Roadcut
The Edwards Plateau. Thick layers of Edwards limestone are exposed in a roadcut on Interstate 10 in West Texas. —texashillcountry.com
Tom Jones
The Edwards Plateau
The Edwards Plateau is a geographic region at the crossroads of Central, South, and West Texas. It is bounded by the Balcones Fault to the south and east, the Llano Uplift and the Llano Estacado to the north, and the Pecos River and Chihuahuan Desert to the west. Renowned for its beautiful water carved canyons and steep hillsides forested with oak and juniper, the Hill Country—as the Edwards Plateau region is known by conservation scientists and most Texans—encompasses 36,680 square miles in Central Texas. —Wikipedia
A roadcut is a cut through a hill or mountain for the purposes of building a road through it rather than over it. Above is a West Texas roadcut on Interstate 10 near Iraan. The white rock layers are Edwards Limestone, and the Pecos River Valley is at the top of the photo. —Richard Nilsen, Driving Across Texas
I recently took a road trip, driving to California to visit family. My route went west from Austin to El Paso with a 400+ mile drive along Interstate 10. It was a perfect opportunity to see the many roadcuts along the Interstate. Roadcuts are a great way to see Texas geology as you drive west from the central Hill Country to the expansive Edwards Plateau. Fortunately, rockhounds can hunt for rocks and fossils legally on public easements along Texas roads, and roadcuts can prove useful for collecting. When I talk about collecting, I am not referring to collecting large rocks from the side of the road to use as major landscaping pieces. What I’m talking about is strictly hobbyist rockhounds collecting mineral specimens.
Along I-10 east of Junction, Texas, the outcrop has thick rock layers from the Upper Edwards limestone. —Texashillcountry.com
Before you head out for some rock collecting, here are some things you need to know:
Never collect specimens in state or national parks.
Stay safe. Make sure that you are not in any danger of being hit by a car, and be careful of loose stones, steep inclines, and falling rocks.
Respect wildlife. The rocks in a roadcut make good homes for spiders, snakes, scorpions, wasps, ants, ticks, and chiggers.
Do not dig on roadcuts. The Texas Highway Department works hard to control erosion and to make the rocks safe for passing cars.
This roadcut on I-10 near Balmorhea exposes distinctive red-brown flaggy (thin-bedded) limestone of the Eagle Ford (or lower Boquillas) formation as well as underlying tan-to-gray limestone of the Buda formation—all mid-Cretaceous (~90 to 100 million years old). —San Solomon Spring and Toyahvale, Bureau of Economic Geology
One of the most famous roadcuts in Texas is located nine miles north of Llano, on Texas Highway 16 as it slices through the world’s only known outcropping of llanite, a rare form of rhyolite flecked with blue quartz crystals. Collectors flock to this roadcut to chip away at the tough llanite boulders. I remember lugging a 6-lb. hammer to the outcrop and was barely able to break off a small piece.
Llanite outcrop along Texas Highway 16 near Llano
—https://www.digforcrystals.com
Cut section of llanite with blue quartz crystals —Wikipedia
Llanite Roadcut/Outcrop
The only public access to an outcrop of llanite is the roadcut on Texas Highway 16 at the crest of a ridge about 9.0 miles (14.4 km) north of the intersection of Texas Highways 16 and 29 in Llano, Texas.
Blue quartz is uncommon and is usually colored by inclusions of unusual minerals, such as crocidolite, tourmaline, or dumortierite. The purplish-blue quartz here, from north of Llano, Texas, is colored by inclusions of ilmenite (iron-titanium oxide). This rock is called llanite for its occurrence in the Llano Uplift of central Texas, and although similar rocks are found in other parts of the world, the variety name llanite really only applies to this location. On a sunny day, the blue quartz in the rocks has an opalescent sheen that sometimes seems to “wink” at you from the outcrop.
Roadcuts on Texas Highway 306 near Canyon Lake
A small roadcut of Edwards Limestone on Texas Highway 306 near Canyon Lake shows hard limestone with a karst/collapse feature. The limestone beds on the right have distinctive cross-bedding which was created by water currents in a shallow Cretaceous sea.
Another small roadcut of the Upper Glen Rose formation just below the Edwards Limestone near Canyon Lake exposes the soft and easily eroded layers of clay often referred to as micrite or lime mud.
Available on Amazon
Exploring for minerals and fossils can be difficult since they are typically beneath the surface. In many areas, Texas roadcuts are a great opportunity to see rocks that are normally hidden. They can reveal different layers of rock, faults, fossils, minerals, and many other geologic features that are often difficult to see. One of my earliest tools to learn Texas geology was Darwin Spearing’s Roadside Geology of Texas. This guidebook was originally published in 1979, and the geology is still relevant today. A copy is always in my glovebox.
“The geologic panorama of Texas is as wide as the state is big, sweeping from volcanic mesas and thrusting mountains in the west to red canyons of the Panhandle, along tropical sand barriers of the Gulf Coast, and across central limestone plateaus onto hard granitic terrain in the center of the state. Texas is blessed with rocks of all ages, as well as an incredible array of natural geologic resources. Darwin Spearing describes the rocks as you come to them, describing what they are, when they formed, what they mean, and how they fit into the big picture of the geology of Texas.” —Review on Amazon.com