Petrified Forest National Park in Apache County, Arizona — The American Mountains (Southwest)

Over the Edge

Petrified Forest National Park

Photographed by Cosmos Mariner, May 21, 2016

1. Over the Edge Marker

Inscription. Below you lies a black basalt which forms the rim of the plateau on which you stand. This hard basalt, called the Bidahochi Formation, was deposited by local volcanoes between 16 and five million years ago, and forms a protective cap over the soft, red rocks of the much older Chinle Formation. The Bidahochi basalt shelters this area from the effects of weather like an umbrella, while the rest of the desert is left exposed and vulnerable to erosion, carving the badland topography you see today.

As you look at the cliff face to your left, the black basalt of the Bidahochi Formation stands in stark contrast to the red sedimentary rocks of the Chinle below. The yellow line in the image above highlights the change between these two formations which divides rocks of two vastly different ages. This thin line represents a gap in the physical rock record called an unconformity. Geologists believe there are more than 1,000 feet (305 meters) of missing rock layers that were deposited and then eroded over 200 million years between these formations.

Erected by National Park Service, U.S. Department of the Interior.

Topics.

Paid Advertisement

Click or scan to see
this page online

This historical marker is listed in this topic list: Natural Features.

Location. 35° 4.864′ N, 109° 47.015′ W. Marker is in Petrified Forest National Park, Arizona, in Apache County. It can be reached from Petrified Forest Road 2 miles north of U.S. Route 66 (Interstate 40), on the right when traveling north. Marker is located at the Tawa Point overlook in Petrified Forest National Park. Touch for map. Marker is in this post office area: Petrified Forest Natl Pk AZ 86028, United States of America. Touch for directions.

Regionally, this marker is in the American Southwest,

in the Mountain West,

in Colorado Plateau,

and at the Four Corners.

Globally, it is in North America,

the Western Hemisphere,

the Western World,

and the Anglosphere.

Historically, it finds itself in what was once New Spain

and also Mexico’s Alta California.

Other nearby markers. At least 8 other markers are within walking distance of this marker: Tawa Trail (about 300 feet away, measured in a direct line); At Home in the Painted Desert (approx. 0.3 miles away); Painted Desert Inn (approx. 0.3 miles away); The Stone Tree House

Marker detail: Wait! Did you say volcano? image. Click for full size.

2. Marker detail: Wait! Did you say volcano?

Aerial map view of the Painted Desert Mesa. This mesa is composed of basalt that flowed from a now dormant volcano.

(approx. 0.4 miles away); Badlands (approx. 0.4 miles away); Get Lost (approx. 0.4 miles away); A Painted Park (approx. 0.6 miles away); Painted Desert Community Complex Historic District (approx. 0.9 miles away). Touch for a list and map of all markers in Petrified Forest National Park.

Another marker is no longer nearby. Painted Desert / Finding Fossils (was approx. 0.6 miles away but has been replaced with another marker now near it).

Also see . . . Petrified Forest National Park (Wikipedia). An unconformity (break in the rock record) of about 200 million years occurs within the park, where erosion has removed all the rock layers above the Chinle except geologically recent ones. The Bidahochi Formation, laid down only 4 to 8 million years ago, rests directly atop the Chinle, and rocks laid down in the Jurassic, Cretaceous, and much of the Tertiary are absent. Although much of the Bidahochi has since eroded, a small part of it outcrops in the northern part of the park — along the rim of the Painted Desert between Pintado and Tawa points. (Submitted on December 25, 2020, by Cosmos Mariner of Cape Canaveral, Florida.)

Photographed by Cosmos Mariner, May 21, 2016

3. Over the Edge Marker

Tawa Point Overlook image. Click for full size.

Photographed by Cosmos Mariner, May 21, 2016

4. Tawa Point Overlook

(looking northeast from marker)

Photographed by Adam Margolis, May 26, 2024

5. Over the Edge Marker

Credits. This page was last revised on September 20, 2024. It was originally submitted on December 25, 2020, by Cosmos Mariner of Cape Canaveral, Florida. This page has been viewed 520 times since then and 22 times this year. Photos: 1, 2, 3, 4. submitted on December 25, 2020, by Cosmos Mariner of Cape Canaveral, Florida. 5. submitted on September 4, 2024, by Adam Margolis of Mission Viejo, California.