🌎 That's NOT Mine Tailings! 🌏 EarthCache

This is an EarthCache. 🌏 As such there is no physical container or physical log. To earn credit for finding this cache, you must visit the posted coordinates, make personal observations, and formulate conclusions based on your observations and careful reading of the cache description. Then, send me the answers to the questions listed under "Logging Requirements" via my geocaching.com profile per the guidelines. Do not post your answers or any pictures that would disclose answers online. You don't need to wait for my response to log your find. I will contact you if there is a problem with your answers.

1. Cache title, date, time, and with whom did you make this observation?
2. What do you think were the primary processes responsible for creating the desert pavement? Why?
3. Describe the size, shape, and color of the clasts forming the desert pavement at Observation Point #1. A tape measure or other measuring device would be helpful.
4. Observation Point #2 brings you to a volcanic boulder approximately 18 x 24 inches in size. Is the desert pavement the same in this area as the first location? If not, what is the difference?
5. Do you believe the coloring of this boulder is original or does it reflect desert varnish? Why or why not?
6. Send a picture of you (or some personal identifying item) at the posted coordinates with your response to me. This picture is REQUIRED in accordance with current guidelines for EarthCaches. Do not post any pictures to your log that show the desert pavement.

This EarthCache is located north of Interstate 10 at exit 26 - Gold Nugget Road. There is room for RVs and trucks to turn around at the posted parking coordinates assuming the ground is dry. The observation points are a short walk away. The posted coordinates will bring you to an obvious marker with the two Observation Points less than 100 feet from the posted coordinates. Permission for this earthcache has been granted by BLM's Yuma Field Manager. Please DO NOT drive on, or displace in any manner, the undisturbed desert pavement!

I once had an individual tell me in all seriousness that all of these relatively smooth, bare, slightly sloped areas around Quartzsite were mine tailing. He may have been well meaning, but he was certainly misinformed! Desert pavement, as it is known in the southwest, is a layer of tightly packed or interlocking pebbles that protect smaller sediments and silt below. It is a stony surface without sand or vegetation that often develops a coating of rock varnish. It is generally considered to be thousands to hundreds of thousands of years old. There are four explanations related to the formation of desert pavement.

1. The deflation explanation assumes the wind blew all the granules, sand, and silt away, leaving a lag surface of pebbles.
2. The sheet flow explanation assumes water moved all the granules, sand, and silt away, leaving a lag surface of pebbles.
3. The heaving explanation suggests that expansion and contraction of the soil moves the stones upward and the fines downward. The expansion and contraction could be caused by heating, cooling, frost heaves, or salt crystals. In some cases, two or all three of these explanations could work in concert.
4. The “born at the surface” explanation is the newest explanation presented by Wells & McFadden in 1995. This explanation suggests that the pavement surface is of the same parent rock as the underlying solid rock, with the soil layer between the two growing over time from dust carried by the wind.

Rock varnish forms very slowly. Surfaces of some rocks rarely exhibit thick coats of varnish because they weather and erode faster than varnish can form. If rock surfaces resist weathering, varnish coatings become increasingly thick and dark over time. Because of this development over time, rock varnish provides geologists with a valuable tool for determining relative ages of different alluvial fan deposits.

Ancient inhabitants of the Sonoran Desert used varnished desert pavements as dark-colored canvases on which they rendered gigantic artistic impressions. By removing the dark varnished stones and exposing the underlying light- colored soil, prehistoric peoples created images of human figures, animals, and abstract forms known as intaglios such as the Bouse Fisherman intaglio.

Aeolian: relating to or arising from the action of the wind. Aeolian processes are important in arid environments such as deserts.

Boulder: rocks larger than 10 inches in diameter

Clast: fragment of preexisting rock

Cobble: rocks from approximately 2.5 inches to 10 inches in diameter

Deflation: erosion by wind of loose material from flat areas of dry, uncemented sediments such as those occurring in deserts, dry lake beds, floodplains, and glacial outwash plains.

Lag Deposit: : coarser material left in place after the removal of fines by wind or water

Pebble: small rocks from approximately 1/8 inch to 2.5 inches in diameter

Sheet Flow: Water moving fairly uniformly with a similar thickness over a sloping surface

Web Resources

• Aeolian Landforms: What Is A Desert Pavement?, John Misachi, WorldAtlas, https://www.worldatlas.com/articles/aeolian-landforms-what-is-a-desert-pavement.html, accessed November 22, 2020
• Desert Soils, Joseph R. McAuliffe, Arizona Sonora Desert Museum, https://www.desertmuseum.org/books/nhsd_desert_soils.php, accessed November 20, 2020
• Earth Science Picture of the Day, February 12, 2015, Desert Pavement, https://epod.usra.edu/blog/2015/02/desert-pavement.html, accessed December 2, 2020
• Theories of Desert Pavement, Andrew Alden, October 2, 2019, ThoughtCo, https://www.thoughtco.com/theories-of-desert-pavement-1441193, accessed December 2, 2020