Geological History of Petrified Forest National Park
The Petrified Forest National Park is particularly well known for it’s fossils, fossils of fallen trees mostly, which lived a long, long time ago, round about 225 million years ago. At this time the area where the Petrified Forest National Park now stands was close to the equator, on the southwestern edge of Pangaea, the super-continent with a sub-tropical and humid climate. Streams flowed across this area from the highlands depositing lots of different sediments and organic matter, lots of trees, animals and plants which had fallen into the water, much of this organic matter was buried so quickly that it didn’t have the time to decay or be eaten by the other organisms of the time, which is how it became fossilized. The sediment which actually holds the fossil logs is part of the Chinle Formation.
The Chinle is very colorful, hence “The Painted Desert” and is up to 800 feet thick in parts of the park, ain’t that incredible? It’s made up of sedimentary rocks including mudstone, claystone and siltstone, as well as harder stuff like sandstone, limestone and congolmerate. Erosion has shaped this land, with millions of years of wind and water creating cliffs, gullies, buttes, mesas and rounded hills. This area is particularly susceptible to weathering because of the sheer lack of plant cover. You will notice a distinct lack of plant life in the Petrified Forest National Park (live ones anyway), largely due to the bentonite clay which swells up when wet and then shrinks back when it dries out causing large cracks and surface movement. The poor plants don’t really stand a chance.
The Painted Desert is part of the Colorado Plateau, which was uplifted around 60 million years ago due to tectonic movements within the Earth’s crust. Some parts of this plateau eventually rose to around 10,000 feet higher than sea level, and the warping movement of the surface of the Earth led to a gradual yet constant destruction of this plateau by our old friend – erosion. There is actually about 200 million years worth of rocks missing from the park, 200 million years worth of rock just eroded away and lost forever. The rock which lies at the top of the Chinle is only around 4 – 8 million years old.
For the last couple of millions years or so, much of the Chinle has been covered by windblown sand and alluvium. Higher elevations towards the north of the Petrified Forest National Park have dunes which are around 500,000 years old, whereas the more sandy, drainage areas like Lithodendron Wash have much younger dunes, around 10,000 years old. In some areas there are sand dunes in the park which are only 1,000 years old, in areas where grasses and vegetation will grow and therefore help to stabilize the forces of erosion.