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Earth Sense: Stone Mountain one of many inselbergs in US
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Easy to reach from Gainesville via Tucker, Stone Mountain Park makes for a fascinating weekend trip.

The exposed part of the mountain stands about 750 feet tall above the north parking lot. Its whaleback shape gives it a unique appearance, but Stone Mountain isn’t the only feature of its kind in the southern U.S.

It’s one of many “inselbergs,” a term derived from German. Literally translated, it means a mountain standing out like an island in the sea. To see others, driving 130 miles will take you to Looking Glass Rock in North Carolina. In the same state, 300 miles from Gainesville, is another Stone Mountain near Winston-Salem. Virginia, too, has several inselbergs.

Inselbergs aren’t volcanoes. Magma formed deep in the earth’s crust 300 million years ago. The molten rock pushed its way upward to a depth of about 8 miles. There it became stuck and cooled for several thousand years, becoming a so-called pluton. During the time that has elapsed since then, the surface landscape was worn away until just the top of the pluton became exposed.

The Southern inselbergs consist of very hard granite (quartz monzonite, in Georgia’s case). With no soil cover to saturate it with water and acids, it doesn’t wear away chemically like rocks do while they
are underground.

But other forces act on it. Taking the 1.5-mile walkway to the top from the northwest parking lot, one can view massive slabs of granite that have popped away from the main rock mass. With the overlying weight gone that used to compress it for millions of years, Stone Mountain is expanding upward very slightly. This makes shells of granite become detached, similar to the way an onion peels. On the south side, large pieces have slid down and remained standing upright.

The grooves in the rock are also intriguing. They show the effect that rainwater has over time, forming and deepening its pathways. Where water accumulates in small depressions, bowl-shaped cavities develop.

Although nature purists condemn the rock carving on the almost vertical north flank, it is nevertheless a technical accomplishment. Overall, the sculpture is larger than the one on Mount Rushmore in South Dakota. The Stone Mountain sculpture was started in 1923 but couldn’t be completed until 1972 when oxy-jet torch-welding technology made it possible to melt the relief into the rock mass.

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