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EarthCache

Blue Mesa - Desert Geomorphology

A cache by lagrac
Hidden : 11/1/2006
In Arizona, United States
Difficulty:
2 out of 5
Terrain:
2.5 out of 5

Size: Size: not chosen (not chosen)

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Geocache Description:

There is a small parking on the far end of the one-way road the circles Blue Mesa. Follow the earth-packed trail down the short escarpment. Note that this description uses the revised stratigraphy nomenclature in Woody, 2006.

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 :

  1. The text "GCZ5H7 Blue Mesa - Desert Geomorphology" on the first line
  2. The number of people in your group.
  3. 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


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Current Time:
Last Updated: on 10/18/2014 11:05:30 AM Pacific Daylight Time (6:05 PM GMT)
Rendered From:Unknown
Coordinates are in the WGS84 datum