STATE TRUST LAND -- PERMIT REQUIRED
Permits can be obtained for a small fee ($15 individual, $20 family) from the Arizona State Land Dept, 1616 W. Adams, Phoenix, AZ 85007. Permits can be obtained in person, by mail or online. The State Land Dept asks that you remain on established trails at all times. Permission has been obtained for this earthcache.
Large, flat areas with sparse vegetation and covered by a layer of tightly packed small dark stones are conspicuous features of arid landscapes like our Sonoran Desert. This is known as desert pavement.
These pavements are older formations not found in geologically young deposits. The most well developed pavements are those that have formed over the passage of several tens of thousands to a few hundreds of thousands of years. Among the best ones in Arizona are those outside of Quartzsite and Apache Junction.
Desert pavement is defined as a surface layer of closely packed or even cemented pebbles and small rocks from which fine material is absent. Approximately 50% of natural arid lands in North America has desert pavement This is a distinctive surface feature where at least 65% of the soil surface is rock covered. The closely packed surface rocks range from generally coarse gravel to cobble-sized rock fragments one to two deep that rest on or are embedded in underlying soil. In the Sonoran Desert, the rocks have a dark desert varnish on them as well. Desert varnish is a hard, dark-colored patina of accumulated iron and manganese oxides. If there is white quartz included among the rocks, they stand out as desert varnish does not form on quartz.
Desert pavements appear barren. The pavement surface is not easily penetrated by water and it funnels rainwater towards the unpaved areas nearby. The clusters of vegetation reflects this funneling of water.
Research done on desert pavements show that beneath the rocky surface is a fine layer of soil called a” vesicular A (or Av) horizon”. “A” denotes its position as the uppermost mineral layer of soil. This layer, or “horizon” is typically a few centimeters (about an inch) thick, and contains mostly silts and clays. Coarse grains are largely absent. This research led to the most recent theory of how desert pavements are created and is discussed below.
What Creates Desert Pavement?
There are three traditional theories that can explain how desert pavement was created. Recently a fourth explanation was developed based on the soil underlying the pavements. First we will look at the traditional theories.
Deflation theory states that the rocks are what is left behind after the wind has blown away the fine grained material. This erosion process is called deflation. While this could be so in some circumstances, in many areas there is a thin crust that binds the surface and prevents deflation.
Sheetflow theory involves the use of moving water. A thin layer of rainwater (called sheetflow) carries off the fine particles around the rocks.
Heave theory claims that repeated cycles of wet and dry cause larger stones to make its way to the surface, leaving behind the smaller particles. This heave process can occur with frost and with salt crystals as well.
The newest theory came about as the result of research at Cima Volcanic Dome in the Mojave Desert. Their research showed that the fine soil below the pavement was not the result of erosion from the overlying rocks and was younger than the pavement rocks themselves. They developed a theory in which the rocks are present at the surface to begin with and fine soil builds up under them, creating the even pavement. The rocks remain above due to heave while the fine soil builds up below the pavement.
In prehistoric times along the Colorado River, local natives created geoglyphs out of the desert pavement. Geoglyphs are large designs produced on the ground. The best known ones in North America are the Blythe Intaglios as seen in this photo from Wikipedia. https://en.wikipedia.org/wiki/File:Blythe_Intaglio_(4858).jpg
Look west-southwest to answer the questions.
1) What is the size range of the rocks found here?
2) Are they all varnished or are some lighter in color?
3) Which method do you think formed this pavement?
4) Is there any vegetation on the pavement?
5) How large is this section of pavement.
Pictures are optional but would be nice to see if there are changes to the area over time.
This earthcache is within the guidelines of recreational use permit on state trust land and is acceptable to the state land department. Permission was specifically granted for this earthcache.