Hill Country Geology, Part One

This past Thursday, March 6th, Dr. Julie Bartley, professor of Geology at the University of West Georgia, spoke at Rico Community Center on the geologic history of the Georgia, particularly our section of the Piedmont. The presentation, which drew a dozen attendees, marked the third month of the first year of our program (yes, already plans are afoot for 2009!), and the last month that our Thursday evening presentations will be held at in Rico. Next month, we will be "on the road" at StudioSwan, a lovely art studio nestled in Serenbe Community (http://www.serenbecommunity.org/). After that, evening talks will be held in the elegant, wood-floored room known as The Studio, also in Serenbe. The Studio offers forty comfortable chairs, bright white walls for PowerPoint projection, and the convenience of being closer to the homes of many of our participants.

Even while I am very excited about upcoming move to The Studio (definitely a step up in refinement and comfort), I will still miss Rico Community Center. There is something about the old elementary school, with its history (recent and not-so-recent) evident in piles of things scattered about (artwork, books, etc.), its donated furnishings, and its quirks, that make it a humble and inviting place to be. For this particular presentation, the site was excellent; Julie placed abundant rock specimens on each table, for participants to classify. It was like being back in school again, a feeling made more tangible by literally being back in school again.

In her carefully prepared talk (broken into sections by time participants spent examining the rock samples), Julie explained to us how the Piedmont is the most complex part of Georgia to interpret geologically. Here, we are in the middle of the Piedmont, far removed from the generally level, younger sediments of the Coastal Plain. Instead, underlying the Hill Country are metamorphic rocks, some deformed several times during various plate tectonic events, intruded by igneous rocks, some of which have themselves been deformed, perhaps even shortly after being emplaced. The result is, well, a mess. And it is a mess that very few people seem to want to tackle. Even Julie prefers to study, in her own fieldwork, Precambrian carbonate reef structures (in such places as Morocco and Siberia). Julie explained that much remains uncertain about Piedmont geology; a new digital geologic map of the Atlanta metro region was recently released, but doubts have already been raised about its accuracy.

The geological story of the region is a story of many tectonic events, including a first collision of land masses, the subsequent opening and then closing of the Iapetus (father of Atlantis) Ocean, convergence of continents to form Pangaea, and the opening of the Atlantic Ocean. Pieces of other land masses, and of island arcs that were once far at sea, now comprise the Eastern seaboard of North America. Our local landscape contains, in the foliation of its gneisses, the tracings of global forces, the pressures that produced mountains. And the story is not a shallow one, either. Exposed now at the surface, the result of millions of years of weathering, are granites and gneisses that were once miles underground. In some places around Atlanta, like at Stone Mountain, the granite that intruded the country rock is a lot more resistant to weathering than the adjacent rock. Given time, the result is a mountain of granite, left behind while the adjacent rock was ground away and carried to sea. In the Hill Country, that granite often outcrops as stretches of rock pavement, more resistant to weathering than the adjacent metamorphic units, but not with a difference in weathering great enough to make our own Stone Mountain here.

We all had a lot of fun, though, just appreciating the complexity of the story, and not worrying too much about the details. Our challenges were, first, to identify which rocks belonged to which of the three major physiographic regions of Georgia: Coastal Plain, Piedmont & Valley and Ridge, and the Blue Ridge. Later in the evening, we worked on a number of black and white rocks, some foliated (schists and gneisses), others not (granites). Wordplay possibilities abounded throughout the evening; at several points, I was sorely tempted to make a remark about not taking everything for granite -- a pun that is rather cliche, I suppose, but is nonetheless tempting to share with novice geologists.