Mesa (The parking area)
The Blue Mesa is capped with the Flattops One bed of the Sonsela
Member of the Chinle Formation. The terms bed, Member, and
Formation are just ways to specify individual groupings of rocks in
a larger grouping, somewhat similar to the way biologists group
things by Phylum, Order, Class, etc.
On Blue Mesa, the Flattops One bed is composed of a relatively
hard sandstone. It sits above the mudstone of the Blue Mesa Member
of the Chinle Formation. The mudstone erodes away much more easily
than the sandstone, leaving the sandstone at a higher elevation
than the surrounding area.
The mesa would have been much more extensive in past, but
erosion in the form of cliff retreat (See Newspaper Rock
Earthcache) slowly shrunk the size of the mesa.
Cross-bedded Sandstone N34 56.364 W109 45.505
As wind or water moves over loose sediment the
sediment eventually forms mounds. Above water these mounds are
called dunes and underwater these mounds are called ripples.
In this case the cross-beds were formed in an ancient
river.
In the ancient river the water pushes individual
grains of the sediment until the grain piles up on each other.
This forms ripples in rows that are perpendicular to the
direction of current is flowing.
As the grains pile up, ripple eventually gets too steep and
collapses in the direction of flow. This creates angled layers in
the ripple and slowly inches the ripple in the direction of
flow.
Diagram Source USGS:
http://www2.nature.nps.gov/geology/usgsnps/dune/dune.html
Over time, as more sediment is transported down the river,the next
ripple migrates over the first, burying the first and preserving
the crossbeds. The USGS Western Coastal & Marine Geology
website has some downloadable movies to demonstrate the process
(http://walrus.wr.usgs.gov/seds/Movie_list.html).
The size, angle, and direction of the crossbeds can be used to
estimate the minimum water depth, direction of flow, and original
ripple height.
Conglomerate N34 56.357 W109 45.434
One of types of sedimentary rocks is a conglomerate. A conglomerate
contains mostly coarse (greater than 2 mm) rounded grains. In order
for water to move such large grains, it must have been moving
rather quickly. The rounded shape of each individual grain implies
that the grains had been transported a long way so that there was
enough time to smooth out the rough edges it had when it first
broke off.
As you can see the thickness of the conglomerate layer is
relative thin compared to the sandstone, so it likely represents a
flood event in the ancient river.
Bentonite N34 56.236 W109 45.564
The Blue Mesa Member is made up mostly of mudstone. This mudstone
has quite a bit of altered volcanic ash called bentonite. Bentonite
contains clay minerals that when they get wet, they swell up to
seven times their original volume. Then when it dries out, it
shrinks and cracks, forming a network of deep cracks.
These deep cracks then channel future rains and may help form
pipes.
Piping to Badlands N34 56.293 W109
45.592
Piping is a natural emotional process that creates tubes through
the ground, otherwise known as pipes. These features are very short
lived, so you may not find any at the coordinates. You will have to
locate some on your own.
Initially a pipe will form as a seep or spring at the base of a
hill (A). Clay is eroded out of the spring to form the pipe.
Portions of the pipe will then collapse forming a line of holes
(B). As the holes enlarge, only bridges are left (C). After the
bridges collapse, a steep walled gully is left.
This type of erosion is one of the ways badland topography is
developed.
As part of the logging requirement, get a picture of one of the
stages of piping that includes your gps. STAY ON THE TRAILS
Ventifacts N34 56.173 W109 45.650
Ventifacts are rock fragments that have been shaped by the wind. In
dry arid environments with lots of wind, dust and sand are blown
against the rocks on the ground. This naturally sandblasts the
sides of the rocks forming sharp edges. Often they have two or
three faces caused by changes in wind direction that twist the rock
around on the ground. Upon close examination you may find tiny pits
in the rock face.
Desert Pavement N34 56.245 W109 45.649
Desert pavement also forms in dry arid
environments with frequent high winds. Desert pavement refers
to the area that is covered by small rocks that protect the
smaller grains underneath. The frequent winds of the area blew
away the smaller grains leaving the larger grains.
Logging requirements:
Send me a note with :
- The text "GCZ5H7 Blue Mesa - Desert Geomorphology" on the first
line
- The number of people in your group.
- Provide a short explanation for the different colors in the
mesa cliffs
The above information was compiled from the following
sources:
- Bezy, John V. and Arthur S. Trevena, 2000,
Guide to Geologic Features at Petrified Forest National Park,
Arizona Geological Survey Down-to-Earth 10
- Woody, Daniel T., 2006, Revised Stratigraphy of
the Lower Chinle Formation (Upper Triassic) of Petrified Forest
National Park, Arizona, in A Century of Research at Petrifed Forest
National Park, Museum of Northern Arizona Bulletin No. 62, Parker,
W.G., Ash, S.R, and Irmis, R.B., eds., 2006
- NPS, Field Signage
- Ash, S.R. 1987, Petrified Forest National Park,
Arizona, Geological Society of America Centennial Field Guide –
Rocky Mountain Section, 1987
Placement approved by the
Petrified Forest National Park
