Two weeks ago we took a break from Indiana’s nature and traded it for Texas’ nature. The warm temperatures were a nice change, and we had an opportunity to visit four of Texas’ state parks: Blanco, Pedernales, Lost Maples, and Enchanted Rock.
Enchanted Rock is a batholith, a dome of solidified magma. It’s quite impressive when you see it from a distance. It gets more impressive as you start to climb it and the view from the top is spectacular. The dome was formed approximately one billion years ago deep underground and as the land rose and erosion removed the soil above it through the years since, eventually slightly less than one square mile of the rock dome has been exposed above ground. It towers 425 feet above the surrounding land and covers an area 62 miles square when the underground part is taken into consideration. That’s one big piece of granite!
As the soil erodes and exposes the top of the dome, there is a reduction of pressure on the dome, which causes it to expand. This expansion causes the granite to break into layers, similar to an onion, and the layers peel off slowly through the centuries in a process called exfoliation. The cracks in the layers eventually trap moisture and windblown soil, which then allows plants to grow. Although the dome at first glance looks like bare rock, as you get closer you can see the plant growth along the cracks. In places you see gray shield lichens, formed through a symbiotic relationship between fungi and algae. These lichens grow only 0.004 of an inch a year, so a four-inch patch might be over 1,000 years old! No wonder they caution you to not walk on the lichens. Other plants you might find are mosses and ferns.
An interesting feature of the exfoliation process is the “mushroom rocks.” Because water collects at the base of the rock and on the sides with less sun exposure, the rock does not erode evenly, leaving an overhang at the top. Although they look unstable, the guidebook said that a picture taken of them 50 years ago compared with today shows no change. Erosion of granite is a slow process.
At the top you find “weathering pits.” These are places where little bits of water have eroded the rock, allowing more water to gather, which then causes more erosion. Some of the pits have very shallow vernal pools – places where the water remains long enough for plants to get a foothold. First spores of algae are blown in by the winds and shortly thereafter zooplankton (microscopic animals) appear. Some of the pits contain fairy shrimp, one-inch long crustaceans that live only from December to May because their pools may dry up in the hot summer winds and sun. We didn’t see any of them, but if they only hatch in December, it’s likely they might still be too small to see easily with the naked eye. Mosses, liverworts, and rock quillworts can be found around the vernal pools along with other wildflowers and little bluestem grass. There is even one stunted live oak tree clinging stubbornly to a crack in the rock at the summit.
Enchanted Rock is called the “geological heart of Texas” and if you’re winded when you reach the top, that’s because you have climbed the equivalent of a 30- to 40-story building. The closest town is Fredericksburg, settled by German immigrants. At the foot of the exposed dome is Sandy Creek, a 42-mile-long stream whose waters eventually reach the Gulf of Mexico. By Enchanted Rock, Sandy Creek is only an intermittent stream, meaning it doesn’t have water in it year-round, but a few miles downstream it does become a constant stream. Little nodules of stone that have eroded from the granite are called gruss and Sandy Creek carries them downstream, bumping and rolling along the stream bed until they become fine sand by the time they reach the Gulf Coast.
Visiting and climbing Enchanted Rock is an opportunity to expand your knowledge of geology and the erosion process, and also a chance to appreciate the beautiful central Texas Hill Country.