In Jeff Davis County near Fort Davis, many of these
mountain peaks exceed 6,000 feet in altitude, and include majestic Mt. Livermore,
which at 8,382 feet, ranks as the fourth highest summit in Texas.Visitors to the Trans-Pecos are often surprised by the
presence of rugged mountains that encompass a triangle formed by the towns
of Fort Davis, Alpine, and Marfa, Texas. In Jeff Davis County near Fort Davis, many of these
mountain peaks exceed 6,000 feet in altitude, and include majestic Mt. Livermore,
which at 8,382 feet, ranks as the fourth highest summit in Texas.
Most of this rocky Texas high country was built from volcanism that began
approximately 48 million years ago during the Tertiary period on the geologic
time scale. But few people realize, for example, that by driving on Highway
90 between Marfa and Alpine they are, in fact, traveling through the center
of what remains of the collapsed caldera of the enormous Paisano Volcano
that erupted approximately 35 million years ago!
Geologists believe that plate tectonic motion caused subduction
(a diving under) of the dense Farallon oceanic Plate where it meets the
less dense continental North American Plate off the western coast of Mexico.
This collision resulted in tremendous pressure deep within the earth’s
mantle that produced magma, or molten rock. And the titanic forces
that were released by this event caused widespread volcanism stretching
from Mexico’s Sierra Madre, through Chihuahua, into southwest Texas,
and to points as far north as Montana!
During the initial 10 million years of volcanism across
the Trans-Pecos, activity occurred primarily in the form of magma seeping
to the surface through fissures vents in the earth’s crust. This extruded
magma called lava, flowed over a wide area, overlapping to build
low and broad dome-shaped mountains called shield volcanoes. The famous
Mauna Loa in Hawaii is an example of a shield volcano. However, this quiescent
form of volcanism gradually evolved into a more violent expression of eruptions
throughout the region.
All kinds of volcanic strata make up the Davis Mountains
of which vertical lava flows, with their columnar jointed palisade appearance,
and ash-flow tuffs are the most common. As the volcanic activity began to
release its energy more forcefully, the composition of the extruded magma
changed from mostly basalt to a type high in silica content called rhyolite,
a very fine-grained version of granite. Rhyolite is fine-grained because
it came from molten rock that was extruded to the surface so rapidly that
no large mineral crystals had time to form.
Ash-flow tuffs (a hot mixture
of gas and ash that cools to a solid) began to form from powerful eruptions
and were deposited as sheets of welded material. Subsequent weathering and
erosion has produced a totem-like columnar appearance. This type of rock
forms the Sleeping Lion formation that is the distinctive backdrop to Fort
Davis and is visible throughout nearby Limpia Canyon.
A third type of light-colored igneous rock, trachyte, is also present. Trachyte is fine-grained lava that contains feldspar and other minerals but little or no quartz.
Using Fort Davis as our starting point, let’s begin
our tour by traveling northwest on Highway 118 and then south on 166, a
circuit known as the “scenic loop.” Given a reaso
nably early
departure time, it is possible to traverse the scenic loop and all three
“sides” of our triangle as a single day trip. If possible, I recommend
dividing the sojourn into separate excursions to allow time to stop and
explore the many interesting sites along the way.
Fort Davis is located between the remnants of two huge
volcanoes: the Buckhorn Caldera and the Paisano Volcano. Both of these volcanoes
are estimated to have been eight to ten miles in diameter or three times
the size of Mount St. Helens! The debris fields of these eruptive centers
comprises many layers of imbedded volcanic rock covering a vast area.
As we pass the Fort Davis National Historic Site and curve
into Limpia Canyon, we are entering a system of hills and valleys cut through
the debris fields’ layers by stream erosion. Along this stretch of
Limpia Creek are many large Cottonwood trees that add to the beauty of the
talus (boulder-strewn) slopes below the canyon cliffs. Look for large,
weathered, gray rhyolite boulders on your left by a roadside picnic area.
Some of these rocks contain large feldspar crystals “zoned” among
the normally fine-grained rhyolite. This mixture formed when slow-cooling
magma was erupted from deep within the earth, accelerating its cooling process.
Approximately 4 miles from Fort Davis is the entrance to
Davis Mountains State Park. Inside the park is a drive up to a scenic overlook
that has several excellent roadcuts. Together, the cuts expose a variety
of colors, textures, and compositions of most of the volcanic rock in this
section of our tour.
Continuing on, our route follows Limpia Creek and hilly,
grass-covered slopes appear in front of us as the canyon begins to widen.
The white domes of the University of Texas McDonald Observatory can be seen
on the horizon to the north. And as the road straightens and heads to the
west, Mount Livermore looms high in the distance, and is easily identified
by its distinctive knobbed-tipped summit. The grassy slopes in the foreground
are sporadically crowned by pinnacle-shaped outcrops of volcanic rock. Casket
Mountain is the aptly-named peak on our right to the north of the Prude
Ranch.
Soon the road turns to the north again as we pass the Central
and Southwest Solar
Park and begins to climb the south face of Mount Locke where the Observatory
is perched. About halfway up our ascent, there is a wide pullout and scenic
overlook that is a popular stop for amateur geologists and school field
trips. Here, a large wall of lava flow is exposed resting on a ash-flow
tuff, turned red in a soil zone. Bisecting the lava wall is a near vertical
white, siliceous dike, turned quartz at its baked edges. Dikes result
from magma that is forced upward along fractures and cracks to produce walls
of hardened rock.
Highway 118 junctures Spur 78 at the entrance to McDonald
Observatory and continues to the west, climbing into the pinon and ponderosa
pines at over 6,300 feet. Spectacular vistas of Sawtooth mountain, Pine
Peak, and Mt. Livermore come into our alpine picture before we abruptly
descend into Madera Canyon and approach the Lawrence E. Woods roadside park,
a popular picnic and birdwatching spot.
A few miles past Madera Canyon is another “Y”
juncture at Highway 166 and 118. If we continue north on 118 toward Kent,
we would skirt the Buckhorn Caldera on the east side of the road. Instead,
our journey continues south on 166, passing the granite “rockpile”
landmark and onward to the foot of Sawtooth Mountain (7,800) and a look
at the west face of Mt. Livermore. Looking to the west, we notice that the
terrain is a relatively-flat, high desert plain.
The Davis Mountains create an uplift in the westerly winds,
cooling the air and condensing the moisture within it. This dynamic results
in higher rainfall amounts beyond the eastern face of Mt. Livermore and
directly influences the types of vegetation and animals that inhabit the
area.
The loop soon reaches its southern limit and curves east
past WTU’s Wind Generator Array and the Davis Mountains Resort. Approximately
five miles further is Point of Rocks, an outcropping of pinkish-gray granite
boulders that shelters a roadside picnic area and served as a stagecoach
and covered wagon stop during the heyday of the Overland Trail.
The high-sloping, ridged summit ahead on our left is Blue
Mountain (7,400). There is a vineyard located at the southern foot of Blue
that takes advantage of the rich volcanic soil that has accumulated at its
base. Highway 166 soon ends where it meets Highway 17 two miles south of
Fort Davis. One may stop here and travel the remainder of the triangle on
another day (especially if staying in the vicinity of Fort Davis), or continue
southwest to Marfa, east to Alpine, and northwest back to Fort Davis.
The Fort Davis-to-Marfa leg of our volcano drive is on
Highway 17, where the road descends several hundred feet over the 21-mile
distance. Heading south out of Ft. Davis, Blue Mountain catches our eyes
to the west as we pass the greenhouses of an extensive hydroponic tomato
farm. Ahead on our left are a few lower peaks that jut from the flats -
The Puertacitas Mountains. From here, the road climbs a low grade through
a short pass where basalt lava flows form low, knobby hills. The view then
expands before us to take in a basin to the southwest and, if the sky is
especially clear, the Chinati Mountains can be seen over 50 miles in the
distance.
As we approach the outskirts of Marfa, notice the gravel and terraced ledges that are a result of stream erosion throughout this alluvial plain. After spending some time exploring Marfa, the seat of Presidio County, head east on Highway 90 to Alpine. This is the section of our triangle drive that bisects the collapsed caldera of the Paisano Volcano.
Leaving Marfa, we cross Alamito Creek (“alamito”
is Spanish for cottonwood) where more of the alluvial fan (streambed conglomerate)
is exposed. The next feature that draws our attention is Cathedral Mountain
(6,800) on the horizon to the southeast. Cathedral Mountain is the erosional
remnant of a sequence of lava flows and sediments. Cathedral was affected
by an earthquake which measured 5.6 on the Richter Scale in the spring of
1995. The quake caused landslides that subtly altered its shape. A second
earthquake of magnitude 3.6 struck nearby in 1998, proving that not all
of the geological dynamics of this region are ancient history.
On the south side of the road, approximately 9 miles out
of Marfa, is the pull-off shoulder area dedicated for the viewing of the
Marfa Lights. From this location, a fine view of Chinati peak (7,700) is
possible across the Mitchell Flats to the southwest. Approximately half-way
to 
Alpine, the mountains of the Paisano
Plateau appear on the skyline. Most notable are Paisano Peak, Twin Sisters
(also known as Twin Peaks), and Ranger Peak. We are now entering Paisano
Pass where we can examine a series of roadcuts that document evidence of
a massive volcano that erupted some 35 million years ago.
Following the removal of magma from beneath a volcano, the caldera itself often collapses, forming a broken jumble of angular rocks called breccia, that jam the volcano’s neck. The road we’re traveling runs directly across this collapsed zone - the center of the Paisano Volcano. As mentioned previously, dikes are magma bodies that are forced upward through fractures and cracks in overlaying rock. When we approach the Baptist encampment on our right, a 3 mile-long dike cuts through the breccia and is exposed in several roadcuts. Light-colored rhyolite is also present in roadcuts just beyond the encampment area where it is neatly cleaved by darker dikes.
Just ahead on the left is a roadside picnic area, and as the road exits
the pass, dark, reddish-colored lava flows become visible as craggy cliffs
and vertical joint columns that are embedded with shallow caves. This area
marks the eastern edge of the Paisano volcano and these lava flows are a
very distinctive and recognizable type of igneous rock.
U.S. 90 then enters the alluvial plain where Alpine is
situated. Alpine is the largest city in the three counties our circuit traverses,
and is the seat of Brewster County. There are many sites to explore here,
including Sul Ross State University. Sul Ross is located on a hill on the
east side of the city, and from this vantage point, it is possible to look
back down U.S. 90 to the west and see the profile of the Paisano Volcano
outlined against the sky.
Alpine-to-Fort Davis is a scenic drive that approaches
a sheltered canyon of volcanic rock from a 10-mile flat, where before cattle
ranching, tall desert grasses covered the alluvial plain. As we head north
from Alpine on 
Texas Highway 118, we can see the rocks and cliffs of
Musquiz Canyon ahead, the Glass Mountains of the much older Permian Reef
to the east, and volcanic mesas to the west. The very obvious, sharply-pointed
rock to the northwest that comes into view as we near the canyon is Mitre
Peak (6,100). Mitre Peak is a striking example of an intrusive mass - hard,
emplaced rock that weathered clear from its surrounding encasement of softer
rock.
As we enter Musquiz Canyon, there is another intrusive
mass, called the Barillos Dome, that is visible on our left. Its core of
harder rock is exposed in places where rock and soil have eroded away. Also
seen at the entrance to the canyon are roadcuts of hard lava resting on
ash-flow beds. Musquiz Canyon is walled by columnar-jointed cliffs that
have talus slopes at their bases. Musquiz Creek (wet weather) is on our
left and crosses the road at a hunting camp where there is a stand of cottonwood
trees. High, ash-flow mesas flank the canyon.
As the highway bends more to the west, we climb higher
and pass a stand of tall cottonwood trees at a roadside picnic area on the
right. The road curves north again and climbs abruptly through a long roadcut
that exposes rhyolite, ash-flow tuff, and trachyte before cresting at an
alluvial plain 5 miles south of Fort Davis.
Immediately on our right is the entrance to the Chihuahuan Desert Research
Institute. The Institute is the regional center for interpretation of the
natural components of the Chihuahuan Desert region, and where public access
is available for hiking into Modesta Canyon. The canyon reveals a contact
zone between two distinct geological formations - the Sleeping Lion and
the Weston Stock.
This completes our drive through the Davis Mountains. The geology of this area is quite unique and the mountainous terrain is often referred to as the “Alps of Texas,” although Mount Livermore may not really be in the same league as the Matterhorne. Thirty-five million years ago things were quite hot indeed. Today, altitude and geography make this area one of the few predictably cool refuges in Texas during the summer and a great place to examine the inside of a volcano!
For More Information on Volcanoes Visit these Sites:
| CDRI, Box 905, Ft. Davis, TX 79734 |
e-mail: choyt@cdri.org |