Texas Parks & Wildlife Magazine   


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Inks Lake Rocks!

Find some of the oldest geology in Texas at this Hill Country state park.

On a warm day at Inks Lake State Park, you might see dozens — if not hundreds — of visitors hanging out on the shore or floating in the water of Devil’s Waterhole. Multiple generations of Texans have made the short hike to this classic swimming hole.  

How many of those visitors have known the ancient story of change and resilience told by these rocks, some of the oldest in Texas?

The park’s geologic story starts around 1.2 billion (yes, with a B!) years ago. Inks Lake State Park sits on the northeastern edge of the Llano Uplift, a 1,290-square-mile island of Precambrian igneous and metamorphic rock surrounded by younger Paleozoic and Cretaceous sedimentary rock. These rocks were formed by the violent collision of tectonic plates, leading to volcanic eruptions, sedimentary deposits along an ancient coast, metamorphism and granite intrusion that came all the way from the Earth’s mantle (perhaps hundreds of miles below the Earth’s surface).

After the formation of the Llano Uplift, a second tectonic event called the Grenville Orogeny formed a giant mountain chain comparable to the Himalayas, stretching from southern Mexico through the central U.S. into Canada. Over millions of years, most of these mountains eroded, and these older rocks are found in the present-day Appalachian Mountains and the Texas Hill Country.

Evidence of these ancient events can be seen at Devil’s Waterhole, if you know where to look. Perhaps the easiest feature to see is right along the shoreline — the difference between the igneous granite and metamorphic gneiss (pronounced “nice”). Because both rocks can be pink, the main way to distinguish between the two is to look at the arrangement of the minerals — granite has a random pattern, whereas the minerals in gneiss are “foliated,” or arranged into layers.

Most of the rock at Inks is gneiss (dating back more than 1.25 billion years), older than Enchanted Rock of the Llano Uplift. In fact, Devil’s Waterhole contains the oldest rock in Texas that is accessible on public land, according to Sharon Mosher, emeritus dean at the Jackson School of Geoscience at the University of Texas at Austin.

If you want to explore further, a hike up Valley Spring Creek to a seasonally running waterfall will reveal something called “partial melting,” which occurs when rock is heated to very high temperatures and some minerals melt and collect into bands or pockets. This structure is called a “migmatite.” When this semi-melted pocket is compressed and shifted, it can form a fold, an interesting loop or a wave shape.

“Inks Lake is actually the best place in the Llano Uplift, if not the entire world, to see these migmatites,” Mosher says.

The next time you visit Devil’s Waterhole, you’ll know the cove is not just a place to swim but a veritable buffet of geologic treasures. Its rocks provide unique testimony to the awesome forces that helped shape our continent.

Don’t forget the old joke: “Our rocks are gneiss; please don’t take them for granite.”

  Sonja Sommerfeld | TPWD

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