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 :
- The text "GC15FHN Portneuf Narrows and an Ancient Lake Flood"
on the first line
- The number of people in your group.
- 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
- Approximate how much wider the valley is at this point than at
the narrows
- 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,
http://www.ugs.state.ut.us/online/PI-39/pi39pg01.htm
- Utah Geological Survey, Great Salt Lake,
http://www.ugs.state.ut.us/utahgeo/gsl/index.htm
- 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.