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Rudi Kiefer: Inselbergs are unique mountains formed by volcanic rock in crust
Rudi Kiefer
Heading south on Interstate 85, after passing Exit 107 to Duluth, an interesting landscape feature comes into view. The round shape of Stone Mountain rises 825 feet above its surroundings.

Farther north, Ga. 184 connects Homer with Toccoa. Just before reaching U.S. 17 and Toccoa, another rock formation stands out 800 feet from the foothills. This is Currahee Mountain, widely visible due to the assembly of tall antennas at its top.

A drive from Cleveland to Helen on Ga. 75 opens views of Yonah Mountain. Its top stands an impressive 1,500 feet above that highway.
Geologically, these features are called inselbergs. That’s a term derived from German, meaning that the mountain stands out alone, like an island. The term “monadnock” is sometimes applied to the same feature.

Most inselbergs in the U.S. South consist of granite. They formed from magma, or molten rock, deep inside the earth’s crust. The process is the same as the beginning of a volcanic eruption. Hot magma rises and melts its way through the earth’s solid crust, which is only a few miles thick. But then it gets stuck. If it were under more pressure, it would produce a volcano. But stuck in the middle of solid rock, it cools and becomes a pluton.

A few hundred million years later, the area above has been wearing down enough to expose the pluton. Because it’s so solid, it doesn’t rot away as quickly as its surroundings, and a dome-shaped or sugar-loaf shaped mountain emerges. Currahee isn’t quite the same geologic type, as it’s made of the same rock as its general neighborhood. But Stone Mountain and Yonah fit that pattern.

Another interesting inselberg is Looking Glass Rock near Brevard, North Carolina. It rises 1,600 feet above the valley floor. A three-hour drive from Gainesville takes you to U.S. 276 and the trailhead near Looking Glass Falls. Two hours on a long, winding, but fairly comfortable trail lead to the top of the mountain. Caution is advised at the trail’s end, because there’s no safety fence like at Stone Mountain. Inselbergs have rounded edges that are deceptively steep.

Currahee Mountain requires a comfortable hike of the same length, using a public gravel road. One can drive up, but four-wheel drive is advisable and there’s no public parking lot. At the top, magnificent views of the Piedmont and the Georgia Mountains await.
 

Rudi Kiefer, Ph.D., is a professor at Brenau University, teaching physical and health sciences on Brenau’s Georgia campuses and in China. His column appears Sundays and at gainesvilletimes.com.

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