Get Started – Build Your Own Trip
Typical field trips attended by professional geoscientists involve a set itinerary with a time schedule that may include dozens of locations over several days. The purpose of these virtual field trips is to allow anyone to visit interesting localities of their choosing on their own timetable. Typical field trip guidebooks that describe important rock outcrops are written for experienced geologists so, naturally, they are filled with technical jargon and are meant to stimulate discussion about advanced concepts. The following locality descriptions are instead written for the general public so they focus on what can be observed without special knowledge. Technical jargon is introduced only to discuss what can be observed. The locality guides can be downloaded so you can easily take them with you. This website began in August of 2022 and is evolving. The first set of virtual field trips will focus on areas of Texas but other states will follow eventually.
Your approach to visiting these localities can be done in several ways: take the dog for a walk at one of them for example. If, on the other hand, you are going to make some of the classic hikes in Big Bend National Park, those field trips will help you see new things. Some of the trips will be more about observing rocks as part of a road trip. If you want to commit more time, pick a geographically-grouped set and see more than one rock formation (a formation is an officially-named body of rock that has a specific age). If you want to see more of a specific formation, visit multiple sites within that formation. Some of the places below are grouped by formation but other localities are grouped geographically. All sites are open to the public and specific details about access are included. You might want to download the free phone app ROCKD which allows you to capture and share your observations and photos with other users. It also provides you with local information about the geology wherever you are in the world. You might also want to check out the Useful Links section on my Resources page. Happy observing!
Austin Chalk Virtual Field Trip
The Austin Chalk is an important part of the scenery in central Texas from San Antonio up to Dallas. It is a white or gray limestone deposited during five million years of the Cretaceous Period beginning around 90 million years ago. The Gracywoods Neighborhood Park locale allows one to see the base of the Austin Chalk and some very interesting features in the lower part after making a short hike. The top of the Austin Chalk can be seen by parking at Dottie Jordan Park and taking a short walk. It is very easy to get a quick look at the Austin at Ricky Guerrero Park and to learn about how animals create what geologists call “trace fossils”. At McKinney Falls State Park, the Austin Chalk was greatly influenced by the volcano that formed just to the south at Pilot Knob. Visit the falls to see how different the Austin Chalk is there compared to the other locales. The documents below provide information about how to find these rocks and interesting things to observe.
For more detailed information about the Austin Chalk in the vicinity of Austin, Texas, the following publications are useful:
- “Down To Earth at McKinney Falls State Park, Texas” by Jay A. Raney (Bureau of Economic Geology, 1997)
- “Austin Chalk – Stratigraphic, Geophysical, Hydrogeological and Hydrocarbon Exploration/Production Characteristics” by Jason Lundquist, Mustafa Saribudak, Robert Mace, Charlotte Sullivan and Patricia Wood Dickerson (Austin Geological Society Guidebook 36, 2015)
- “Cretaceous Volcanism in the Austin Area, Texas” by Keith Young, S. Christopher Caran and Thomas E. Ewing (Austin Geological Society Guidebook 4, 1982)
- “Guidebook to the Geology of Travis County” by Diana Grunig, M.A. Jordan, Donald F. Parker, Bill Williams and Keith Young (University of Texas Student Geology Society, 1977)
Gracywoods Neighborhood Park Field Trip Stop
Dottie Jordan Neighborhood Park Field Trip Stop
Ricky Guerrero Park Field Trip Stop
McKinney Falls State Park Field Trip Stop
Big Bend National Park Virtual Field Trip
Summary of Big Bend Geology
Big Bend National Park has spectacular scenery because the region has been a geological “crossroads” for over a billion years and the desert climate provides excellent exposure of the rocks. Only cacti and small shrubs cover the rocks except in the highest elevations in the Chisos Mountains and along the Rio Grande where trees are more abundant. Plate tectonics and time have produced a landscape that rises 7000 feet from the Rio Grande in Boquillas Canyon to Emory Peak. The map below from Geological, geochemical, and geophysical studies by the U.S. Geological Survey in Big Bend National Park, Texas has the right level of detail for our needs.
The oldest rocks exposed in the national park are small Paleozoic outcrops in the northeast. These rocks have been through a lot of plate tectonic drama so they are highly broken. The Persimmon Gap Field Trip Stop below describes their history. The vast swaths of green on the map are the outcrops of Cretaceous limestones that formed when a shallow sea covered Big Bend. You can see some of these up close at the Santa Elena Canyon Field Trip Stop below. Around 70 million years ago, plate tectonic forces began to push the limestones up into mountains with the biggest range in the eastern part of the park where today you see the Deadhorse Mountains and Sierra Del Carmen. Mesa De Anguila on the western edge of the park, is another of these mountain ranges that includes Santa Elena Canyon. Mariscal Mountain, in the southernmost part of the park, is another limestone mountain range that extends up through a thick pile of younger rocks.
As these limestone mountains began to rise in the park, other ranges were forming throughout western North America. Major river systems formed that delivered sand, gravel and mud to the Big Bend region until the dry land replaced the limestone sea. The Hot Springs Field Trip Stop shows how the ancient landscape began to change from ocean to dry land. A topographically-low area known as the Tornillo Basin formed between the rising Deadhorse Mountains and the Mesa De Anguila (yellow area on the map) and it filled with sandstone, conglomerate and shale from about 70 to 50 million years ago. The famous Big Bend dinosaurs lived on that land. The Fossil Discovery Exhibit is a good place to see these rocks up close.
Around 46 million years ago, igneous rocks began to form in Big Bend when volcanos erupted lava flows and huge clouds of ash. In Big Bend, lava flows produced basalt and rhyolite and the ash became tuff. Some good examples can be seen at the Burro Mesa Pouroff Field Trip Stop. The volcanos were fed by huge underground reservoirs of molten rock which eventually cooled to form intrusions that now form some of the most picturesque peaks in the park (pink on the map above). Igneous activity ended around 33 million years ago within Big Bend National Park. Younger igneous rocks can be found elsewhere in western Texas and throughout New Mexico.
As the last igneous rocks were forming, the final geologic chapter began to be written when numerous faults (black lines on the map) began to break the landscape into the terrain we see to day. They are part of a much larger system of faults that extends from Big Bend up through New Mexico into Colorado: the Rio Grande Rift. These faults control the location of the Rio Grande from the Colorado Mountains until it flows out of Big Bend. You can see one of the biggest faults on the hike described in the Santa Elena Canyon Field Trip Stop.
For more detailed information, check out the National Park Service’s Geology of Big Bend and these great books:
- Big Bend Vistas: Journeys through Big Bend National Park by William McLeod
- The Big Bend of the Rio Grande;: A guide to the rocks, landscape, geologic history, and settlers of the area of Big Bend National Park, by Ross A. Maxwell (University of Texas. Bureau of Economic Geology. Guidebook 7)
- Geologic Wonders of West Texas by Donald P. McGookey
Things To Avoid in Big Bend
When you take the hikes described in the field trip stops, you will see some great rocks and you will probably pick some up to get a better look. Looking is good but taking is bad and actually unlawful. Leave the rocks where you found them so everyone can enjoy them. All of the hikes described below are easy but you must be aware of the hazards associated with the desert environment. First, plan ahead for exposure to sun, cacti, and critters. It is easy to get scratched, poked, sunburned or dehydrated. Its also easy to avoid doing that with appropriate clothing, shoes, sunscreen and water. I have spent a lot of time in Big Bend and have almost never seen any snakes even though there are a lot there. I never put my hands inro place where I can’t see and I don’t jump into brushy areas unannounced. Check out the park website for safety tips and use your common sense.