Deborah Painter (USA)
In the area east of the small community of Bagdad and on the northeast edge of the Arrastra Mountain Wilderness of central Arizona in the USA, my friends, Terril, Yvette and David, stood with me at the base of a vision in the desert of a rockhound’s dream. This was a colourful, irregularly shaped hill, standing alone in the arid wildlands, its bright whites, reds and greens standing out against a blue and white March sky. The entire hill seemed composed entirely of loose stones of quartz, caliche (a mineral deposit of gravel, sand and nitrates found in dry areas of the USA), basalt, travertine, green quartzite, tuff and gabbro. One whole side of the hill was white from quartz.
We had attempted to climb this amazing thing. But, like wonderful things in a dream, most of it eluded us. We could climb but a metre or so, before we slid back down, unable to secure a foothold. However, the four of us collected about a bucket full of the rocks on this Bureau of Land Management land.
Just across the roadway to the south, we had hiked a short distance across an arroyo (a Spanish word for a dry creek or stream bed). Here, the rocks visible at the surface were quite different and we saw less travertine and more quartzite. No titanic mounds of loose stones met our seeking eyes. What we did see were large basalt boulders, some reddish scoria and gabbro, and some sand, as well as desert pavement along the Santa Maria River. How could the geology appear so different on either flank of a relatively narrow highway?
David Hawk and I had only been in Arizona for one day and we could already see that the geology of the state was very complex. Our ride on a commuter shuttle from the big city of Phoenix, north along Interstate 17 to Prescott, treated us to views of dark rocks and many mines. Upon arrival in Prescott to stay for a short while with Terril and Eve, we had a chance to see the varied geography, and also the animal and plant life of this region. Many habitats intersect here.
In Prescott, there are forests of ponderosa, juniper and pinon pine, as well as acreages of short grass prairie upon which graze small herds of pronghorn, a unique species of ungulate related to sheep but closely resembling antelope. David and I had never seen these animals before. Eve and Terry knew this area well, and both are in the environmental education field and tireless advocates for the Arizona ecosystem, so we knew we had great guides in our quest for the ‘real’ Arizona, that is, the part of the state seldom seen by tourists.
Our first day trip of our too-brief sojourn in the Grand Canyon State had us on Iron Springs Road, west out of Prescott. This took us to areas approximately 1,219m above sea level. Scrub oak and scrub chaparral-type shrubs were the principal vegetative cover here. As we proceeded west, the land became more arid and we saw gigantic spheroidally weathered white granite boulders almost everywhere. We stayed on Valley Connector Road for a number of kilometres and Skull Valley was the evocative name of the first town we reached.
White tufa towers as high as a tall person lined the banks of Skull Valley Wash and the railroad grade that paralleled the roadway. Tufa (not to be confused with tuff, an extrusive igneous rock) is a kind of calcite travertine-like deposit. Continuing west on State Route 15, we passed through the town of Kirkland, a less colourfully named village than the previous one. Now, the countryside opened up into rolling tracts of sandy areas and red boulders with agave, prickly pear cacti and the tall saguaro cacti that many people the world over associate with the American West.
The only creatures we saw were black Angus cattle and ravens. I wondered aloud how these black-hued animals survive in the summer with little shade from the relentless sun. “They just cope,” was Terry’s reply. Next was Arizona State Route 96, still proceeding west. We continued down a dirt road. When we saw Finley’s Iron Horse Ranch, we knew we were almost at our destination, one more swing gate to the west of the ranch. We were on the south flank of the roadway. My party opened and closed the gate behind us. Campers were visible approximately 400m away. Beyond the boulders, approximately one-half kilometre away, were copper mines that we would not be entering. This sandy road wound around and crossed deep arroyos, and eventually turned west closer to the shores of the Santa Maria River.
Along the southern flank of State Route 96, the terrain was less sandy, and here and there along the Santa Maria were sugar loaf-shaped outcroppings of basalt and other igneous rocks. The place was a wonderland of ocotillo shrubs, as well as squat teddy bear chollas we did not dare touch, despite their appealing name. These small cacti have thin spines that give the appearance of a plush teddy bear. Barrel cacti, saguaros, tiny lizards, the traces of rattlesnakes and cactus wrens were everywhere.
Here and there were large packrat middens and burrows up to 1.2m in height. We did not disturb these because we wanted to leave them, as the Native Americans had, for scientists of the present and future to study. Scientists have come to understand changes in the vegetative communities of the deserts since the close of the Wisconsinian Ice Age pollens and seeds within ancient packrat burrows and middens. At our feet were pieces of rose and milky quartz, green gabbro-diorites, basalts, green quartzite, and pockmarked red scorias. We chose some rocks but left the prettiest ones behind. To take everything we saw that looked attractive would take something of what made this area so special. The Santa Maria River stretched before us. David and I were both born in Virginia, in the humid Mid-Atlantic region of the United States, and in Virginia this wide and shallow river would be considered a creek.
We crossed Arizona 96 on foot. There was a barbed wire gate across the road which we were obliged to go through. David and Eve spied a rattlesnake in a burrow dug into the short bluff along the dirt road. However, he withdrew his nose from his burrow entrance. Rattlesnakes generally are not aggressive. It is my personal policy to respect their territories and give them and other pit vipers as wide a berth as possible, so that there will be no misunderstandings.
The geology of the areas on either flank of the road differed. The northern flank, where we beheld the vision of loose minerals and rocks described in the first sentence of this article, was flatter and had only a few teddy bear chollas, ocotillo shrubs, sand, the desert pavement, and the amazing white and green hill. Beyond the hill, on the southern horizon about a kilometre away, was the dark and brooding Bismarck Mountain. This area of Yavapai County is within the Central Highlands of the state and has been uplifted and eroded many times. About 1,700 million years in the past, during the Proterozoic, a crustal plate collided with the plate that is present-day Arizona, and granite and basaltic batholiths were intruded. Massive mountains were created and are known as the Yavapai series.
The area in the vicinity of Santa Maria River is part of the Proterozoic Bridle Formation. This activity accounted for the boulders rising above our heads like loaves of bread or sugar. As time passed, seas moved across the eroding mountains. The Palaeozoic Era brought much deposition of sediments and sedimentary rocks. Late in the Mesozoic, another mountain building episode followed. This was on a vast scale. After the Mesozoic Era ended, another period of erosion erased most of the Mesozoic history of the Central Highlands, but some sedimentary rock layers remained. A Pacific Ocean arc rammed what we call Arizona and resulted in more uplift. This uplift consisted of whole blocks lifted in their entirety, rather than collisions. Cenozoic Era (Gila Formation) volcanism created the scorias, basalts and tuffs seen at the site we visited.
The Arrastra Wilderness is named for the arrastra – or Mexican rastra – a crude but effective mule or donkey-powered timber and dragstone mill that was used in the 1800s and early 1900s to separate gold from the commonly occurring quartz in the area. We only found pyrite and not gold in our quartz. In the 1800s, the gold seekers somewhat wistfully named pyrite “fool’s gold”, since it occurs in veins and has a superficial appearance of gold.
But what of the markedly different north flank? The hill that stood alone, seemingly made entirely from loose minerals, sedimentary rocks and igneous rocks, was a mystery I was determined to solve. Here again, we saw the common but still economically useful and attractive basalts, scorias, rose and milky quartz, granulated quartz, green gabbro-diorites, and green quartzite. One piece particularly drew my interest, but I left it in place. It was a complete copy of a commercially baked chocolate cupcake (the chocolate colouring was the scoria), with a coiling ribbon of white travertine across the top. Perhaps you may see it. If so, please leave it for the amusement of others who come after you.
We knew from the US Geological Survey geologic mapping that the igneous rocks on this northern flank of the roadway dated from the middle Miocene epoch and were less than 15 million years old. Quartz often occurs in such areas and this accounted for the quartzite, metamorphosed through contact with superheated igneous material. But what were travertine and caliche, calcareous sedimentary rocks, doing here, and taking on so many colours (grey, white and yellow) and so many textures (ribbons, inserted pieces and framing structures around a larger igneous matrix, and blobs)?
Back home in Virginia, geologist colleagues, Fred Davis and Jim Creighton, had good, geologically sensible explanations that we were able to verify in our readings of geological papers and books (see especially Anderson, Scholz and Strobell’s paper listed in the references section at the end of this article). Fred and Jim explained that the hill was actually a small volcanic neck, all that remains from the erosion of a Miocene epoch volcano common in this area of Yavapai County in Arizona, and the calcareous rocks were of the Gila Formation.
The caliches and travertines, both calcareous, were the result of cementation from an ancient landslide. The north flank of Arizona State Route 96 has this distinctively different bedrock. The caliches and travertines were formed from sedimentary rocks that dated originally from the Mesozoic, when seas inundated the region. The neck was buried under the Miocene landslide and groundwater cemented the grains of sand, calcite and larger fragments. Travertines and caliches were formed beneath the surface, composing the Wilder Formation of Pleistocene time. Holocene erosion accounts for the dispersion of the sand within the landslide and the smoothing of the landscape in the wide valley like feature below Bismarck Mountain. Thus, a rather unusual feature of the Central Highlands desert wilderness came into being.
About the author
Deborah Painter is an ecologist and general environmental scientist specialising in transportation and industrial development planning to minimise deleterious environmental impacts. She lives in the United States.
Anderson, C.A., E. A. Scholz and J. D. Strobell, Jr. 1955. Geology and Ore Deposits of the Bagdad Area, Yavapai County, Arizona. Geological Survey Professional Paper 278. United States Government Printing Office, Washington.
Arizona Bureau of Mines/Arizona Geological Survey.1958.Geologic Map of Yavapai County, Arizona.
Arizona Geological Survey. 2019. Interactive geologic map of Arizona.
Beatty, Barbara,and P.A.K.Wilkinson.1986. Frontiers in Geology and Ore Deposits of Arizona and the Southwest. Volume 16 of Arizona Geological Society Digest. Arizona Geological Society. 554 pages.
Cooke, Ron. Plumas County Adventures. California State University at Chico. https://www.csuchico.edu/~rcooke/rastra.html
Creighton, Jim. Personal communication.
Davis, Fred. Personal communication.
DeLorme. 2002. DeLorme Atlas & Gazetteer. Arizona Detailed Topographic Map Fifth Edition. 76 pages.
Schnoeker-Shorb, Yvette. Personal communication.
Shorb, Terril. Personal communication.