Geology Rocks

Paul Parmentier, guest columnist

Driving or walking around Valencia County, black-stained rocks can be seen on llano surfaces and along the edges of the many old lava flows. When broken up, the rocks clearly show that the rocks are coated with a thin layer of greasy-looking black, sometimes orange coating called desert varnish.

Desert varnish is composed mainly of clay mixed with oxides of manganese (darker varnish) and iron (more orange or red varnish).

Bacteria takes manganese out of the environment, oxidize it, and cement it onto rock surfaces. In the process, clay and other particles also become cemented onto the rock. These bacteria microorganisms live on most rock surfaces, and desert varnish has been observed in Antarctica.

Varnish surfaces tend to be shiny when the varnish is smooth and rich in manganese.

Although we know that desert varnish takes a long time to form, (on average 50,000 years to form a varnish thickness of a piece of paper), the chemical composition of the varnish or its thickness cannot be used to accurately date the desert varnish, because changes in climate will result in change in varnish formation, and lichens can chemically erode rock varnish. We just know that thick desert varnish is very old.

The desert varnish also has provided many surfaces for native people to scrape off the dark desert varnish and expose the underlying unweathered rock, thus creating long-lasting records called petroglyphs. These include abstractions like spirals, dots and geometric patterns, or more recognizable forms like animals, humans and handprints.

Whatever they represent, these curious figures provoke within most people the desire to understand. A figure on horseback suggests a relatively recent date since horses only came with the Spaniards. The portrayal of an atlatl (spear-throwing device) recalls a much older archaic cultural period. A line of ghostly figures holding snakes with birds or other animals hovering above them may suggest an otherworldly experience.

We have petroglyphs on El Cerro de Los Lunas and Tomé Hill, and other areas of the county.

Scientists at Los Alamos Laboratory are studying Mars data from the NASA explorations to confirm whether the presence of desert varnish on Mars points to the presence, or at least the former presence, of life on that planet.

(Paul Parmentier, a certified professional geologist retired from California and living in Los Lunas, shares the rich geologic features in Valencia County. He earned a Bachelor of Science degree in geology from Belgium and a master’s degree in geochemistry from Japan. The Geology Landscapes of Valencia County are featured monthly.)

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Paul Parmentier, guest columnist