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 :
- The text "GC14YVP The Basalt of the Narrows " on the first
line
- The number of people in your group.
- The approximate height of the columnar jointing on the road
side of the canyon (DO NOT CROSS THE ROAD)
- 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