PETRIFIED FOREST Guidebook 1961

THE GEOLOGISTS' EXPLANATION

The Forests

About 160 million years ago, in the Triassic period, much of northern Arizona was swampy lowland where shifting streams spread sand and mud on vast flood plains. Dense beds of ferns, giant horsetails, clubmosses, and swamp-growing trees grew in the marshlands and along the streams. Trees of the most common fossil species grew in scattered copses on the occasional hills and ridges that were above the water. This species resembled our modern native pines, but it is most closely related to the araucarian "pines" of the Southern Hemisphere. Remains of several other kinds of primitive trees are also found here.

Painted Desert badlands.

How the Trees Were Buried

Natural processes, including fires caused by lightning or volcanism, ravages of insects, and fungus, or rot, are believed to have killed the trees. Most of the trees that grew on the hills and ridges of the broad flood plain simply rotted away upon the ground. Most of those that were preserved were carried by flooding streams from the south and southwest to be deposited in bays or on sandbars where rapid burial by mud and sand prevented their decay. The deposits, in which these trees were buried eventually hardened into the sandstones and shales that make up what is now called the Chinle formation. These deposits were buried under at last 3,000 feet of sand and silt that was laid down later by shallow seas.

How the Logs Petrified

The sediments in which the logs were buried contained a large amount of volcanic ash, rich in silica. This silica was picked up by ground water, carried into the wood, and deposited in the cell tissues. The mineral filled the wood solidly, forming the petrified logs. Varying amounts of the original organic matter of the wood remain. The mottled color patterns were caused by oxides of iron and manganese, creating the predominant types of minerals known as agate and jasper.

How the Forest Was Uncovered

After the forest was buried, there were several periods of great mountain-making. Forces from deep within the earth slowly thrust the Rockies and Sierras upward several thousand feet, and the land between these ranges was lifted far above its former position near sea level. As a result of this mountain growth, certain areas became arid and desertlike. Then wind and rain started wearing down the deposits that covered the region, and large river systems carried away the loose sand, mud, and gravel. 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 rocks that make up the Painted Desert. As the logs washed from hillsides, the broken sections accumulated in piles at the base of the slopes.

Only a small part of the petrified forest is now exposed, for logs are scattered below the surface of the ground to a depth as great as 300 feet.

The Broken Log Sections

Rhythmic vibrations of earthquakes during the periods of uplift and subsidence of the land are believed to have produced the fine cracks that appear 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.