Rare plants thrive on monadnocks and outcrops

By Charles Seabrook – For the AJC

Georgia’s rock outcrops and monadnocks — such as Stone Mountain, Arabia Mountain and Panola Mountain in east Atlanta — appear to be harsh, barren landscapes, but they are superb natural treasures, where some of the world’s rarest and most unusual plants thrive.

That was apparent last weekend when we took a walk on Arabia Mountain, the centerpiece of the Davidson-Arabia Nature Preserve, a 2,200-acre DeKalb County park near Lithonia that also includes streams, two lakes, pine and oak forests and several rock outcrops.

Georgia’s rock outcrops and monadnocks are closely related. An outcrop (or flat rock area) is where the soil has eroded and exposed the underlying bedrock, often granite, at the Earth’s surface. An extreme outcrop is a monadnock, where hundreds of feet of soil and rock surrounding the bedrock have worn away, leaving a dome-like mountain of very hard rock rising conspicuously over the landscape.

The bedrock itself was formed some 290 million to 375 million years ago, when hot magma intruded into existing rock and cooled and solidified deep beneath the surface.

Geologist Bill Witherspoon, co-author of the book “Roadside Geology of Georgia,” explained this last weekend as we followed him on the Bradley Peak Trail to the top of Arabia Mountain.

He noted that not all of Atlanta’s monadnocks are made of granite, as is commonly thought. Arabia, for instance, is composed mostly of migmatite, a mixture of two major rock types. The mountain is also 50 million to 100 million years older than Stone Mountain.

On the way down, our leader was biologist Leslie Edwards, co-author of the book “The Natural Communities of Georgia.” Despite the mountain’s desert-like environment, she explained, it harbors some of the world’s rarest and most endangered plants, such as the black-spored quillwort.

Most of the plants grow in “solution pools,” shallow depressions in the bare rock that are lined with thin soil layers and hold rainwater temporarily. During our walk, one plant, the elf-orpine, was a stunning sight — growing in lush, red patches in the solution pools, as it does every spring.

In the sky: The moon is first-quarter Saturday, said David Dundee, Tellus Science Museum astronomer. Venus is in the west at dusk and sets about two and a half hours later. Mars sets in the west at dusk. Jupiter, in the west at dusk, sets around midnight and will appear near the moon Sunday night. Saturn rises in the east around 10 p.m.