Chinle Formation The colorful badland hills, flat-topped mesas,and sculptured buttes of the Painted Desert areprimarily made up of the Chinle Formation. Thesedimentary rock mainly consists of fluvial (riverrelated) deposits. Within Petrified ForestNational Park, the layers of the ChinleFormation include the Blue Mesa Member, theSonsela Member, the Petrified Forest Member,and the Owl Rock Member.

The Blue Mesa Member consists of thickdeposits of grey, blue, purple, and greenmudstones and minor sandstone beds, the mostprominent of which is the Newspaper Rock Bed.This unit is best exposed in the Tepees area ofthe park. The Blue Mesa Member isapproximately 234 million years old.

The Sonsela Member consists of three parts: 1)the lower Rainbow Forest bed consisting ofwhite cross-bedded sandstone and cobbles andcontaining the logs of the Rainbow Forest, 2) themiddle Jim Camp Wash beds of blue, grey, andpurple mudstones and numerous small grey andwhite sandstone beds, and 3) the upper FlattopsOne bed, which consists of a thick cliff-formingbrown, cross-bedded sandstone. This sandstonecaps Blue Mesa, Agate Mesa, and the mesa northof the Rainbow Forest. The Sonsela Member isapproximately 227 million years old, based offossil pollens and vertebrates.

The Petrified Forest Member consists of thicksequences of reddish mudstones and brownsandstone layers. This member is exposed in theFlattops and the red part of the Painted Desert.The Black Forest Bed, part of the Petrified Forest

Petrified Forest National Park ServiceU.S. Department of the Interior

Petrified Forest National ParkPetrified Forest, Arizona

Geology and the Painted Desert

Part of the Painted Desert, Petrified Forest National Park features a strangely beautiful landscape.Erosion has sculpted and shaped intriguing landforms, revealing a treasure trove of fossils withinmulti-colored layers. The rocks reveal an enthralling chronicle of time that is unfolding and ever-changing.

What can the rocks tell us? Think of the colorful layers of the Petrified Forest as pages in a massivebook. As the pages are turned, we discover that the words are a language we don’t completelyunderstand. The pictures help, but we must put together the story of this ancient book withfragmented clues. The first chapter of the park’s geological text is the Chinle Formation.

Member north of Kachina Point, isapproximately 213 million years old, based onradiometric dating.

The Owl Rock Member consists of pinkish -orange mudstones mixed with hard, thin layersof limestone. This member is exposed onChinde Mesa at the northernmost border of thepark. The Owl Rock Member is approximately205 million years old, based on its stratigraphicposition and vertebrate fossil record.

During the Late Triassic, this region was locatedon the southwestern edge of the supercontinentPangaea and just north of the equator. Evidencefrom ancient soils as well as fossil plants andanimals indicates that the climate was humidand sub-tropical during the Late Triassic. Agreat river system deposited the sandstone,mudstone, and conglomerate of the ChinleFormation.

The colorful bands of the Chinle Formation,which give the Painted Desert its name,represent ancient soil horizons. While the redand green layers generally contain the sameamount of iron and manganese, differences incolor depend on the position of thegroundwater table when the ancient soils wereformed. In soils where the water table was high,a reducing environment existed due to a lack ofoxygen in the sediments, giving the ironminerals in the soil a greenish or bluish hue.The reddish soils were formed where the watertable fluctuated, allowing the iron minerals tooxidize (rust).

June 2006

Bidahochi Formation

Quaternary Sediments

During the Miocene and Pliocene of the TertiaryPeriod (4-8 million years ago), a large lake basinwith ephemeral lakes covered much ofnortheastern Arizona. Fine-grained fluvial andlacustrine (lake related) sediment such as silt,clay, and sand represent the lower part of theBidahochi Formation. Volcanoes, both nearbyand as far as the southwestern Nevada volcanicfield, spewed ash and lava over the land and intothe basin. Many of the volcanoes werephreatomagmatic, explosive volcanic eruptionsresulting from the interaction of surface orsubsurface water and magma. The resulting ashformed fine-grained deposits, interbeddedwithin the lake sediments. After a few millionyears of erosion, most of the Bidahochi

Formation has been removed from the park area,exposing the volcanic landforms known asscoria cones and maars (flat-bottom, roughlycircular volcanic craters of explosive origin).The vent from one of these maars is exposed inthe Painted Desert Rim across the park road tothe east from Pintado Point. The Hopi Buttesvolcanic field, which can be seen from thenorthern overlooks of the park extendingnorthwest, is considered one of the largestconcentrations of maar landforms in the world,covering about 965 sq. miles (2,500 sq. km). Theerosion-resistant lava flows, such as Pilot Rockand the Hopi Buttes, protect the softer lake-beddeposits beneath.

Quaternary Period (1.8 million years ago topresent) deposits of windblown sand andalluvium (deposited by flowing water) nowcover much of the older formations of the park.500,000-year-old dunes are located at higherelevations in the northern part of the park.Younger dunes, around 10,000 years old, arefound in drainage areas that contain sand suchas Lithodendron Wash. The youngest dunes are

Preserved and protected for future generations, our national parks and their resources needeveryone’s care. Every year, up to a million people visit Petrified Forest National Park.Permanent damage to this area occurs quickly from such impact but is slow to mend. Pleaseleave the park undisturbed—take only pictures and memories.

E X P E R I E N C E Y O U R A M E R I C A http://www.nps.gov/pefoPrinted on recycled paper

Colorado Plateau The supercontinent Pangaea began to break upabout 225-200 million years ago due to tectonicmovement of the earth’s crust, eventuallyforming the continents of today. About 60million years ago this region began to rise. Theuplift process raised some areas as much as10,000 feet above sea level. Over millions ofyears, erosion stripped away many layers ofrock. It is thought that erosion stripped away therocks of the Jurassic and Cretaceous Periodsbefore the Chinle rocks were covered byyounger rocks of the Bidahochi Formation,creating a break in the rock record—anunconformity. The unconformity between theLate Triassic Chinle Formation and theMiocene-Pliocene Bidahochi Formation can beseen from Whipple Point. In Petrified Forest,

this gap represents about 200 million years ofmissing geological history! Geologists studyother areas in the region to learn about thelayers absent in the park.

Present and Future The Little Colorado River and its tributaries,including the Puerco River, have cut their ownvalleys into the soft Chinle Formation of thePainted Desert. Water erosion is the majorgeologic process that removes the exposedsedimentary stone, revealing more of thepetrified wood and other fossils. While erosioncan be relatively fast in the Painted Desert, theactual rate varies due to the material, the slope,and the presence of vegetation and pebbles.Bentonite clay, one of the main components ofChinle Formation rocks, swells as it absorbsmoisture, then shrinks and cracks as it dries,causing surface movement that discouragesplant growth. This lack of plant cover rendersthe sediment susceptible to weathering. Heavyrains of the summer monsoons remove as muchas ¼ inch of rock each year from the steep, barrenslopes. In addition, water creates small tunnels(pipes) in the hills, which widen into largegullies through time. This gullying carves thecanyons that exist between the mudstone hills,giving the badlands their dissected appearance.

Where capstones of harder sandstone or basaltprotect the softer layers, mesas (broad flat-topped hills) and buttes (narrow flat-toppedhills) form. These landforms can erode at asubstantially decreased rate due to the protectivecapstones. Eventually the capstone also breaksapart due to gravity and erosion, exposing thesofter layers to the ravages of time. Similarly,where vegetation grows, erosion is muchslower. In some areas, strong winds carry awaythe dry, loose soil leaving only a covering ofsurface pebbles, called desert pavement, whichact as a protective crust.

The ancient book of Petrified Forest’s geology isfar from over. Even as the first pages are turnedand studied, new pages are being formed. Thesame natural processes of the past are ongoingin the present and will continue to shape thedramatic landforms of Petrified Forest NationalPark.

found throughout the park, in all settings,deposited around a thousand years ago. Thesedune deposits are largely stabilized byvegetation, especially grasses. While not asnumerous as the fossils of the ChinleFormation, fossils found in the Quaternarysediments include fragments of an ancestralproboscidean (elephants and their relatives, suchas mammoths).