Friday, December 12, 2008

The Geology of Georgia

Part One of an Occasional Series:
The Appalachian Plateau

I blog about a lot of things here, but please don't you forget (because I do occasionally) that I'm actually a geologist. Master's Degree, Boston University, 1980. My geology's gotten fairly rusty, but occasionally, very rarely, I get a chance to practice actual geology, instead of just arguing environmental policy with EPA, industries, and engineers at some grease spot behind an old, industrial complex.

Today, I got the opportunity to get out in the field for a client who needed a geologic reconnaissance of a piece of property in northwest Georgia. I used the opportunity to also provide myself with a refresher tour of the geology of the Appalachian Plateau.

In the map above, the Appalachian Plateau is that little yellowish area way up there in the upper left-hand corner of the state. In Georgia, the Appalachian Plateau includes the Lookout Mountain District. Lookout is a nearly flat-topped mountain, capped by sandstones of Pennsylvanian age. It is separated from a spur, Pigeon Mountain, visible in the map above as that piece of the plateau that looks like it's trying to break away. Between Lookout and Pigeon Mountains is McLemore's Cove, which is underlain by limestone of Ordovician age. The escarpment on the southeastern side of Lookout and Pigeon Mountains abruptly drops 800 to 1,000 feet to the Chickamauga Valley and is breached by numerous small streams which reach the valley floors through deep notches in the cliffs.

Sound interesting? If so, then come on, I'll take you on a geologic tour. First, you'll need a scorecard, a chart of the geologic formations that make up the more interesting mountains and ridges (we'll leave those boring valley-floor limestones alone for now).

Driving north out of Atlanta on I-75, we cross the Cartersville fault somewhere around Lake Allatoona, and the bedrock abruptly changes from the hard crystalline rocks of the Georgia Piedmont to those softer, Paleozoic strata of the Valley and Ridge Province. We leave I-75 at Adairsville and head toward Armuchee, and shortly after Armuchee, we come to Taylor Ridge, the first major hill of our trip.

Taylor Ridge is supported by dense cherts of Devonian and Mississippian age. Chert is an extremely fine-grained, insoluble residue of silica that generally forms from limestone deposits. Alert readers will have already correctly concluded, based on the chart above, that these Devonian and Mississippian beds are in fact the Armuchee and Fort Payne Cherts. There are good exposures of these cherts on U.S. Highway 27 past the town of Gore, just as you start to climb the ridge. Here's a picture of those exposures:

The Armuchee Chert was named by noted geologist C.W. Hayes in 1902 for the town of Armuchee, Georgia (which we just passed through). The Armuchee is a black or medium- to dark-grey, thinly-bedded, fossiliferous chert. The Fort Payne Chert was named by E.A. Smith in 1890 for Fort Payne, Alabama, and occurs in layers ranging from several inches to several feet thick and tends to weather into nodules or blocks. Getting out of the car, and mindful of the traffic whizzing past, you can get a closer look and see some of the structure of the chert nodules.
During the Devonian, when the Armuchee Chert was formed, the closing of the proto-Atlantic (Iapetus) Ocean, which had begun in the Late Ordovician, was almost complete, resulting in the continental collision that produced the folding of the Appalachian Mountains along the east coast, or leading edge, of the North American continent. As the two plates collided, deposits along the leading edge of North America were uplifted and the debris was shed onto the continent to the west. Closure of the Iapetus Ocean was completed during the Pennsylvanian Period, completing the assembly of the supercontinent Pangea.
Continuing on U.S. 27 to the top of Taylor Ridge, you can get a good view of northwest Georgia and the Chickamauga Valley terrain that lies ahead.
The first large town on 27 after Taylor Ridge is Summerville. Here's a picturesque storefront in Summerville:

As we travel toward the northwest and start to ascend Lookout Mountain, we move "up-section," higher up the chart near the top of this post. Soon, we encounter the Floyd Shale and other Mississippian formations that overlie the Fort Payne Chert west of Taylor Ridge. In contrast to the preceding Devonian and following Pennsylvanian Periods, the Mississippian appears to have been relatively stable tectonically, although the closing of Iapetus and assembly of Pangea were more-or-less constant processes.

The Floyd is the thin-bedded black shale at the bottom of the outcrop. The formation was named for Floyd County, Georgia by C.W. Hayes in 1891. East of Taylor Ridge, all Mississippian rock above the Fort Payne Chert is identified as the Floyd Shale. In the Black Warrior Basin of western Alabama, the Floyd is a good producer of natural gas.

Finally, as we get near the top of Lookout-Pigeon Mountain we also find the top of the Mississippian Series - the Pennington Formation, a hard siliceous sandstone named for Pennington Gap, Virginia. The Pennington is predominantly a gray shale with some beds of sandstone and limestone, and which forms the cliffs on the sides of the mountain.

The top of Lookout Mountain affords great views of the agricultural McLemore's Cove:

As previously noted, the top of Lookout Mountain is capped with Pennsylvanian sandstones, and driving along the plateau, we come across low, flat outcrops of the Lower Pennsylvanian Gizzard Formation. The Gizzard is about 150 feet thick and consists of fine-grained sandstone and gray shale.

We're getting near my client's site now, so I have to go to work for a little while. Due to client confidentiality, I can't show you too much of the site, but I can show you the dirty old underwear someone left nailed to the gate at the front of the site.

The Mississippian and Pennsylvanian Systems together comprise the Carboniferous Period, named for the carbon typically present in the form of coal. There's coal in the Gizzard and other Pennsylvanian deposits, although not in the underlying Mississippian. So as we drive across the Pennsylvanian plateau on the top of Lookout Mountain, we see evidence of former coal mining, in things like road names:

And old mine shafts:

(Actually, in not sure that's a mine shaft, but it is an odd tunnel that I found coming out of the bottom of Lookout Mountain on the floor of McLemore's Cove. It might be a ventilation shaft for an old mine, or just a very elaborate culvert.)
And that's the end of your tour. We drive back to Atlanta by the same route we came, whizzing past the now familiar outcrops at which we had stopped on the way up. Someday, sometime, this series on the geology of Georgia might continue.


Mumon said...

I wondered what your training was!

Well, science.

Hats off to you, sir!

Edward Fletcher said...

Thank you. Please share more of your geological adventures!