In DeKalb County, around East Mountain Street and 2nd Street, back and front yards seem to grow rocks. Some residents build little walls with them. Others make neat rock gardens. Some pick them up and try to hide them to preserve the suburban lawn look. But the more rocks one removes, the more seem to emerge from the ground. Pieces of light grey stone are everywhere.
The town is Stone Mountain. One mile away, a giant rock by the same name rises almost 800 feet above the surrounding area. Tourists often talk about this unique feature.
It’s picturesque but not unique, actually. Near Cleveland, between Ga.75 and Ga.384, Mt. Yonah tops out at 3,164 feet. That’s 1,700 feet higher than Lake Laceola, 2 miles away.
From the observation platform on top of Brasstown Bald, Yonah and other cone-shaped mountains stand out from the surrounding area like islands in a sea. This impression has earned them the German term “inselberg” in geology. On a clear day, 33 miles to the southeast, you can see another of these “island mountains” near Toccoa. Currahee Mountain rises 850 feet above nearby Ga.184. Just like Stone Mountain and Yonah, Currahee shows curiously rounded rock surfaces. Unlike most mountains in North Georgia, dominant shapes seen on inselbergs are arcs instead of sharp, jagged cliffs. The rock that forms them is granite, different in color and composition than the bedrock forming the gentle rolling landscapes nearby.
The granite developed deep underground. Some 300 million years ago, hot bubbles of magma from the earth’s molten interior were poking their way upward. Before they reached the surface, they cooled too much to proceed any further. Over time, they solidified and remained embedded in other rock as solid, dome-shaped chunks called plutons. Millions of years passed, with rain, rivers, landslides and other events wearing away at the bedrock overhead. Because granite consists mainly of quartz, it’s very dense and hard. Erosion of the landscape takes place more quickly in the surrounding area, making each inselberg stand out.
A close look at the exposed rock faces shows another process at work. With the weight of mile-thick overlaying cover removed, the inselbergs expand as if they were taking a deep breath. Thick shells of granite become detached. The south side of Stone Mountain has the most spectacular examples. They demonstrate that even the oldest landscapes in Georgia are still under active development.
Rudi Kiefer, Ph.D., is a professor emeritus of physical science at Brenau University. His column appears Sundays and at gainesvilletimes.com.