Hill Country Geology, Part Six

Continuing up the trail beyond the first granite pavement outcrop, we came upon the ruins of one of the mills that give Cochran Mill Park its name. The mill building itself was made of wood, and burned as a result of arson in the 1970s, though it had been starting to fall apart before then. The mill operated into the 1950s -- I have read accounts by local residents of trips to the mill to get their wheat ground into flour. One of the mill stones still remains on the site, partly covered by mosses, grasses, and fallen leaves. The stone itself is Panola granite -- we could tell right away from the large feldspar crystals dotting its surface. Local stone, it might even have been quarried right there in the park. I appreciated seeing that stone, carved meticulously out of 350-million-year-old rock, shaped into a tool for human beings to make their bread. I thought of all the ways that granite traveled through the environment -- as exposed outcrop, as soil, as home for potter's wasps, and now here, as human technology centuries old.

We clambered up onto the second outcrop; the trail was much steeper than the access to the first. In-between hunting for xenoliths, I paused to look back down at the mill ruins, picturesque against the backdrop of Bear Creek, on this blustery late winter day.

Chilled by the winter wind, we started back down the trail toward the warmth of Cochran Mill Nature Center, where we would lunch in the main gathering room, amongst the various reptiles and amphibians that call the Center home. Nearly at the end of the trail (where it meets Cochran Mill Nature Center's entrance road), I found a spider's web in the branches of a shrub that had not yet leafed out. Water droplets in the web had turned to ice in the cold morning air, and hung glittering like the jewels in Indira's Net. The spider was nowhere to be seen -- I pictured her (or him?) relaxing in a miniature lounge chair, somewhere in south Florida, sipping Bloody Marys on the rocks. I took several photographs, trying to convince my auto-focus Kodak not to ignore the web in favor of the nondescript branches in which it was nestled. At last, my camera relented, and the result is below.

That was the last photograph I took that afternoon. The trip didn't end there, of course. The eight of us (including Julie's enthusiastic future geologist son of five years, Bruce) all enjoyed lunch together at Cochran Mill Nature Center. Then we drove out the nature center access road, passing to the right side of a series of boulders of Palmetto granite on our exit. We drove maybe an eighth of a mile south, then parked and walked in on a trail that was the original route of Cochran Mill Road, in use probably until the 1950s. Walking a short way down the flat, pine-needle-covered trail, we came to a roadcut that predates the ones along the South Fulton Parkway by perhaps a century. The rock had weathered heavily, and was mostly covered with mosses, lichens, and unlichenized algae (including an abundant free-living green alga that was, strangely enough, 1970's yellow-orange instead of green). We quickly identified the rock as highly weathered gneiss, or saprolite. Bruce enthusiastically demonstrated the auditory quality of a saprolite: a geologic hammer striking it makes a dull thud, rather than the metallic ring of "fresh" rock. We continued down the path toward the wooden pedestrian bridge over Little Bear Creek (the location of one of our school's Adopt-a-Stream sampling sites, but that is another story, to be told in November). Along our way, Julie would stop to examine rocks along the side of the trail where the cut had been made to put in the road. A few hundred feet further along from our first saprolite encounter, we stopped at a boulder of what Julie thought would turn out to be granite. Though heavily weathered, it was still possible to discern the foliation pattern in the original rock, once it was broken open. More gneiss.

Still further down the trail, approaching the creek, we came to another rock, this time only a large cobblestone in size. Breaking this open, Julie was fairly confident that she was seeing granite instead. So the Contact was probably nearby, though it would elude us this time around. We did not examine any more rocks, and I was left feeling that my knowledge of the area's geology was patchy and inconclusive at best. I knew there were granitic rocks (the Palmetto granite) intruding into schists and gneisses (the Atlanta Group), but the story was still murky, the details nearly indiscernable with the passage of time. I could consult a geologic map -- perhaps even the new one -- but I expect that only months on the ground, walking the land, picking up rocks and peering at them with a hand lens, sampling them for microscopic thin sections and even x-ray diffraction analyses -- only all of this would help reveal more of the story. Or maybe, as I have come to suspect through my years as a geologist, all of that would simply lead to new and different questions to ponder.

Hill Country Geology, Part Five

It takes a trained eye to notice granitic xenoliths. Xenoliths, as I have noted elsewhere, are literally "strange stones", bits of other rock that had gotten caught up in flowing magma and managed to maintain aspects of their original character, even if (as in the case of the photo above) they had become partially absorbed into the surrounding melt. Anyway, at first, the granite at my feet looked all alike, except where the solution pits poc-marked its surface, or where the joints ran that linked the pits in a geologic connect-the-dots. But then Julie pointed out a xenolith, and we were all fascinated at the discovery. After all, xenoliths can't be that common, can they, if it had taken us so long to find one?

The reality was that xenoliths of all sorts were all over both of the pavement outcrops we explored. Some were darker than the granite, and were probably true bits of the "country rock" that had been intruded, while others were lighter granitic phases. The novelty wore off, though I was still entranced by some of them -- particularly a large (maybe eight inches long) xenolith of finer grained granite that had weathered out a bit from the surrounding rock (see below). Julie explained to me that the xenolith, which was just downhill from a zone of finer-grained granite, was probably part of the finer-grained phase that, having cooled early in the history of the melt, somehow got ripped away from the rest and incorporated into the remaining

magma. Now, it rises from the granite pavement surface like a miniature Stone Mountain, channelling the flow of rainwater down the rock, causing a small solution pit to form just downhill. I took several photographs of it, thinking about how small a difference in erodibility can, in turn, can cause such noteworthy topographic expression. I was beginning to appreciate the subtle nuances of granite bodies, though the relationship was strictly plutonic.

Hill Country Geology, Part Four

We hit the road again, down the South Fulton to Cochran Mill, and soon thereafter onto the entrance road to Cochran Mill Nature Center. We parked the vehicles, and set off down the trail to a couple of granite pavement outcrops in the adjacent Cochran Mill County Park. This was a different experience -- walking atop the rock, rather than looking at it up above you, from a viewpoint at the base of a roadcut exposure. Here, we could see more of the granite itself, locating enormous feldspar phenocrysts and even noting a slight orientation to them, a flow foliation resulting from the very slow rate of movement (akin to silly putty) of the gradually solidifying granitic magma. It is easy to imagine walking on the very top of a pluton (body of granite), but Julie reminded us that granite weathers by exfoliation, peeling off in layers like an onion, a process caused by the removal of all the overlying material. In fact, the actual pluton would have been much less smooth, and much more irregular, with fingerlike protrusions of granite penetrating adjacent rock units.

We noticed joints -- cracks in the granite where no lateral motion has occurred. These were probably release joints, formed as the overlying rock eroded away. At a number of spots along one of the joints, solution pits had formed in the rock; some were empty, others still held water, and one even held a pine tree. Julie explained to us how the fine red soil in those more vegetated solution pits was probably not transported there from elsewhere on the rock face, but instead formed right there, through chemical and biological weathering processes. That clay originated as feldspars in the granite, now weathered into fine grains, deep enough in places for a loblolly pine to take hold. An ecological successionist would describe the solution pit development as a succession sequence, from bare rock to solution pit to pit with lichens and mosses to pit collecting soil to pit with loblolly pine. This process as I am describing it here has, no doubt, happened many times in such outcrop environments; but the story does not end there. The pines get tall, are blown over by the wind, and the soil on their roots washes away, leaving bare rock again....

I was distracted from my reveries by two round objects in the pine's branches. They turned out to be two beautiful tiny pots, still sealed shut, the living places of the larvae of potter's wasps. I was struck by how the clay had been reworked; the same feldspars, broken down into soil, were reanimated, made into a home for an insect. We are not the only organisms who build elaborate structures, using the natural environment as source of raw materials, and perhaps even inspiration.

Hill Country Geology, Part Three

We piled back into the min-vans, and headed east down the South Fulton Parkway, toward Cochran Mill Rd. The drive lasted only a couple of miles, but it was time enough to recover somewhat from the harsh wind and cold of the rather wintery late Saturday morning. By the time we parked the vehicles and crossed the road, the sky had clouded up, and a few flurries were falling, and it felt colder than ever. We paused in front of the outrcop again, to try to make sense of it all, in word or gesture (above).

Julie quickly discovered another possible Contact, and this one was much more intriguing. Below was the old standby, Palmetto Granite, with its phenocrysts (large crystals) of feldspar. Above, a series of layers, black and white and intermixed -- a melange of metamorphic material. It was beautiful, and fascinating. It was a Contact where much had happened. The surrounding rock had already been metamorphosed from gosh-knows-what parent rock (shall we take it for granite?), into a well-foliated (layered with minerals) gneiss. Then the granite intruded the gneiss, most likely during the uplift of the Appalachian Mountains, roughly 350 million years ago. The process was an example of regional metamorphism, because it involved the deformation and transformation of rock over a large area, as the result of tectonic forces. The hot magma "baked" the zone where it met the gneiss, causing contact metamorphism, a kind that is localized and usually the result of a magma coming into contact with much colder rock. It was a sort of geological epiphany, to see a metamorphic rock that had, in turn, been metamorphosed again, but in a very different way.

I felt like a college student, back on an introductory geology field trip, getting my eyes opened and seeing rock units in more profound ways than before. It was the kind of experience that is all too rare in book reading (for all the books that I polish off every year) -- a deep knowing that comes from direct encounter, from the experience of witnessing the outcrop and feeling amazement at the story it is telling. This is why geology needs to be taught as a field science, not something that can be learned through "virtual field trips" and computer simulations. What is actually going on out there is much more powerful than what one simply reads about. Maybe T.S. Eliot was right: "Human kind cannot bear very much reality." That discovery, that metamorphosed rock could also be metamorphosed through contact with granitic magma, lingered with me throughout the remainder of the trip, and stays with me now as I type these words. I still recall fondly that moment of enchantment and bliss, there in the bitter wind on that second outcrop of the day, gazing at the Contact and into its secrets.

Hill Country Geology, Part Two

It was the first outcrop of the day, and we piled out of the mini-vans and into a brutal north wind that reminded me of Upstate New York or maybe somewhere along the coast of Baffin Island in the high Arctic. The snow had stopped, at least for the moment. Julie had awoken that morning (Saturday, March 8th) to a half inch of snow blanketing the ground in Carrollton, and had sent me an email even, wondering if we should consider cancelling, on account of the weather. But no, we were all hardy folk, and why would we let freezing temperatures and gusts to thirty miles an hour or more slow us down? So we bundled up, and hit the outcrop: a fairly recent roadcut (one of few left that has not been "tragically" revegetated) along the South Fulton Parkway.

I jumped across the obligatory Swampy Area at the base of the outcrop, through the entirely expected Zone of Briars and Generally Nasty Vegetation, up to the rock face. Granite, mostly -- the good old Palmetto Granite, the stuff of which large boulders are made (such as the ones beside the entrance road into Cochran Mill Nature Center, and in the entrance road, too -- the driveway splits into two one-way routes at a couple of large, well-exfoliated (rounded) chunks of granite). But wait: what was metamorphic rock -- gneiss, probably -- doing here at the granite outcrop? Was it a "Contact"?

I need to pause here and explain for non-geologist readers the sacred nature of the Geologic Contact. In simplest terms, it is the point where two lithological units (rocks of different age or origin) come together. But at those boundaries, amazing things can happen -- stories can be read by a geologist's trained eye, whether of extinctions (the most famous Contact of all, the K-T boundary, marks where the dinosaurs died off) or perhaps an episode of contact metamorphism (a molten granite intruding and baking the surrounding rock) or perhaps even a basaltic flow erupting from a fissure and flowing over the landscape. Bodies of granite are interesting, certainly -- one can describe their textures: aphanitic (fine crystals), phaneritic (large crystals) or porphyritic (a mixture of the two, with some very large crystals). One can further classify them in terms of their mineralogy: is that black speck hornblende, or biotite? One can look for xenoliths, bits of country rock encorporated into the slowly flowing melt (with a viscosity like that of silly putty). But basically, granite is fairly homogeneous stuff, and it gets boring. One starts looking instead for intriguing things growing on the granite, such as lichens and mosses, and even, if desperate, one tries to discern the words in a bit of grafitti covering part of the rock surface. The same is true, for the most part, for metamorphic rocks -- though they occasionally deform into fascinating folds, and some gneisses and schists contain impressive garnets. But the true quarry, the field geologist's quest, is the point of meeting, the Contact.

In the case of this first outcrop of the day, the drama was short-lived. Yes, there was a location on the rock face where granite gave way, rather unspectacularly, to gneiss. But the gneiss was not true country rock, but only an enormous xenolith, perhaps the size of a VW microbus, surrounded by the granite. Matching the spot Julie first found, a distance over on the rock face, a participant discovered another point of meeting, evidence that the rock was just a xenolith, and that the Contact lay elsewhere.

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.