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Portneuf Narrows and an Ancient Lake Flood

A cache by PurplePaws Send Message to Owner Message this owner
Hidden : 8/27/2007
In Idaho, United States
2.5 out of 5
1 out of 5

Size: Size: not chosen (not chosen)

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Geocache Description:

At the Portneuf Narrows, the floodwaters from the ancient Great Salt Lake were constricted into this narrow gap concentrating the erosional power of the water.

There is a dirt pull off next to a series of informational panels. Certain information, such as the name of the lake and date is purposefully left off this description to try to keep the integrity of the virtual cache at this location. All this information can be obtained from the informational panel. This cache will examine the effects of the water that powered through this narrow gap during that ancient flood.

East of this location where the I15 makes an almost 90 degree turn, the flood waters backed up behind this constriction. A large deposit of gravel collected north of the freeway.

Traveling through the narrows, the speed of the water increased. This eroded all the material down to the basalt bedrock. Many of the alcoves and channels in the basalt flows along the I15 east of the coordinates were eroded out during this short flood.

At these coordinates, the width of the valley is narrowest. As the width decreased, the speed of the water increases. This happens because the same volume of water passes through the smaller cross-sectional area of the narrows as the larger cross-sectional area up stream. In order for that to happen, the speed of the water must increase.

As the speed of the water increases, the erosional potential of the water increases. The surface material was eroded and alcoves were eroded into the basalt east of the coordinates where the water was moving slower. Here at the narrowest point, the flood waters ripped up all the basalt and carried it away. Then as the valley widened up again, the water slowed and no longer had the erosional potential to rip up the basalt.

These blocks of basalt were deposited further down the valley in Pocatello. (That would be the subject of yet another earthcache should someone want to set it up)

Logging requirements:
Send me a note with :

  1. The text "GC15FHN Portneuf Narrows and an Ancient Lake Flood" on the first line
  2. The number of people in your group.
  3. from the coordinates, travel west back to I15 until you can see outcrops of basalt again. You are looking for black rock along the road. Send me the coordiates at this location
  4. Approximate how much wider the valley is at this point than at the narrows
  5. General description of the outcrop, smooth, rough, potmarked, etc

Please begin your e-mail with the name of the earthcache and make sure your log includes the number of people in your group.

The above information was compiled from the following sources:

  • Willis, Grant C. et al, Geology of Antelope Island State Park, Utah in Geology of Utah's Parks and Monuments, 2003 Utah Geological Association Publication 28 (second edition) D.A. Sprinkel, T.C. Chidsey, Jr. and P.B. Anderson, editors
  • Utah Geological Survy, Lake Bonneville, PI-39 Commonly Asked Questions About Utah's Great Salt Lake and Ancient Lake Bonneville,
  • Utah Geological Survey, Great Salt Lake,
  • Link, Paul Karl, Darrell S. Kaufman and Thackary, Glenn D. 1999; Field Guide to Pleistocene Lakes Thatcher and Bonneville and the Bonneville Flood, Southeastern Idaho in Hughes, S.S., and Thackray, G.D., eds., Guidebook to the Geology of Eastern Idaho, Idaho Museum of Natural History.
  • Bright, Robert C. and H. Thomas Ore, Evidence for the spillover of Lake Bonneville, southeastern Idaho, in Geological Society of America Centennial Field Guide, Rocky Mountain Section, 1987.

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Additional Hints (Decrypt)

fbhgu bs V-svsgrra haqrecnff

Decryption Key


(letter above equals below, and vice versa)



212 Logged Visits

Found it 207     Didn't find it 1     Write note 3     Publish Listing 1     

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Current Time:
Last Updated: on 9/27/2017 3:33:59 PM Pacific Daylight Time (10:33 PM GMT)
Rendered From:Unknown
Coordinates are in the WGS84 datum

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