Near Holbrook in Navajo County, Arizona — The American Mountains (Southwest)
A Gap in the Geologic Record
Below this layer of basalt, a horizontal line cuts across the face of the mesa and separates rocks of two different geologic periods. The pink mudstone below this line belongs to the Chinle Formation deposited about 225 million years ago. The brown mudstone and basalt layers above the line represent the Bidahochi Formation deposited only 5 to 8 million years ago.
How is it that only this thin line represents more than 200 million years of geologic history? This gap is known as an "unconformity." Geologists believe the missing layers were more than 1,000 feet (305 m) thick and that they were eroded over the years by running water and wind - forces that continue to shape this landscape. The emplacement of the basalt has temporarily stopped erosion on the Painted Desert rim, while it has continued in the valley below.
Erected by National Park Service, U.S. Department of the Interior.
Location. 35° 4.14′ N, 109° 47.677′ W. Marker is near Holbrook, Arizona, in Navajo County. Marker is on Petrified Forest Road, on the left when traveling east. Touch for map
Other nearby markers. At least 8 other markers are within 2 miles of this marker, measured as the crow flies. Early Explorers (approx. 0.2 miles away); The Painted Desert (approx. 0.6 miles away); Painted Desert Community Complex Historic District (approx. ¾ mile away); Painted Desert / Finding Fossils (approx. 0.8 miles away); Desert Vistas (approx. one mile away); Painted Desert Inn (approx. 1.1 miles away); Badlands (approx. 1.1 miles away); Route 66 (approx. 1.4 miles away). Touch for a list and map of all markers in Holbrook.
Categories. • Natural Features • Notable Places •
Credits. This page was last revised on June 16, 2016. This page originally submitted on September 29, 2013, by Denise Boose of Tehachapi, California. This page has been viewed 422 times since then and 26 times this year. Photos: 1, 2, 3, 4, 5, 6, 7, 8. submitted on September 29, 2013, by Denise Boose of Tehachapi, California. • William J. Toman was the editor who published this page.