At the exit for Clairmont Road off northeast-bound I-85 in DeKalb County, a huge, looming mass of exposed rock — or road cut — is clearly visible to motorists at the end of the exit ramp.
I have driven by this geologic standout countless times because Clairmont is a main route to my home in Decatur. I have often wondered about the road cut: What type of rock is it? How old is it? What minerals are in it?
Well, wonder no more. I have received a copy of a superb new book, “Roadside Geology of Georgia,” which provides fascinating glimpses and details of Georgia’s road cuts and other geological diversity, from the ever-changing barrier islands on the coast to the sandstone ridges in the remote northwest corner of the state.
The authors are geologists Pamela J.W. Gore, a professor at Georgia Perimeter College, and Bill Witherspoon, who teaches at Fernbank Science Center. In their book, they call the road cut along Clairmont “an excellent exposure of biotite gneiss.” Gneiss is a type of metamorphic rock with a striped appearance due to layers of light and dark minerals. In this case, biotite, sometimes called black mica, is the dark mineral.
In general, all rocks belong to one of three basic rock families — igneous, sedimentary and metamorphic.
Igneous rocks originate from the cooling and solidification of molten rock called magma. They are formed either underground, like granite, or above ground, like basalt. Sedimentary rocks form by the hardening of material (sediment) deposited by erosion or biological processes, and include sandstone, shale and limestone.Metamorphic rocks, such as gneiss, schist and quartzite, are igneous or sedimentary rocks that have been transformed into other rock types by immense heat, pressure and other forces over millions of years.
During a half-day field trip last weekend to explore the geology of DeKalb County, Gore and Witherspoon told me that gneiss and schist dominate the Atlanta area. Some of the area’s most prominent geologic features, however, are composed of different rock types. Arabia Mountain in DeKalb, for instance, is mostly a partially melted gneiss, or migmatite, rock. Neighboring Stone Mountain is solid granite.
For a full lineup of upcoming walks and talks on Georgia’s geology led by the authors, visit: georgiarocks.us/book/TalkTourListing.com.
IN THE SKY: The moon will be first quarter on Monday, said David Dundee, Tellus Science Museum astronomer. Venus is low in the west just after dark and sets two hours later. Jupiter and Mars are low in the east just before dawn. Saturn is high in the west at dusk and sets around midnight. It will appear near the moon on Tuesday night.
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