Like the sheepherders he studies, Mallea-Olaetxe immigrated to America from the Basque country of Spain. Unlike them, he was not under contract to a big sheep company. "My big company was the Catholic Church," he says. Raised on a remote mountain farm, Mallea-Olaetxe was sent to a Catholic boarding school when he was 9 and entered a seminary at 16. There he clashed with the oppressive regime of Franco, who ruled Spain from 1939 until his death in 1975. The schoolboys were forbidden to speak Basque. Every time Mallea-Olaetxe did, the teacher tightened a wire painfully wrapped around his finger. Mallea-Olaetxe's eyes still blaze with fierce Basque pride when he recalls Franco's clumsy attempts to destroy his language. It was an early sign of the activism that would define his life.

In 1964, the Catholic Church sent Mallea-Olaetxe to a monastery in upstate New York to learn English so that he could become a parish priest. After he was mugged over a pizza in front of the church, though, Mallea-Olaetxe left for Elko, Nev., in 1968, where he began ministering to the Basque herders who lived in the mountains. He saw his first arborglyphs by chance, riding horseback to a sheep camp in a remote region in northeastern Nevada. "You couldn't miss them, really, because the horses took us inches from the aspen trunks." Names, dates and drawings covered the trees from their bases to as high as a man could reach. But it was the extreme loneliness of the landscape that most impressed the newcomer, not the carvings. He mentioned it to the herder, who replied, "Remote, you say? God has yet to arrive here."

Mallea-Olaetxe's tenure as a priest in Elko was cut short in 1970 after he got in trouble with his bishop for officiating at a wedding in a Methodist church. So Mallea-Olaetxe began a decade of wandering -- leaving behind the books of his church-bound childhood to follow the life of a workingman. He tended bar in Winnemucca, kept bees in Arizona, and supported his wife and three children by growing and selling steak tomatoes in a back-to-the-land venture in Arkansas.

Ultimately, though, Mallea-Olaetxe returned to books. His graduate studies culminated in 1988 with a doctoral degree from University of Nevada, Reno, where he specialized in 16th century colonial Latin America and studied the life of Mexico's first Catholic bishop, who was Basque. He rediscovered arborglyphs by accident. During a break from his Ph.D. dissertation, he and his family went hiking among the aspens on Peavine Mountain at the edge of Reno. Mallea-Olaetxe's memory of the day is vivid: "The aspens were blazing gold, the sky was bluer than blue. And these words on trees were leaping out at me -- names and dates and places in Basque. I was staring at a history nobody knew anything about."

That day Mallea-Olaetxe decided to dedicate his academic life to piecing together this history and presenting it to the world. He began inventorying the carvings in hundreds of aspen groves, organizing the arborglyphs by their general message, the carver's place of origin and whether he wrote in Basque, Spanish or French. Names generally identify the carvers as Basques, but they also used a distinctive calligraphy, Mallea-Olaetxe says. He records the carvings with a digital camcorder, circling the tree while translating the words and commenting into a microphone. He has taken thousands of still photographs, which are combined with hundreds of hours of videotape in a carefully catalogued computer database. The work has taken him four-wheeling, backpacking and horseback riding into the mountains of 10 states from California to Montana -- wherever there are sheep camps in aspen groves.

The vast majority of the carvings are names and dates. Others express the pent-up longings of isolated men, often provoking a written dialogue with later herders. One carver wrote, "Wine and women are both good," and a second carver responded, "Yes, but they are hard on your pocket." Other arborglyphs are soliloquies, the intimate, private expressions of men who never expected anyone else to see them. Finding these carvings is like looking into someone's soul, Mallea-Olaetxe says, so he withholds their names when he describes what they wrote. "I cannot tell all. I don't want to betray these men."

He has identified and analyzed consistent themes involving daily life, politics and symbols -- some religious, some from Basque mythology. He has also examined the bread ovens and other artifacts the herders left in their camps, and interviewed hundreds of carvers. What has emerged is a rich account of the sheepherders' experience. Others have researched and recorded Basque carvings but Mallea-Olaetxe has taken the study beyond "folk art" to a comprehensive cultural history. "It's the difference between arrowhead collecting and an archaeological dig," says William Douglass, emeritus coordinator of the Basque studies program at the Reno university.

The sheepherders' cultural chronology is emblematic of the history of the American West, where emigrants flocked from around the world to reinvent themselves. Here, herders from Basque country joined other newcomers as individuals in a self-made society, where they could freely reflect on the lives they had left and the new ones they were creating. Their canvas was the smooth white bark of the quaking aspen. Arborglyphs are the happy coincidence of trees, loneliness and time, says Mallea-Olaetxe: This could not have happened in Europe.

These days, Peruvian, Chilean and Mexican immigrants tend to herd sheep in the high lonely country. Those Basques still in the business are often camptenders who have access to trucks and road systems that help them avoid the solitude. The last Basque arborglyphs Mallea-Olaetxe knows of were carved in the 1980s.

In the aspen stand above Lake Tahoe, he scrambles from one tree to another in search of a priest and a naked lady he saw carved on a tree here 10 years ago. He stops at a carving of a man smoking a cigar. As he translates the words, an off-color joke about masturbation, the former priest blushes and mutters something about American Puritanism. Then he is back on hands and knees, crawling over a jumble of downed trunks in hopes that the elusive priest and lady may be among them. They are gone, he sighs. Old age is claiming the aspen groves, here and throughout the West. Many are not regenerating, a problem scientists largely blame on fire suppression and warming temperatures. Mallea-Olaetxe now works as fast as he can to catalogue the undocumented arborglyphs before their trees succumb. With his goatee already flecked with gray, he is realistic about his odds of success. "I'd have to live 200 more years," he says. "We cannot help it. Nature will claim them."

Then, with a rueful shrug and a smile, he picks up his camcorder, focuses the lens on a carving of a man on a horse, and launches into an explanation for the historical record.