This high, dry tableland was once a vast floodplain crossed by many
streams. To the south, tall, stately pine-like trees grew along the
headwaters. Crocodile-like reptiles, giant fish-eating amphibians,
and small dinosaurs lived among a variety of ferns, cycads, and other
plants and animals that are known only as fossils today. The tall
trees—Araucarioxylon, Woodworthia, and Schilderia—fell and were
washed by swollen streams into the floodplain. Silt, mud, and volcanic ash
then covered the logs. This blanket of deposits cut off oxygen and slowed the
logs decay. Gradually, silica-bearing ground waters seeped through
the logs and, bit by bit, replaced the original wood tissues with silica
deposits. As the process continued, the silica crystallized into quartz, and
the logs were preserved as petrified wood.
That was about 225 million years ago in the late Triassic Period. After
that time, the area sank, flooded, and was covered with freshwater sediments.
Later the area was lifted far above sea level and this uplift created
stresses that cracked the giant logs. Over time, wind and water have worn
away the layers of hardened sediments, exposing the fossilized remains of
ancient plants and animals. The hills will continue to expose fossil
treasures as ceaseless weather patterns sculpt the soft sediments of the
Painted Desert.
Today the ever-present forces of wind and water continue to remove
sediments. Erosion continues to break down the exposed logs and unearth
the logs and other remaining fossils still buried below the surface. In
some places, up to 300 feet of fossil-bearing material remains.
The petrified logs, other fossils of plants and creatures that
lived in the area, and the rocks locking them in place all testify to
changes in the environment through millions of years.
There are many stories here in the Petrified Forest. Evidence of
early human occupation is readily visible on the landscape. Sites
throughout the park tell of human history in the area for more than
2,000 years. We do not know the entire story, but there were separate
occupations, a cultural transition from wandering fainlies to settled
agricultural villages—pueblos—and trading ties with surrounding
villages. Although evidence of these early people fades about 1400, their
story remains through their dwellings, pot sherds, and petroglyphs.
