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Enchanted Rocks, Prehistoric Plants and Exfoliation

One of my first outings of 2024 was to visit Enchanted Rock, a famous geological feature of the Texas Hill Country. It is a massive dome of pink granite dominating the landscape all around.   The climb to the top is satisfyingly steep, I really felt like I’d earned the panoramic view and the chance to peer at the very special environment up there.  At the top, there are vernal pools which fill with rainwater and although they dry out in the burning Texas sun, nevertheless they provide habitat for a variety of unique or at least rare organisms. I was very keen to find out more about the romantically named fairy shrimp. These tiny creatures somehow persist in rainwater pools on top of a granite outcrop under the Texas sun. Their eggs survive when the pools dry out and are ready to hatch when the pools fill.

Rock Quillwort, only found in Texas, grows in these pools too. Although it resembles a grass and is therefore easily overlooked, it is actually more closely related to spike mosses and clubmosses. This fascinating group of plants, like ferns, reproduce through spores. While its early ancestors had a worldwide distribution due to emerging on Pangea (the landmass which broke apart to form the seven continents we know today) Rock Quillwort is only found in these seasonal vernal pools on granite and gneiss outcrops of the Llano Uplift rock formation in Central Texas… which is to say it is rare. There are only 10 populations, in 4 Texas counties! Quillworts in the Jurassic period were much bigger than their modern descendants, in fact they were tree-like and were a big proportion of the plant matter which eventually formed coal.

The granite itself is impressive, and comfortingly rough underfoot - I have a tendency to end up on my backside whenever I walk downhill and that only happened once! The name granite actually comes from the Latin granatus for grain (just like the garnet and the pomegranate). It is named for the grainy surface where you can easily see the minerals.

If you’re looking for granite facts, here are a few…

  • It is an igneous rock which is to say it formed from magma with a high silica content cooling slowly and solidifying underground.

  • It is one of the most commonly seen rocks on earth and is mostly formed of quartz and feldspar.

  • All granites contain some amount of Uranium and therefore emit radon gas.

  • Aberdeen in Scotland is known as the Granite City as many of its buildings are built from local granite which is extremely durable as a construction material.

  • Sticking with Scotland, most curling stones in the world are made from the Blue Hone Granite and Common Green Granite from a tiny uninhabited volcanic plug in the Firth of Clyde.

  • The highest granite mountain is Kanchenjunga in the Himalayas (8586m / 28,169 feet). Only Everest (limestone) and K2 (gneiss) are higher.

Almost as fun as climbing to the top was the hike around the base of Enchanted Rock, a chance to view the enormous formation and also appreciate (or be concerned by!) how much of the granite has made its way down from the top. The rock is something known as  an exfoliation dome. When the overlying soil eroded away the rock expanded slightly as it no longer had the great weight on top … and it developed curved cracks, forming layers like an onion.  Over time, the rock breaks into smaller pieces which slide down. and the process continues.

It’s a wonderful image to think of the massive weight being lifted. But in that relief and expansion cracks can appear which allow shedding of external layers… Is that a good or bad thing?

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