Introduction to Virtual Geological Field Trips
This is another one of my ongoing projects. It began as a resource for my
geology students at Texarkana College. I took them on geology field
trips into the Ouachita Mountains area of Arkansas, which was not too far, and
there was a lot of geology to see there. However, that area has plenty of
vegetation to hide the geology. Places that we went to - road cuts, spillways,
hillsides, etc. - became more and more overgrown as the years went by. Hence,
I began using pictures from my Big Bend trips in the classroom and eventually
putting them online with explanations so that students could look at them at
West Texas, and especially
the Big Bend area, which includes Big Bend National Park, has long been of
great interest to me. Unfortunately, it is a long way from the places I've
lived over the past several decades, and I haven't been able to get out there
nearly as often as I would like. The current material is from trips my
brother, Randy, and I have been taking since May, 2006. We have been driving
and hiking through Big Bend, examining the geology on the way. We both take
pictures with our digital cameras, so quite a few of the photographs shown here
are his, although not always credited. Now that I'm retired and generally
useless otherwise, I've continued and hope to continue my trips to Big Bend and
expand this work.
The geology of West Texas is fascinating - even unique. And,
because of the dryness
of the Chihuahua desert, which is nevertheless subject to infrequent episodes
of intense erosion, the geology is exposed and readily visible. Erosion is the
real star of the show in Big Bend National Park and its environs. Millions of
years of weathering and differential removal of the rocks emplaced by igneous
and sedimentary processes, has resulted in the creation of the area's incredible
topographic features and the revelation of its unique geological history. This
virtual field trip is a sort of slide show with notes. I have made
considerable effort to be as accurate and up-to-date as possible.
The geologic history of Big Bend can be roughly divided into five
- The Ouachita-Marathon mountain-building episode, when the South
collided with the North American plate. This occurred (roughly speaking) about
300 million years ago. Mountains were raised as the collision pushed sediment
on the order of a hundred miles up onto the North American plate. The remnants
of this episode can be found in the
Marathon uplift and the Solitario in Big
Bend Ranch State Park. The "roots" of the old mountains can be found there, as
well as in southwestern Arkansas and southeastern Oklahoma. The effects of this
mountain building can also be seen in the geologic structures of the
Persimmon Gap area in the northern
part of the park.
- Rifting as South America and North America
separated. The mountains were worn flat and Big Bend became a shallow sea
in which limestones and other marine sediments were deposited. All this occurred
between about 200 million and 85 million years ago. These rocks can be seen in
the mountains on the east and north side of the park, Mariscal Mountain,
Mesa de Anguila, and even in parts of
"The Basin". The Gulf of Mexico formed,
deepened, and widened during this time due to seafloor spreading.
- The Laramide
orogeny, which affected much of western North America between 70 and 50
million years ago as the North American and
collided. This mountain-building event produced large
faults in Big Bend.
Both can be seen on Persimmon Peak at
the north entrance to the park. The folds found in
Mesa de Anguila, Mariscal Mountain, and
the Sierra del Carmen all owe their existence to this orogeny. The marine
sedimentation transitioned into continental sedimentation as the region
including the park rose in altitude.
- Between 46 and 28 million years ago, periods of
affected the park and its environs.
Igneous rocks due to
this activity are ubiquitous in the central and western parts of the park,
Chisos Mountains, the Rosillos Mountains, the
the McKinney Hills, among other
areas. The volcanism is thought to be due to the shallow
subduction of the
Farallon Plate beneath western North America.
- The park continues to be affected by the crustal extension of the
Basin and Range
province activity, which began about 25 million years ago (and which is
decreasing as little activity has been noted over the past two million years).
The cause of this
crustal extension is not entirely clear, but it may have to do with the action
of the Pacific tectonic plate as it moves northwest and grinds against the
North American plate. The Basin and Range Province is elongated roughly in that
direction, and the extension is also roughly parallel to that direction.
Crustal extension has led to numerous high angle faults in the
park, including the ones that bound the crustal block containing the Chisos
Mountains on the east and west and the one that resulted in the spectacular
cliffs of the Sierra del Carmen in Mexico.
- Finally, erosional forces remain at work to continually alter the
landscape. The Rio Grande came into being during the last 2 million years, as
it established its present course and ate down into the Cretaceous limestone,
Santa Elena, Mariscal,
Boquillias, and other canyons.
Throughout the park, as less resistant rock is worn down, the topographical
relief between it and more resistant rock grows. However, even rock highly
resistant to erosion is eventually worn down. Without further mountain-building
activity, Big Bend will one day be a flat plain once again.
Constructive comments and criticisms are welcome - even solicitated
I hope you enjoy the show.
BIG BEND NATIONAL PARK AND WESTERN TEXAS
Virtual Field Trips