Hiking A Batholith

Batholith. I love that word. It is a portmanteau of “depth” (bathos) and “rock” (lithos), literally meaning “deep rock,” but sounds like you’re trying to say “basilisk” after having burned your tongue on hot coffee.

On Sunday, Racy of the Racy Mind, VirgoTex (whom you all know by now as she who puts the Town in Back Of Town) and I hiked the Enchanted Rock granite batholith in the Llano Uplift area of central Texas. It took us approximately 2.5 hours to hike up and down a deceptively easy 425 feet; as Virgo said, “You think you’re getting close to the top, and then there’s more top after that.” We could have accomplished it faster, I suppose, but the point of a hike is hiking and not a race to the top and back.

As I get older, I am more a consummate scientist (“Consummate Vs!”) and less a geologist, and increasingly piece together the immediate and more nuanced relationships between the earth and the things that live in and on it. When I was a geology undergraduate, my approach was “Cool rock! FIND ALL THE SAMPLES. KNOW ALL THE THINGS.” This attitude has now morphed into “Cool rock! What does it tell me about the larger geologic history of this place? Also, notice how it has eroded and broken down to its constituent minerals and how some minerals form one type of soil on which X species of plants grow, and house these specific animals and birds, while others weather or are transported elsewhere to form different deposits and soils.”

Mineral type, grain size, chemical content, distance from source, chemical breakdown, clay content, soil formation, that the cacti and peaches love a certain type of acidic soil and the peach pie served at the cute little cafe in Llano. It’s a contained system, this earth, and I don’t bring up this observation from a standpoint of hippy-kumbaya-interconnectedness or simply being older, but one of practicality: Being open to different inputs and practicing the art of making the right connections among them makes you a better-equipped human, much less a scientist. Even as a scientist, detective work yields better results if you step back and focus your eyes beyond your area of specialty. This is why I still want to be a crime scene forensic geologist when I grow up [insert mental image of me whipping off my sunglasses all Caruso-styleeee].

Back to Enchanted Rock. If it’s one of twelve batholiths and surrounding metamorphics that constitute the Llano Uplift, why is it up here at the earth’s surface? In fact, why is it higher than surrounding rocks for miles and miles? Rob Reed’s Llano Uplift site has very detailed descriptions of the (debate surrounding the) mechanics and timing of these preCambrian granites and schists exposed in the middle of a much younger Texas geology. My Cliff’s Notes version goes like this:

1) Uplift by Metamorphism, Deformation and Intrusion: Mesoproterozoic (~1.5 billion years ago) sedimentary rocks metamorphosed into schists and intruded by granites, including Enchanted Rock and surrounding granite batholiths, during the Grenville orogeny around 1.1 billion years ago. This resulted in the formation of very thick continental crust in this area. So, the “Uplift” has always been relatively high with respect to its surroundings. (Can you say “positive gravity anomalies,” kids?)

Vein (with Virgo's finger for scale)

2) Continued Uplift by Tectonics and Erosion: Deposition of sands and limestones occurred through much of the Paleozoic (540-300 million years ago). Erosion of the Uplift’s rocks probably continued in this time. The late-Paleozoic Ouachita orogeny seems to have contributed to another re-Uplift and exposure of the preCambrian rocks. The deposition-erosion cycle chugged along through the Miocene (15 million years ago) movement along the Balcones Fault Zone, which pushed the Uplift’s rocks higher than the rocks to the southeast of the fault zone.

3) Even More Uplift and Weathering/Erosion: All through this time, from around 1 billion years ago to now, the rocks have experienced varying degrees of uplift and weathering/erosion. Uplift or not, two phenomena you can count on beyond death and taxes are weathering and erosion. (Weathering is the in-place breakdown of a rock by physical and chemical means and erosion is the movement of weathered material from its source.)

The view from Enchanted Rock

So, from when they were first exposed to the present time, the granites and metamorphic rocks of the Uplift have moved upward in response to the forces of buoyancy, tectonics and burial. And the moment you move rock relatively up, *BAM!* wind and water begin to eat into it. When we were on the top of Enchanted Rock on Sunday, it was very windy and heavy rains had come through the night before – we saw chemical weathering, gravel formation and sediment transport before our very eyes. Granites display an additional weathering feature in that they exfoliate (yes, think skin exfoliation) on the removal of overburden or pressure, which is why you see sheets/sheaths of granite that look like they’re ready to slide off the surface any minute now.

Exfoliation

Wait a moment here. Take the geological action that we saw in 2.5 hours and begin to envision a billion years of it. Think of how much sediment this group of rocks has generated and how far it must have traveled and been buried by now to possibly be re-intruded by igneous rocks and be metamorphosed elsewhere some day. Tell me it doesn’t make you trip backwards in awe of space and time and this ball of rock that makes all of this possible. I cannot think of doing anything else in life besides geology. At a crime scene. With cool sunglasses to take off.

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If you would like to visit Enchanted Rock and the surrounding Texas Hill Country, here is a handy-dandy set of geology links to peruse before you head out.

1. The aforementioned Llano Uplift site (which, hilariously, describes everything we’ve covered above as “an island of rock excitement adrift in a sea of Cretaceous limestones whose only redeeming value is the oil in them.” Not true. Hill Country = pretty + aquifer.)
2. An indispensable Fredericksburg to Enchanted Rock Road log which you Must Have in the car. The log narrates the geology along Highway 965 between the town of Fredericksburg and Enchanted Rock Park and beyond to the town of Llano. If for nothing else, follow along so you know when to look for the caliche pit and the “gorgeous, panoramic view of Enchanted Rock and the surrounding countryside laid out from skyline to skyline.”
3. My Geology of the Llano Uplift and Texas Hill Country pre-read guide, which consists mainly of maps, photos and overviews to get you quickly oriented to central and southeastern Texas geology. Each page has links to the source(s) for even further reading. Nice thing about this Google doc is that it is a very specific and breathing document that can be crowd-edited if I open up permissions.
4. The Enchanted Rock State Natural Area topographic and trail map. Take water, wear boots, look out for rattlesnakes and *sadface* don’t beat on the rocks to take samples home.
5. My Enchanted Rock Flickr photo set containing the photos you see here and more. All of my photographs are available under a CreativeCommons BY-NC-SA 3.0 license, so feel free to use them for teaching and/or open research purposes.

Little Rock (L) and Enchanted Rock