Marker Logo HMdb.org THE HISTORICAL
MARKER DATABASE
“Bite-Size Bits of Local, National, and Global History”
“Bite-Size Bits of Local, National, and Global History”
Near Holbrook in Navajo County, Arizona — The American Mountains (Southwest)
 

A Gap in the Geologic Record

 
 
A Gap in the Geologic Record image. Click for full size.
By Denise Boose, September 2, 2013
1. A Gap in the Geologic Record
The graphic on the right is entitled "Geologic map of Petrified Forest National Park." See below for the graphic on the bottom of the marker.
Inscription. The black basalt that caps the cliffs before you stands in stark contrast to the colorful Chinle Formation visible throughout the Painted Desert.

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
A Gap in the Geologic Record Marker image. Click for full size.
By Denise Boose, September 2, 2013
2. A Gap in the Geologic Record Marker
. Marker is at or near this postal address: Petrified Forest Road, Holbrook AZ 86025, United States of America.
 
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. 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 FeaturesNotable Places
 
A Gap in the Geologic Record Marker image. Click for full size.
By Denise Boose, December 13, 2012
3. A Gap in the Geologic Record Marker
Cycle of Erosion and Deposition image. Click for full size.
By Denise Boose, September 2, 2013
4. Cycle of Erosion and Deposition
First graphic: The 225-million-year-old Chinle Formation was buried under younger rock layers.

Second graphic: Erosion cut away younger rock layers exposing the Chinle Formation.

Third graphic: Starting eight million years ago, lakes and volcanoes reburied the Chinle Formation under the much younger Bidahochi Formation.

Fourth graphic: Recent erosion has removed most of the Bidahochi, re-exposing the Chinle Formation.
Painted Desert image. Click for full size.
By Denise Boose, September 2, 2013
5. Painted Desert
The upper black layer is basalt that flowed from a volcano that erupted here. Below the lava an "unconformity" (see arrows on marker photo) indicates missing layers, or a gap in the geologic record.
Painted Desert image. Click for full size.
By Denise Boose, September 2, 2013
6. Painted Desert
Painted Desert image. Click for full size.
By Denise Boose, September 2, 2013
7. Painted Desert
Painted Desert image. Click for full size.
By Denise Boose, September 2, 2013
8. Painted Desert
 
 
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 444 times since then and 48 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.
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