On Aug. 18, 1846, Gen. Stephen W. Kearny and the Army of the West completed their march over the trail from Fort Leavenworth, Kan., and occupied Santa Fe.

Exactly one month later, Kearny sent a subordinate, Lt. Col. Congreve Jackson, with a detachment to examine conditions in the Navajo country. En route, the troops camped overnight near Laguna Pueblo.

In his journal, one of the soldiers wrote: "In the mountains nearby were large quantities of petrified timber. At some places entire trunks of trees, the remains of an extinct forest, were discovered on the steep slopes."

Thereafter, as Americans explored New Mexico, they would find here and there, much to their astonishment, an abundance of "petrified timber."

The widest and most spectacular field of the stuff was located in north-central Arizona, a territory detached from New Mexico in 1863. There today, visitors to Petrified Forest National Park can see numerous stone tree trunks richly colored in yellow, orange and red.

Most people want to know how wood petrifies. A fallen and buried tree becomes a candidate for fossilization when all conditions are right. The wood becomes something of a mold, since its structure and cells are replaced by silica and other minerals deposited by percolating ground water. The replacement of the wood often produces brightly colored quartz, like agate and jasper.

Examples of petrified wood can be found around the state, but it is perhaps most common in New Mexico's northwest quadrant. The extensive Bisti Badlands south of Farmington contain spectacular groupings of such wood.

Prominent paleontologists Barry Kues and Keith Rigby in 1977 stumbled upon a true fossil forest just east of the Bisti, containing dozens of petrified stumps "in their living position." Kues afterward wrote that the forest remnant "will probably undergo strip mining in the future."

A significant quantity of petrified wood exists in the vicinity of the Galisteo River that runs from Glorieta Pass down to its junction with the Rio Grande at Santo Domingo Pueblo. The stone forest is concentrated between the towns of Galisteo and Cerrillos, virtually all of it now on private land not accessible to the public.

Lt. Richard Smith Elliott, another member of the Army of the West, gave us one of the earliest references to fossilized trees along the Galisteo. He and a military company had been sent with the main herd of army horses to find grazing south of Santa Fe.

As they approached the town of Galisteo, wrote Elliott in a letter, "we saw four pieces of the petrified trunk of a tree, each about four feet long and 18 inches in diameter. The fractures were apparently caused by some violent effort of nature. The wood was evidently cedar." In fact, the majority of the trees were probably conifers, cone-bearing evergreens.

That was merely the first of innumerable petrified trees the lieutenant observed while guarding horses. He would come to declare: "This New Mexico is a paradise for geologists."

Now jump ahead five decades to the year 1900. Then a young Kansas man, Clarence Sweet, arrived on the scene and bought a 542-acre ranch on the river. In time, he would add another thousand acres or so.

Sweet farmed and raised cattle on what we would term a hardscrabble operation. From the beginning, he was fascinated by the multitude and size of the petrified logs that littered his ranch.

The largest tree was 4 feet in diameter and 185 feet in length. Some of the smaller logs had quite distinct growth rings.

Before long, Sweet decided to capitalize on this natural geological asset. He printed up brochures and flyers and went into the tourist business.

A large crude sign placed on a state road a mile away advertised "Sweet's Petrified Forest, Admission 50 cents." Before long, he was attracting up to 75 sightseers a day.

They were treated to a two-hour tour that showed off the best specimens of petrified wood. At one of these, their guide paused and pointed to a log he said displayed teeth marks of an ancient beaver who had cut down the huge tree millions of years ago.

Sweet always added, with a twinkle in his eye, that beavers back then were much larger than now.

On March 9, 1958, the Denver Post ran a story on his off-the-wall business venture. It appeared under the headline, "Rancher Finds Fossil Logs Pay Better Than Live Steers."

Not long afterward, Sweet suffered a heart attack and moved to Albuquerque to be near a hospital. His death within a short time was thought to have been hastened by the enforced departure from his beloved petrified forest.

Historian Marc Simmons is author of numerous books on New Mexico and the Southwest. His column appears Saturdays.