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The Basalt of the Narrows

Hidden : 08/06/2007
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Geocache Description:

The Yellowstone River has cut down through the layers of sedimentary and volcanic rock of the area revealing a portion of the geologic history of the area along with characteristic columnar jointing in the basalt flows.

The pullouts on the northbound side of the road provide parking for a couple of vehicles. If you miss the one specified by the coordinates, one of the other pullouts will do. If you are southbound, it is safer to go a short way down to the Tower Falls store and turn around to access the pullouts since the road is narrow and curvy making it difficult to see oncoming traffic.

Across the valley, the almost vertical walls of the canyon provide a natural cross section of the layers of rock that are found off to the east. This layering of the rock is called stratigraphy. Two geologic principals can be seen here: the Law of Original Horizonality and the Law of Superposition.

Law of Original Horizontality simply states that rock layers form horizontally. Lake sediments, flood pain deposits, lava flows, ash falls, etc, will settle down across the earth in a hoi zonal layer. Any change from horizontal is a sign that the rock layers have been disturbed after they formed. Of course there are exceptions, the edges of a valley are sloped and sediment falling on slopes will be sloped, ash falling on the side of a mountain will be sloped. In the case we see across the valley.

The Law of Superposition builds on the Law of Original Horizontality. Since layers of rock are laid down essentially horizontally, the lowest layers must have been in place before the layers of rock above them could be deposited. Thus, the oldest rock is found at the bottom of a series of rock and the youngest rock is found at the top. This is assuming that the series of rock has not been disturbed.

In example across the valley, the rock layers are still in their original orientation and have not been altered. So we can conclude that the oldest rocks are found at the bottom of the canyon. At the bottom of the canyon are Eocene (about 50 million year old) volcanic gravels. Directly above that is a horizontal layer of 1.5 million year old basalt that looks like it is made up of columns (more on that later). Above this layer of basalt is a layer of gravel that deposited by glacial melting followed by another layer of 1.5 million year old basalt. The entire sequence if topped by a relatively thin layer of glacial till. These rock layer in order from bottom to top is called a stratigraphic column.

The stratigraphy on the road side of the canyon is slightly different. There are some of the same rock types, Eocene volcanic gravel, basalt, and glacial gravel, however the actual ages are different. The basalt on the road side of the canyon formed about 2.2 million years ago in an ancient stream channel.

Without crossing the road you can get a good view of the columnar jointing in the basalt. These columns form as the lava cools. As the lava cools it contracts and cracks. These cracks typically meet at approximately 120 degree angles, forming roughly hexagonal columns. In the best examples, these columns can be hundreds of feet long.

Logging requirements:
Send me a note with :

  1. The text "GC14YVP The Basalt of the Narrows " on the first line
  2. The number of people in your group.
  3. The approximate height of the columnar jointing on the road side of the canyon (DO NOT CROSS THE ROAD)
  4. The stratigraphic column of the road side of the canyon. (again, DO NOT CROSS THE ROAD OR CLIMB UP OR DOWN THE CLIFFS, all the rock layers can be seen from the pullout)
    As an example the stratigraphic column for the far side of the canyon would be:
    Glacial Till (youngest)
    Columnar Basalt
    Glacial Gravel
    Columnar Basalt
    Eocene Volcanic Gravel (oldest)

The above information was compiled from the following sources:

  • NPS informational Pannel
  • Fritz, William J., Roadside Geology of the Yellowstone Country, Mountain Press Publishing Company, May 1989.

Placement approved by the
Yellowstone National Park

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Additional Hints (Decrypt)

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(letter above equals below, and vice versa)

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