For years scientists have assumed that desert varnish was composed primarily of manganese and iron oxides, precipitated out of the rock through weathering processes. However, the varnish structure eluded precise analysis, because it is composed of particles too fine to be characterized by x-rays, the main diagnostic tool of mineralogical investigation.
Infrared spectroscopy, a technique which illuminates mineral samples with infrared light and records the pattern of absorbed wavelengths, was recently
The results of the analysis revealed that the main constituent in desert varnish, totaling about 70%, is clay, not manganese and iron oxides. The oxides form the remaining 30%. The red coating on the underside of the varnished rocks, previously believed to be iron oxide, turned out to be 90% clay incorporating an iron oxide stain, similar to the iron in the black finish on the rocks' exposed portions. In addition it was found that all desert varnish, whether it formed on the side of a cliff or on a 10-inch boulder, shares a similar composition.
It was concluded that most of the coating collects from sources outside the rock rather than from material leached out of it, as many geologists had believed.
Fine, windblown clay particles are a critical ingredient in forming the varnish, which first forms on rough, porous surfaces. These surfaces allow dew and other moisture to collect, depositing a thin film of clay when the water evaporates. This film of sediment on the rock's surface encourages water to migrate through tiny pores inside the film, depositing traces of manganese and iron as the water evaporates.
The formation of desert varnish is interdependent upon the clay and oxides. The dry, fluffy clay particles depend on the oxides to form a resistant cementing agent. The oxides, in turn, require clay particles for transportation and deposition. This is the underlying reason why all desert varnish that was examined contained both clay and manganese and iron oxides - never one without the other.
...Caltech.
acquires Baboquivari Ranch
 |
Nature conservancy
has bought the Baboquivarl Ranch, 65 miles southwest of Tucson, for
$155,000. Sellers were Douglas and Kendall Cumming who acquired the 640 acres of patented land plus grazing rights on Surrounding public land nine years ago from the Original homesteaders, Richard and Erma Riggs. Baboquivari Ranch is located in Thomas Canyon, which drains the east side of the 7,730-foot high peak. The area is known for its whitetail deer and mountain lion. The peak is also popular with hikers and rock climbers. The Conservancy bought the ranch in an effort to preserve it in its present, nearly natural condition. "We are holding it for an agency like the U.S. Bureau of Land Management which does not now have the money appropriated to buy it," said Conservancy spokesman Virgil Rule, "The BLM has indicated the ranch is on its priority, list to acquire when the money becomes available." Rule said the ranch has been leased back to the Cumming brothers. "When we bought the ranch from the Riggs in 1969 we tried to keep it in as natural a state as possible," said Douglas Cumming who owns another ranch in |
Peck Canyon, about
15 miles northwest of Nogales. "Both Kendall and I thought it ought to be a
state park or a preserve, and when the Conservancy offered a good price, we
took it." The BLM does not now have the money to buy the ranch, said Dan Beard, a spokesman in the office of the Secretary of the Interior. "We will try to put it in the budget for the 1980 fiscal year," Beard said. The State Parks Department is also interested in acquiring the ranch; It is third on its list of priorities as future state parks. The ranch is in a spectacular setting with the 1,000 foot east face of Baboquivari Peak towering 3,000 feet above the main ranch buildings. Rock climbers and hikers find the area to their liking. Whitetail deer and lions find the rugged country and the vegetation ideal for both feed and cover. Historically, the ranch has been open to both hikers and hunters. Beard said this was one of the main reasons the BLM wanted to acquire the land since the eastern escarpment of the peak, plus a l0 mile long strip of land along the top, is under its jurisdiction. Most of the land surrounding the |
ranch is under the
jurisdiction of the State Land Department. Acquiring private land and holding it for future public management is one of the main purposes of the Nature Conservancy, according to Rule. "Environmentally, the Baboquivari Ranch is an exceptional place," said Rule. "It has been on our list of priorities for some time and when the Cumming brothers indicated a willingness to sell, we bought." The Conservancy plans no developments, Rule said. Douglas Cumming said a caretaker would continue to live at the ranch and only minimum improvements would be made. The road into the ranch was recently almost washed out and it will be repaired he said, but only for truck traffic. If a public agency does acquire the ranch, that would assure continued public access to the peak and the surrounding area. Neither public agency - the State Parks Department or the BLM - has any plans for the future development of the ranch area. "I like that place," said Douglas Cumming, "but it is not big enough to be a cattle ranch." |