at Copalillo has been tight since the site was first excavated in 1983 after looters were found at work with heavy equipment. Villagers alerted Martinez Donjuan, who was working in the Guerrero INAH office. With some 25,000 potential archeological digs of Pre-Colombian sites listed in government files for all of Mexico, INAH officials arrange priorities according to whether the material might be endangered. Copalillo was immediately placed on the rescue list.

"I told them I would be there on Tuesday and arrived instead the previous Saturday, finding the looters at work," she recalled. The highly organized thieves were so confident that they had set up living quarters complete with their own power generator and television set. Martinez Donjuan had two men arrested, who were later set free by authorities in Chilpancingo, the state capital. Before being stopped, the looters, who were looking for items left in the Olmec burials, had dug into at least one ancient grave.

However, they missed one of the most poignant burials. Beneath a sheared-off stela, or free-standing stone column, Martinez Donjuan later excavated the grave of a young Olmec child, who was accompanied in death by two young jaguars, their skeletons perfectly preserved.