Geology 158T: Geology of the Trans-Pecos


During the 2011 spring break, ten Whitman students traveled over 2000 miles to a remote corner of Texas.  Our journey began with crossing over the bulk of the western United States in an airplane.  We arrived safely in El Paso, Texas after hours on different planes and frustration with airport food. Before continuing on our journey by car, to find the wilds of the West, we first had to travel east.

If you have ever looked at a night-time satellite picture of North America, there are few very dark patches on the map.  Regions such as southeastern Oregon and northern Nevada stand out due to their darkness.  On this map, West Texas is a black and dark-purple patch with absolutely no lights.  Once out in the midst of the Chihuahuan Desert of West Texas, the moon isn’t the only thing to keep you awake; the Milky Way is often so bright as to keep any would-be camper up at night.

While Texas now is hot and dry and very little large-scale geologic activity is taking place, as recently as 28 million years ago volcanoes were erupting throughout this region.  These volcanoes compose the Trans-Pecos Magmatic Province, stretching from the southern end of New Mexico all the way through Texas and into Mexico.

Unlike the Cascade volcanoes west of Whitman, the volcanoes of West Texas were formed as the Earth’s crust stretched and fractured, allowing magma to reach the surface.  A modern corollary can be found in East Africa, where the Earth’s crust is being pulled apart.  Volcanoes there are the tallest mountains in Africa (Mounts Kilimanjaro and Kenya).  Fast-forward 30 million years and the volcanoes of the East African rift would appear very similar to those of West Texas.       

Despite the geologic focus of this trip, our group drew from many different backgrounds at Whitman.  Only half of the group were geology majors; we had two environmental studies-sociology majors, one environmental humanities major, one biology major, and one environmental studies-politics major.  Be it a geologic or environmental discussion, we were quite prepared.  This mix of majors helped to ensure that everyone brought different experiences and interests to West Texas.  While stopping at a road cut was the time for the geology majors to go to work, if it weren’t for other people’s areas of interest, we never would have gotten out heads out of the geologic sand and learned more about border issues, alternative energy, and environmental concerns in West Texas. However, our trip would only be on border issues unless we could first get out of El Paso.

Just west of Van Horn, Texas, students examine the flora of the Chihuahuan Desert.  Photo by Matthew Morriss

Just west of Van Horn, Texas, students examine the flora of the Chihuahuan Desert.  In addition to taking numerous different majors to Texas, our goal was to cover ecology, biology, ornithology, and geology while in the field. (Photo by Matthew Morriss)

Still traveling east, we camped in the small town of Van Horn, adjacent to metamorphosed and deformed Precambrian rocks left over from the formation of the continent 1.3 billion years ago.  We planned to visit three distinct volcanic centers on this trip, the oldest and most eroded of which are the Davis Mountains. 

Three hours east of El Paso lies the town of Kent, a single exit on I-10.  The tallest structure in Kent is the Mercantile Building, which conveniently enough has walls made out of a green flow-banded tuffaceous rock.  This small one-house town was our gateway to the Davis Mountains, which lay due south.  After a short stop to look for fossils in Cretaceous limestone, we continued on to our next destination: the Davis Mountains.


students examine the rocks sin the wall of the Kent Mercantile Building. (Photo by Matthew Morriss)

As an exercise in geology, students examine the rocks sin the wall of the Kent Mercantile Building. (Photo by Matthew Morriss)

While the twisting highway we followed brought us up into Ponderosa Pine and Alligator Juniper, every rock we saw beyond this point showed evidence of volcanism.  Roadcut after roadcut showed some form of lava cooled long ago..  These rocks are the very reason for the Davis Mountains elevation above the surrounding countryside.  They are more resistant to erosion than the surrounding rock (limestone). 

These mountains merely marked where our journey through volcanic fields began.  But beyond the geology, we had experiences endemic to the West.  Where else could you see several javelinas walk unconcerned through your camp?  Where else does a desert reach out to poke any animal that passes it by, yet Aspen trees, the “quakies” of the Rocky Mountains, live.  Alongside now cool lavas and exposed volcanic necks, we found oases referred to as “sky islands” by biologists. 

While the moonlight hid many of the night’s wonders, our group found another way to explore the heavens.  We paid a visit to the McDonald Observatory (a research observatory, owned by the University of Texas at Austin, nestled high in the Davis Mountains).  Once a month amateurs and professionals alike pull out their tripods and equatorial mounts, and dust off their various mirrors and lenses.  For two hours, we toured the heavens through various telescopes and gained a sense of direction among the stars.  Our group had just begun to taste the great diversity West Texas has to offer, and we still had two stops to go.  Putting the “Alps of Texas” in our rear view mirror, we journeyed farther south to Big Bend National Park. 

In the Big Bend of the Rio Grande lies one of the least visited national parks in the country, yet the park boasts more diversity of birds than any other in the lower 48 and features numerous species found only in the park or hundreds of miles away.  In the Chisos Mountains, geology has played an important part in affecting the biology seen today.  The most recent volcanism here occurred 31 million years ago.  Similar to the Davis Mountains, the Chisos mountain range stands well above all of the surrounding topography. At 7,825’ tall, Emory Peak is one of the tallest mountains in Texas.  The elevation within the Chisos has created a refugium for numerous species otherwise found in more temperate climates. 


The Mexican Jay is one of over 400 species of birds found throughout the park, making Big Bend National Park a birding hotspot. (Photo by Will Bender)

The Mexican Jay is one of over 400 species of birds found throughout the park, making Big Bend National Park a birding hotspot. (Photo by Will Bender)

During the last glaciation when the climate in this region was cooler and wetter, Aspens, Maples and many other temperate climate species migrated south.  But when the climate warmed again, these species became trapped within the mountain ranges.  They could only move farther up north facing slopes, finding the coolest and wettest corners on mountain slopes.  So in addition to seeing numerous cacti in Chihuahuan Desert, we saw Maple trees!


The Chisos Mountains are filled with numerous tree species not normally found in the Chihuahuan Desert several thousand feet below.  Those trees isolated in the mountains since the last glaciation include Maple trees and Aspen.  (Photo by Matthew Morriss)
The Chisos Mountains are filled with numerous tree species not normally found in the Chihuahuan Desert several thousand feet below.  Those trees isolated in the mountains since the last glaciation include Maple trees and Aspen.  (Photo by Matthew Morriss)

Here in this land of extreme temperatures, geology, and ecology, our group of Whitman students soaked their sore legs in a hot spring, all that remains of the heat from long extinct volcanoes.  The Rio Grande which was flowing by just a few feet away marked a border to another country, but not necessarily a different place geologically.  The same rocks that were on this north side of the river continued to the other, and yet this river has marked violence and death as those struggling to enter the United States wade its shallow waters.  Here on its banks, we had our first encounter with the people of Boquillas del Carmen, a small town just across the river, which was once supported solely by the tourists in Big Bend National Park.  Since the 2002 closure of the border, the town has stagnated and become a ghost town in the middle of the Chihuahuan Desert.  This sad state of affairs on the US-Mexico border pushed itself into our consciousness upon seeing handcrafted trinkets and minerals sold by the people of Boquillas del Carmen.

From our nicely stocked national park campground in Rio Grande Village, we drove farther west.  West toward our third destination of the trip; west into the wild lands of Big Bend Ranch State Park on the “Other Side of Nowhere.”  The journey took us past one of the three spectacular canyons of Big Bend National Park: Santa Elena Canyon.  Thousand foot-tall walls of limestone shaded the Rio Grande; sun-light and shadows danced from wall to wall as if in a Utah desert slot canyon.   Santa Elena Canyon is a beautiful example of what geologists call cross-axial drainage. 


Santa Elena Canyon is one of three canyons found in the national park.  Here the Rio Grande has incised down several thousand feet through Cretaceous limestone. (Photo by Matthew Morriss)
Santa Elena Canyon is one of three canyons found in the national park.  Here the Rio Grande has incised down several thousand feet through Cretaceous limestone. (Photo by Matthew Morriss)
 
The next destination was hotter and drier than the past two. The Bofecillos volcanics (the youngest in the suite of Trans-Pecos magmatic rocks) are in Big Bend Ranch State Park.  The Bofecillos group is typified by basalt and various tuffaceous rocks .  The main event of this trip was saved for last.  A large circular depression in the Bofecillos volcanics can be seen from space; it was this feature which first inspired Bob Carson to visit West Texas: the Solitario. 

The Solitario has jokingly been called a “lacco-caldera” as it formed from an intrusion of magma which culminated in an eruption.  The rim rocks around the resulting caldera date back to the Paleozoic (470 million years ago).  There are no mountains in Big Bend Ranch State Park, only endless horizons where topography drops off in the negative, into small canyons or larger long extinct calderas.   Occasional high points are lava-capped buttes, showing just how much topographic relief has been lost to erosion.  In March, temperatures already top 95°, and at night no bugs interfere with your sleep.  Yet here miles from nowhere, the main visitor center –once a working ranch house- has satellite Internet and a coke machine.  If one were to hike deep into the Solitario, a small shack with a satellite dish and webcam can be found.  While we didn’t journey that far, everywhere we walked in the Solitario there were boot prints from previous geologists.

Despite visiting three of the most remote places in West Texas, the dark blank spots on the satellite picture, we had yet to escape and journey totally away from the hustle and bustle we started in.  Even a hike through the Solitario showed us evidence of repeated visits by various travelers over the years.  It was here that we packed up our bags for the last time in the early pre-dawn light, our souvenirs (rocks and minerals) stowed safely in a cooler, and drove west toward the setting moon.  A long day’s journey back to Seattle lay before us.

In the Bofecillos volcanic field, students take notes on Three Dike Hill.  Here three feeder dikes fed the basalt flow atop numerous tuffs.  (Photo by Matthew Morriss)
In the Bofecillos volcanic field, students take notes on Three Dike Hill.  Here three feeder dikes fed the basalt flow atop numerous tuffs.  (Photo by Matthew Morriss)


Students in front of the Chisos Mountains of Big Bend National Park.  From left to right, Katie Tackman, John Whiting, Will Cooper, Clara Easter, Will Bender, Rachel Alexander, Abbye Neel, Shannon Buckham, Matthew Morriss, and Geneva Faulkner.  (Photo by Bob Carson).
Students in front of the Chisos Mountains of Big Bend National Park.  From left to right, Katie Tackman, John Whiting, Will Cooper, Clara Easter, Will Bender, Rachel Alexander, Abbye Neel, Shannon Buckham, Matthew Morriss, and Geneva Faulkner.  (Photo by Bob Carson).