While here in Albuquerque for an Early Career Workshop with the regional climate adaptation science centers, I fit in a trail run with friends and colleagues at the Piedras Marcadas Canyon. It offers a unique and beautiful insight into the geologic, cultural, and natural resources of this region. There are remnants of volcanic eruptions from about 200,000 years ago in the form of basalt flows. These flows cap the underlying sandstone of the Santa Fe Formation, a group of geologic formations found not only in New Mexico but Colorado, too, containing fossils dating back as far as 34 million years ago!
The underlying sandstone is softer than the lava flows, so it has eroded away with time, resulting in the basalt breaking off and tumbling down the slopes. This formed the volcanic escarpments (cliffs) we see today and is also where the Ancestral Pueblo People carved petroglyphs, from AD 100 to 1600. The Ancestral Pueblo People, or, Anasazi, were primarily farmers, developing a set of beliefs that emphasized the importance of rain. In the arid Southwest, water means life and rivers are the lifelines of people.
Most of the petroglyphs were made by “pecking”, accomplished by striking volcanic boulders directly with a hammerstone and removing the dark, desert varnish on the surface. Later, a more controlled technique was developed by using two stones, in much the way a chisel is used, to peck boulders. This “hammer and chisel” method gave petroglyphs more control and thus more detailed images. The images here include human-like figures, concentric circles/spirals, animal figures, and geometric designs.
Scientists researching this varnish initially wanted to understand how microbial ecosystems in the desert interacted with it. Through a series of advanced analysis and measurements including DNA sequencing and mineralogical analyses, they found manganese compounds deposited in layers, a result of ongoing chemical cycles. What could do that? Life. Scientists found a high prevalence of bacteria called Chroococcidiopsis that use manganese to combat the oxidative effects of the harsh desert sun. They pull it out of the atmosphere and cover themselves with it (as a precipitate) to protect themselves from UV radiation! Eventually this manganese hardens into a varnish, but it takes thousands of years.
photo: Ulyana Peña, at Petroglyph National Monument