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Roger Weller, geology instructor

wellerr@cochise.edu
                                 

San Pedro River
Sara Nelson
Physical Geology
Spring 2006 
                                              The Curious Tourist’s Guide to Arizona Geology:

The San Pedro River Valley
 

            Arizona is famous for its geological wonders.  The Grand Canyon, Painted Desert, Petrified Forest, and Meteor Crater are just a few places easily recognized by tourists.  Yet there is so much more to Arizona geology than the northern part of the state.  Southeastern Arizona’s Cochise County, for example, is filled with many natural wonders.  The San Pedro River Valley within Cochise County attracts many tourists, but a curious tourist wants more than a photo op.  A curious tourist wants to know what geological forces helped shape the unique rock formations and landforms in this area. 
 

            Our tour of the San Pedro River Valley begins east of Benson at a rest area on Interstate 10 named Texas Canyon.  The Texas Canyon rest area is a pass close to the summit at an elevation of 4974 feet between the Little Dragoon Mountains on the north and the Dragoon Mountains to the south.  Of interest are the weathered cliffs with deep cuts and rounded boulders.  The formations are a good example of jointing, spheroidal weathering, and exfoliation.^[1]  Pink granite porphyry, tan quartz monzonite porphyry with large crystals of feldspar, and banded gneiss give the rocks of this intrusion their colors and texture.^[2]  From Texas Canyon, westbound Interstate 10 begins a descent into the San Pedro River Valley.  Deeply eroded ravines are etched into the weathered granite along the Interstate. 
 

            Southeastern Arizona is part of the Basin and Range province, an area of the Southwest with separate mountain ranges and wide valleys.^[3]  The disconnected ranges slant across the landscape in a generally northeast to southwesterly direction.  Descending into the valley on Interstate 10 provides a magnificent view of the valley.  It is easy to see why the mountain ranges in a basin and range province are referred to as sky islands. 
 

            The basins are a deposition area for sediments.  Some basins in Cochise County, including the San Pedro River Valley, are remnants of ancient lakebeds.  The largest, Lake Cochise, was east of the San Pedro Valley.  The Wilcox Playa in the Sulfur Springs Valley is all that remains today.^[4]  The shape and contour of these valleys are still recognizable as ancient lakebeds, and fossil records verify the diverse plant and animal life that once lived here.  Some of the mammal fossils include sloth, jaguar, horse, camel, llama, and mammoth.  Amphibian and reptile fossils indicate a much wetter environment once existed.  Marine fossils are numerous and common, such as, brachiopods, crinoids, and corals.[5] 
 

            The north-flowing San Pedro River appears to be little more than a sluggish creek at some locations in the valley and an ephemeral stream in other places.^[6]  Yet, it is an important waterway, particularly as a migratory corridor for many North American birds.  The river is fighting for its life in a valley with competing water needs and a growing population. 
 

            During the monsoon months of July and August, if rainfall is plentiful, the San Pedro River may overflow its banks.^[7]  Stream channels, gullies, and arroyos feed the river during summer rains and winter snowmelt off the mountains. ^[8]  Both erosion and deposition of sediments are evident geological events.  In the late 1800’s, the grasslands of the San Pedro Valley were home to up to 14,000 head of cattle, according to local historians.  As the valley became over-grazed, grasses disappeared, and the land was subjected to more and more erosion.  Even without overgrazing, arroyos are common in this region that receives most of its annual rainfall of 10-15 inches within a few months of the year. 
 

            Basin fill in the valley consists of angular to rounded cobble conglomerates.  Channel fill is fluvial sand, unsorted debris flow, and silty clay.  Rock types are primarily igneous and sedimentary along with some metamorphic.  Igneous rocks show much weathering.  Throughout the valley, lime and calcium deposits carried downhill have cemented the soil into hard caliche.  Layers of sediment are both permeable and impermeable.  Alluvial fans spread from the base of the mountains, particularly the Huachuca Mountains where the town of Sierra Vista sits.^[9]   The nature of the alluvial fans, plus the permeable and impermeable sedimentation, makes it difficult for hydrologists to assess the valley aquifers.^[10]  How much water is in the aquifers is a subject of much controversy. 
 

            The mountain ranges bordering the San Pedro Valley to the west are the Whetstone, Mustang, and Huachuca Mountains.  To the east, the ranges include the Dragoon and Mule Mountains.  One of the primary tourist attractions is Kartchner Caverns, a living limestone cave in the Whetstone Mountains, about ten miles south of Benson on highway 90.^[11]  Opened in 1999, it is environmentally protected to maintain its humidity levels and beautifully pristine condition.  It is considered to be one of the world’s top ten caves.  Perhaps the cave’s best-known feature is the 58-foot column of dripstone named Kubla Kahn.  The cave also has the world’s longest known soda straw formation measuring 21 feet in length; however, it is deemed too delicate to be on the main tour.  The Karst topography of the area provides the conditions for many smaller caves that are explore-at-your-own-risk adventures.^[12]
 

            Past volcanic and tectonic activity helped carve and shape the mountain ranges.  Old cinder cone and maar craters can be found in Cochise County, particularly in the San Bernardino Valley.^[13],  ^[14]  The San Pedro Valley ranges are an outstanding example of thrust faults and folds.^{[15], [16]} 
 

            The most recent earthquake occurred in 1887.  The epicenter was south of the border in Mexico near Douglas, Arizona.  The earthquake caused one side of the San Pedro Valley to drop as much as nine feet. 
 

            Another tourist attraction in the San Pedro Valley comes directly from the area’s volcanic past.  The Bisbee Copper Queen mine and Lavender Pit mine are evidence of mineralization produced by volcanic and hydrothermal activity.^[17]  Copper, silver, gold, and other mineral ores formed in the contact zone of the volcanic intrusion.  Bisbee malachite and azurite are world famous for their deep, brilliant colors.^[18]  An extensive cave system was a great asset in tunneling the mines.^[19]  Today, tours of the Copper Queen mine, which ceased operation in the 1970’s, are conducted by retired miners.  In addition to the Bisbee copper mine, the legendary old west town of Tombstone, not far from Bisbee, was the site of silver mining in the late 1800’s.  
 

            Fort Huachuca, at the base of the Huachuca Mountains is of historical interest, aside from the geology of its canyons and cliffs.  Petroglyphs and other archeological sites are preserved by the fort.  Ramsey Canyon Nature Preserve protects birds and wildlife habitat in the canyon.  Throughout the mountain range, particularly in Carr Canyon, remnants of mines from the area’s gold rush days can still be found.  At the far south end of the Huachuca Mountains, the Coronado National Monument and visitor center commemorate the route of Spanish conquistador Francisco Vasco de Coronado and his men.  The San Pedro River Valley is rich in history as well as unique and fascinating geology. 
 

            These are some of the sights a curious tourist visiting Cochise County will want to explore.  The Chiricahua National Monument in the southeastern corner is not covered in this paper, but certainly is one of the great geological sights of the county.^[20]  Cochise County is proof that not all of Arizona’s wonders are north of Phoenix.   

 

Bibliography

Callendar, J. F., R. E. Clemons, and Jan C. Wilt.  Land of Cochise, ed. H. L. James.

            New Mexico Geological Society, 1978.

Chronic, Halka.  Roadside Geology of Arizona.  Missoula: Mountain Press Publishing         Co, 1983.

Arizona State Parks, “Kartchner Caverns State Park.”              http://www.kaet.asu.edu/wildaz/caverns/discovery/discovery8.html.

Haney, Richard A., ed. Arizona Soils. “Geological Framework of Arizona.”              http://southwest.library.arizona.edu/azso/body.1_div.2.html.

Hendricks, David and others.  Natural History of the Southwest.  “Basin and Range.”             http://www.fhsu.edu/biology/Eberle/DesertSW/BasinRange.html.  

Weller, Roger. “Geology of Southeastern Arizona,” “Virtual Arizona Geology Field     Trips,” and others. http://skywalker.cochise.edu/wellerr/aawellerweb.htm.
 

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