A story in stone: Arizona's Petrified Forest provides fascinating look at a surreal world

Though once indeed trees, petrified wood has been transformed by time and mineral saturation into quartz. And no, these logs were not cut, though they sometimes appear to have been so. The physical characteristics of cylindrical quartz cause it to break cleanly when stressed.

Why did so much petrified wood end up here? There was a large, prehistoric river system here, with large forests upstream. The mineralogical conditions of the groundwater were simply conducive to the petrification process.

Petrified wood is also extremely heavy — on average petrified wood weighs about 150 pounds per cubic foot.

It is also very hard and rates between 7 and 8 on what is called Moh's Hardness Scale. (Thus, only topaz, ruby, sapphire and diamonds are harder.) Only a diamond-tipped saw can cut it.

This part of Arizona's high desert area was once a vast floodplain, with numerous streams, where large conifer trees grew. Silt, mud and volcanic ash buried the trees after they died and fell. A lack of oxygen underground meant the logs' decay was slowed, and silica-heavy groundwater replaced the original wood — preserving the tree's structure as a petrified mirror of its former state, Some scientists believe the process of becoming petrified wood may have only taken about 100 years.

Petrified Forest's sinuous road winds through plains and arroyos, and among cliffs and strangely eroded badlands formations. Here's a sampling of what you'll see:

• Agate House Trail: Northeast of the Rainbow Forest Museum, this trail leads to a pre-Columbian pueblo, partially reconstructed in the 1930s, composed of petrified wood.

"The Agate walk is quite interesting," Larsen said. "It's about a two-mile walk."

Agate House itself is an eight-room structure. Ancestral Puebloan people also used petrified wood for tools and building material.

• The Crystal Forest: Another attractive stop, the Crystal Forest offers an 0.8-mile path through more petrified wood, featuring an array of colors, textures and shapes. The rolling, badlands-like hills on which the "forest" sits is mostly barren of vegetation, making the petrified trees stand out even more.

• Agate Bridge: This impressively long log-like formation stretches across an arroyo — a precarious situation that prompted early park managers to put a somewhat unsightly concrete bar underneath it in 1911 and again in 1917 for fear of collapse. A sign explains the phenomenon: "The stone log, harder than the sandstone around it, resisted erosion and remained suspended as the softer rock beneath it washed away."