Thursday, December 5, 2013

Petroglyphs of Signal Hill: Geology and Cultural History Come Together in Saguaro National Park West

In the Tucson Mountains west of Tucson, Arizona, lies the unsuspecting rocky outcrop of Signal Hill. Just a short hike from the scenic Bajada Loop Drive through the Sonoran Desert in Saguaro National Park West brings you to the hillock’s rubble-covered crest of sun-baked boulders where a millennium-old gallery of petroglyphs (Greek for "stone-carvings") is on display.

Members of the pre-Columbian Hohokam (HO-HO-kam) created the rock renderings with a stone and hammerstone. By chipping, incising and abrading through a micro-thin, dark coating of desert varnish into the lighter, underlying rock, they fashioned images commonly called rock art. Leaving no written history, the images afford us an opportunity to look into the past and gain insight into the Hohokam's lives and thoughts. 

This High Dynamic Range photograph centers on a one-foot diameter Hohokam spiral geometric.

The Hohokam people thrived in central and southern Arizona from about 400 to 1450 AD. They were farmers, hunters and gatherers, who built over 1,000 miles of elaborate canals in the Phoenix area and cultivated crops of corn, cotton, squash and beans. In the region of Signal Hill, they occupied the river valleys and deserts between the Tucson Mountains and the Rincons.


After thriving in the Sonoran Desert for over 1,000 years, Hohokam society began to decline and collapse over the course of several generations. During the Protohistoric Period, between the Hohokam occupation and Spanish contact, the area appears to have been occupied by Sobiapuri (Upper Pimam) and Tohono O'odham people. 

When Spanish explorers arrived in the 1500's, they found Hohokam villages in ruins. Their fate is both controversial and mysterious, whether related to droughts, soil depletion, warfare, disease or internal strife. Archaeologists that search for clues also look for direct links to indigenous Native Americans like the O’odham Nation that practice Hohokam’s desert traditions such as the annual saguaro cactus fruit harvest. 

The Hohokam’s petroglyphs at Signal Hill are reminiscent of life forms such as snakes, lizards, bighorn sheep, dogs, plants and stick-figure humans, but the majority are abstract geometrics of circles, spirals and lines. Their meaning remains unknown, whether pre-historic graffiti, artistic, symbolic conveyance of a message to passersby or their descendants, astronomical, religious or ceremonial. Perhaps their significance lies in the very act of creating the images rather than the message they convey.

Petroglyphs litter the crest of Signal Hill seen from the trail

Found throughout the Southwest, and in fact worldwide, desert varnish is the canvas on which many Native American cultures engraved their rock art. Pictographs are often confused with petroglyphs, which are literally drawn or painted on rock faces with naturally-occurring pigments made from clays and minerals. Pictographs are not as durable as petroglyphs unless located in protected settings such as rock shelters or caves. 

Desert varnish is typically found on resistant rock surfaces that are subjected to periodic wetting and drying in arid regions of the world. "Rock" varnish is a more appropriate term, since it also occurs in tropical, arctic and alpine environments. Its coating ends with an abrupt boundary on rocky substrates. The sharp demarcation is suggestive that it is derived from external sources. A long standing debate exists as to whether the varnish's formation is microbially-mediated, deposited by inorganic processes or more likely a combination of biological, physical and chemical processes.

Micro-fungi have been cultured from the varnish, but it remains unclear whether fungi or bacteria precipitate the varnish or if microbial components complex with metals in the varnish-forming process. It has been theorized that the varnish’s thin patina protects underlying microbes from exposure to desiccation and UV radiation. The following schematics summarize three popular models for the formation of desert varnish.

Three schematic models of varnish formation showing: evaporative leaching of the underlying rock substrate followed by re-precipitation (a process whereby varnish develops from the inside out); diagenesis of detrital dust particles after accretion to the substrate; and direct chemical precipitation of dissolved atmospheric trace-metal components in rain water, fog droplets and aerosols (a model which explains varnish's conspicuous association with moist surfaces which favor microbial colonization). HFSE's are high field strength elements: Zr, Nb, Hf and Ta. REE are rare-earth elements. See author's article for detailed explanation.
Modified from Thiagarajan and Lee (2004)

Numerous chemical elements are found in varnish but predominantly clays and oxides of manganese and iron, making it appear black and reddish-brown, and at far greater levels than the neighboring substrate and soils. Mixtures of oxides are responsible for intermediate shades of brown. The varnish is almost as hard as quartz and yet extremely thin, commonly a hundredth of a millimeter in thickness (<200 µm). 

It takes thousands to tens of thousands of years to coat a rock with varnish in the arid conditions of the Southwest with growth rates from <1 to 40 µm per thousand years. That qualifies desert varnish as the slowest forming terrestrial sedimentary deposit! Varnish's structure appears micro-laminated in microscopic cross-section, reminiscent of a stromatolite's onion-layered macro-stratification. 

Attempts to utilize varnish for paleo-dating have proven unreliable; however, its usage as a paleo-climate indicator does shows promise. Incidentally, varnish may have planetary analogues, Mars in particular, which has fueled speculation of its potential usefulness in the search for life on other planets. 

The bedrock rock type in the immediate region of Signal Hill is granodiorite, a coarse-grained, intrusive igneous rock intermediate in composition between granite and diorite. These rocks were generated within a volcanic caldera, a collapsed magma chamber, at the end of the Cretaceous from a subduction zone (Sevier-Laramide compression) at the continent’s western margin, a time when at least seven volcanoes were active in southeastern Arizona.

Long after the cessation of volcanic activity and related to continued plate subduction with an altered geometry around 30-35 million years ago, extension (pulling-apart) and heat from within the Earth's crust separated the landscape into linear ranges with intervening basins. Portions of the extinct caldera were faulted upward into a range, the Tucson Mountains, while other portions dropped into the nearby basin and were covered by basin fill sediment.

Bedrock map showing relationship of "upper plate" Tucson Mountains and "lower plate" Catalinas with the Tucson Valley basin intervening. Note the location of the Catalina detachment fault.
Modified from Arizona Geological Survey's Open File Report OFR 06-01

Brittle, faulted-blocks of the Tucson Mountains represent an unmetamorphosed “upper plate” (not to be confused with a tectonic plate) of an arched "metamorphic core complex." Complexes are enigmatic, controversial and only recently described geologic features of Arizona’s Basin and Range province.  The Tucson-upper plate slid off (detached) from the “lower plate” along an intervening, near-horizontal “detachment fault”, which bowed upward into a blister-like uplift.

Uplift occurs in response to tectonic denudation (i.e. erosion) and the inflow of hot, middle crust. Portions of the lower plate are exposed by erosion and others lay buried within the Tucson basin (called a graben, German for grave). The metamorphic core of the “lower plate”, stretched and deformed plastically rather than brittlely, forms the Santa Catalina Mountains across the basin. These domal uplifts are not confined to the Basin and Range of Arizona but follow a sinuous belt from southern Canada to northwestern Mexico. 

Schematic formation of the Tucson-Santa Catalina metamorphic core complex.
Note that the west to east perspective in the diagram is opposite that of the bedrock map.
A, Tucson Mountain volcanics and domal uplift; B, Volcanism and detachment of upper plate from lower plate; C, Basin and range faulting with downdropping of Tucson graben; D, Basin sediment fill.  

The ruggedness and colors, the distinctive look and feel, the topography and climate, and the rocks exposed in Saguaro National Park West are a compendium of the state’s billion year-plus geologic history. On our return hike from Signal Hill, the setting sun and rising moon, the billowy clouds and blue sky, the saguaro and prickly pear, and the spirits of the Hohokam were in exquisite balance.  Please enjoy the following photos as our day gloriously gave way to night.


High Dynamic Range digital photograph

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