Painted Desert Geology – Part 1, Northern Petrified Forest NP, AZ — © Dave Spier
From east of the Grand Canyon, the Painted Desert crosses the Colorado Plateau in a long curving arc southeastward to encompass the Petrified Forest east of Holbrook, Arizona, and then continues into New Mexico and northeastern Arizona. The bright colors, primarily shades of red, are due to iron oxides in the late Triassic Chinle formation deposited as clay, silt and sand in lakes and wetlands that covered a broad plain over 200 million years ago (mya). Most of these sediments are now soft mudstones and related rocks with a few harder layers of sandstone. Color contrast in the northern section of the Petrified Forest is provided by a light-colored band of volcanic ash known as the Black Forest bed.
On top of the Chinle layers there are younger deposits of clay, silt and sand plus lava flows belonging to the Bidahochi Formation, much of which has been removed by erosion. These Tertiary layers can be seen from several of the overlooks north of I-40. Flat-topped mesas to the north are capped by this formation.Photography Notes: On cloudy days, the diffuse lighting makes topographic features look flat and featureless. If possible wait for a sunny day or time and choose locations where the sun will be to your side (left or right). This side-lighting creates shadows for contrast and brings out the texture and details. The effect is maximized soon after sunrise and before sunset, which are the golden times of daylight and give the warmest color tones.
We were on our way to the Grand Canyon, so time constraints reduced our ability to revisit this section under better lighting conditions. There is no campground [or car camping] in the park, [although you can backpack which requires a permit], and the extra travel further reduced time for photography.
Corrections, comments and questions are always welcome at firstname.lastname@example.org or connect through Facebook (Dave Spier, photographic naturalist and northeast naturalist).
Petrified Forest – a Story in Stone, by Dr. Sidney Ash (© 2005), by Petrified Forest Museum Association
Roadside Geology of Arizona, by Halka Chronic (© 1983/reprinted 2002), published by Mountain Press