Apparently the first man to report the “stone trees” was Lieutenant Sitgreaves, an Army officer who explored parts of northern Arizona in 1851, soon after Arizona was acquired by the United States.
The petrified forests remained largely unknown, however, until the settlement of northern Arizona began in 1878 and the Atlantic and Pacific, now the Santa Fe Railway, was completed across northern Arizona in 1883. During the following years, the existence of the petrified forests was threatened by souvenir hunters, gem collectors, commercial jewelers, and abrasive manufacturers. Entire logs were blasted to obtain the quartz and amethyst crystals often found within the logs, and much agate was carried away for making jewelry. The erection of a stamp mill near the forests to crush the petrified logs into abrasives offered the most serious threat. Alarmed, the citizens of Arizona, through their territorial legislature, petitioned Congress to make the area a national reserve “so that future generations might enjoy its beauties, and study one of the most curious results of nature’s forces.”
Following an investigation by Lester F. Ward, of the United States Geological Survey, Petrified Forest National Monument was established by President Theodore Roosevelt on December 8, 1906, under authority of the Act for the Preservation of American Antiquities.
THE GEOLOGIC STORY
The Forests.—About 160 million years ago, in Triassic time, northern Arizona was probably a lowland where shifting streams spread sand and mud over the plains. Scientists believe that the growing forests were upstream, possibly as much as a hundred miles west and southwest of the present petrified forests. The principal tree resembled modern pines, but was more closely related to the Araucarian pines of South America and Australia. Remains of two other kinds of primitive trees are also occasionally found.
The Trees Were Buried.—Natural processes, sometimes hastened by destructive fires and ravages of insects, are believed to have killed the trees. Certainly, many of them decayed on the ground, but others fell into streams and came to rest in bays or on sand bars where rapid burial by mud and sand prevented their decay. The deposits in which these trees were buried eventually turned to the hard sandstones and shales now called the Chinle formation. These were buried at least 3,000 feet beneath layers of sand and silt spread by shallow seas.
The Logs Petrified.—The sediments in which the logs were buried contained a large amount of volcanic ash, high in the mineral silica. This silica was picked up by ground water, carried into the wood, and deposited in the cell tissue. The mineral filled the wood solidly, forming the present petrified log. However, the greater part of the original cellulose remains. The various color patterns were caused by oxides of iron and manganese. This particular type is known as agatized wood. Cavities in the logs were often filled or lined with quartz crystals.
How the Forest Was Brought to Light.—Since the forest was buried there have been several periods of great mountain making. Forces from deep in the earth thrust the Rockies and Sierras upward several thousand feet, and the land between was lifted far above its former position near sea level. As a result of this great mountain growth, certain areas became arid and desertlike; soon wind and rain started wearing down the great deposits that covered this region. Large river systems carried away the loose mud, sand, and gravel which finally found its way to the sea. Thus the sediments that once covered the forest were removed. Finally the layers in which the logs were buried were cut by canyons and ravines, revealing the great petrified logs and the many bands of colored rock that make up the Painted Desert. It is believed that the petrified logs were cracked in many places by the weight of sediments and earthquakes during the mountain uplift period. As the logs wash from the hillsides, they break into sections which accumulate in piles at the base of the cliffs. Only a small portion of the petrified forests is now exposed, for logs occur below the surface of the ground to a depth of about 300 feet.
Broken Log Sections.—Rhythmic vibrations of earthquakes during the period of uplift of the land are believed to have produced fine cracks at more or less regular intervals across the petrified logs. As erosion of the softer material around the logs took place at the surface of the land, the exposed cracks widened, separating the logs into sections.
The Painted Desert.—The badlands of the Petrified Forest areas and the Painted Desert get their color from the ancient volcanic deposits of that region, and the surface forms are typical of desert erosion.