Enchanted Rock Geology: For the Casual Observer or Amateur Enthusiast

Twice a year a couple of my friends and I, all of us geoscientists, teach geology for Texas Parks and Wildlife Becoming an Outdoors Woman program.  Occasionally we auction a hosted trip to Enchanted Rock and wine country near Fredericksburg with proceeds benefiting the Friends of Texas BOW non-profit organization.  For the ladies unable to snag a spot on these trips, for the ladies unable to attend due to scheduling conflicts, and for anyone visiting Enchanted Rock in the future, this is for you.

 

 

I’ve condensed a geologic overview of what we share with the participants on the hosted trips here.  All eligible participants must have taken one of our geology workshops at BOW in the past (condensed version here: Geology Basics: For the Casual Observer or Amateur Enthusiast).  This is not comprehensive by any means, and definitely not too sciencey.  I want participants/enthusiasts to at least appreciate geologic processes, rocks and minerals, and/or fossils when they visit Enchanted Rock and the surrounding area (in this case, the stretch of 290 between Fredericksburg and Hye/Johnson City).  Geologic beauty is everywhere!

 

 

On our hosted trips to Enchanted Rock, we camp and hike in Enchanted Rock Natural Area, and we make a trip to nearby Fredericksburg on our way out on Sunday (or as a backup plan if it rains), so the map above is a regional geologic map of the area including Enchanted Rock and wine country along 290 between Fredericksburg and Hye/Johnson City.  This being a simple geologic overview, the area surrounding Enchanted Rock are older rocks (Pre-Cambrian >541 million years), and the area in wine country are younger rocks (Cretaceous 145-65 million years).

 

 

I shared several versions of the geologic time scale in Geology Basics: For the Casual Observer or Amateur Enthusiast, but my favorite is the pictorial version above (and it’s all you’ll really need as an enthusiast).  So when we discuss gelogic ages (and the map colors on the geologic maps), use the one above as reference.  See how the green areas around wine country on the geologic maps match the Cretaceous period in the timescale above?  The pink areas around Enchanted Rock also match the Proterozoic (Pre-Cambrian) age in the timescale above.  Of course there are shades of green and shades of pink in the geologic maps below, but that’s just subdivisions of the ages.

 

 

Zooming in on the area around Enchanted Rock increases the details, so I’ve included geologic unit descriptions in the table above.  Some labeling on the map may not be entirely legible, but the colors/symbols will correspond to the proper descriptions.  Enchanted Rock is a small (geologically speaking) pink granite outcrop of a larger complex called the Town Mountain Granite, an instrusive igneous rock of Pre-Cambrian in age.  In the southeast of the park, Packsaddle Schist, a medium-grade gray(ish) metamorphic rock, can be found along the creek.

 

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Source: Dr. Ron Blakey

 

Sometimes it’s difficult to visualize the environment/topography at the time these rocks were formed, so I often share Dr. Ron Blakey’s paleogeographic maps.  I think they’re beautiful representations.  I’ve included the ages in the geologic maps of Enchanted Rock and wine country above, and they cycle through the Pre-Cambrian, Cambrian, Ordovician, Cretaceous, and Pleistocene.

 

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Source: Bryce Hixson, Roadside Geology of Texas (link to book below)

 

I also like to share excerpts from Roadside Geology of Texas, as Bryce Hixon does a much better job explaining the geology of Enchanted Rock (and all major roads of Texas, for that matter) than I ever could.  I highly encourage any geology enthusiast living in Texas to purchase this book to keep in their car (I’ve included a link to the book on Amazon below).  The book is organized by region and subdivided by roads, so you could easily follow the geology as you’re driving anywhere in Texas.

 

 

Enchanted Rock was created by molten rock crystallizing deep under the surface about a billion years ago, and in the meantime the material above has been eroded away to expose the granite.  The dome itself has classic exfoliation dome features as the pressure being removed causes the rock to crack and peel off (slowly) like an onion.  Town Mountain Granite in particular is rich in silicates (quartz, feldspar, and dark minerals) and gets its distinct pink color from the abundance of potassium feldspar.

 

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As I mentioned above, Enchanted Rock is a small outcrop of a larger complex called the Town Mountain Granite.  In satellite imagery, you can make out the outline of the Town Mountain Granite.  Typically, different rock/mineral compositions create different soil, so what you’ll see in satellite imagery (and often driving along the highways) is a change in vegetation where geologic boundaries occur.  Can you see it above?

 

 

As they say in Texas, if you don’t like the weather, wait a day.  In our case, Enchanted Rock is about a 5 hour drive from Houston…so if weather is absolutely unforgiving, our backup plan is visiting nearby wineries around Fredericksburg.  And why not enjoy the geology there too?  The geology around Grape Creek Vineyards is Cretaceous age (Early Comanchean ~100 million years) limestone, sandstone, conglomerate, and shale.  In and around the creek are Quaternary alluvium/river terrace deposits of sand, silt, clay, mud, and gravel.

 

 

Driving further east on 290 towards Hye/Johnson City, the geology around Lewis Wines is Cretaceous age (Aptian/Albian ~110 million years) limestone, clay, sandstone, and conglomerate.  In and around the creek are Quaternary alluvium/river terrace deposits of sand, silt, clay, mud, and gravel.

 

 

Now that the geology of the area is covered (as in depth as I’m going to get, anyway), I’m sharing last year’s trip to Enchanted Rock, Fredericksburg, Grape Creek Vineyards, and Lewis Wines in the next post.  Stay tuned!

 

 

INSTRUCTORS

Sheila Echols-Smesny holds a Bachelor of Science in Geology from Sam Houston State University, an Executive MBA from Texas Woman’s University, and works at an oil & gas company in Houston.  Red Shoes. Red Wine. is Sheila’s travel, lifestyle, & hobby blog.

Katy Mainwaring holds a Bachelor of Science in Geology from Texas Tech University and works at an oil & gas company in Houston.  Katy teaches geology with Sheila for Texas Parks and Wildlife Becoming an Outdoors Woman program.

Jenni Jackson holds a Bachelor of Science in Geology from Central Michigan University, a Master of Science in Geology from California State University, Fresno, and works at an oil & gas company in Houston.  Jenni teaches geology with Sheila and Katy for Texas Parks and Wildlife Becoming an Outdoors Woman program.

 

More Geologic Adventures on the Blog:

Geology Basics: For the Casual Observer or Amateur Enthusiast

British Isles Cruise on Royal Princess (Part 2: England, Ireland, Northern Ireland)

Alaskan Cruise: Juneau Part 1 (Mendenhall Glacier)

Alaskan Cruise: Scenic Cruising Tracy Arm Fjord

TPWD Becoming an Outdoors Woman Program

Becoming an Outdoors Woman as an Instructor

Becoming an Outdoors Woman Workshop: Spring 2016

Visiting Big Bend National Park: Something for Everyone

A Return to Big Bend National Park: Lodge Edition

Skyline Geology: A Self-Guided Walking Tour of Downtown Houston’s Buildings (City Hall Loop)

 

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