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Does this look like archaeology to you?

April 28, 2012

So there I was, hardhatted and hiking-booted, perched on a rock in the middle of the pit of an abandoned coal mine and contemplating this beauty of a syncline.

Cross-section of sandstone syncline

The coal miners couldn’t have excavated a better place for studying structural geology if they had tried. They had mined out the stratigraphic horizon of the coal and no deeper, revealing the 3D surface of the fold that before had lain hidden in the earth. And there at the end of the valley was the cross-section of that syncline, showing everything they had cut through. My assignment was to draw it. The problem was that I couldn’t figure out exactly how all the pieces I was looking at fit together. The major syncline was clear enough, but I was sitting on the height of an anticlinal ridge – the Whaleback – in the middle of this valley. How could it have happened that the rocks bent upward at one level in the sequence and downward at greater depth?

My first thought was that the Whaleback was part of the same anticline in the left side of that picture above – do you see how the bedding of the darker rocks bends over sharply? But that would have required a pretty dramatic fold or fault across the axis of the valley, and I would have seen evidence of that in the steeply curving walls. Then, looking more closely, I saw a slightly less dramatic anticline straight ahead of me, cloaked behind scree and trees. Could that have once been continuous with the Whaleback? Thinking about it, I realized that wouldn’t make any sense either – it didn’t leave any space for the coal that had once lain on top of the Whaleback, and the plunging angle of the Whaleback’s ridge didn’t line up right anyway.

It was the thought of the coal that led me on the right track. It’s one thing to know that a hole in the ground was dug as a mine – it’s quite another to see things that are no longer there, to imagine the air filled with the weight of coal and rock bent by mountain-building far away. And once I could imagine that, the soft coal sandwiched between two more rigid rock sequences, I could see how the pieces all fit together.

Under compression, stronger rocks bend at a longer wavelength than weaker rocks. There are a lot of factors that go into rock strength, but a major one is thickness – thick formations are stronger than thinner ones. So if you compress a strong layer and a weak layer, the weak layer will have smaller, more frequent folds than the strong one. How can this happen if the rocks are all connected to each other? That’s where the coal comes in. That layer of coal, mined out years ago, decoupled the strong sandstone above from the weaker shale below, letting them slide past each other as the folds formed at different different frequencies. At first the layers folded in parallel – you can tell because steep shale walls of the valley lie parallel to the limbs of the sandstone syncline on the end. Later in deformation, the weaker shale just kept on folding, at a smaller wavelength this time, with the coal above it faulting and folding and flowing to fill in the gaps.

Steep shale cliff, pockmarked with concretions

The south valley wall, seen from the Whaleback

There are all sorts of cool things this valley. There’s a fossil tree trunk, originally cylindrical and vertical, from which you can calculate the sense of shear. There are conjugate faults on the cliff face – paired faults from which you can calculate the direction of the original principal stresses which caused deformation. The principal stresses from the faulting on the shale valley walls are nearly perpendicular to the principal stresses required for folding, so the shale must have fractured early in deformation. Slickenlines on fault surfaces, caused by sliding and not yet eroded away, give the direction of movement across the fault. The cliff surface is pockmarked by concretions the size of my head – some still in place, some fallen leaving only holes.

It’s more than enough to keep a geology class occupied on a Sunday. Several other field trips cycled through; they’d stop by, be lectured at for a bit, stare up at the cliffs while their instructors waved and pointed. Not us. We drew things. We measured things. We tossed ideas back and forth and compared drawings and calculations. I learned how to use a Brunton compass, an instrument of great beauty and precision; I remain convinced that you have to be a Zen master to get a bearing from one in less than three minutes. Our professor wouldn’t give us any answers, but gradually, we coaxed the story out of the Earth.

The valley was also populated by a healthy population of dirtbikers from the area, because apparently dirtbiking in abandoned coal mines is just the cool thing to do on summery Sunday mornings. As I sat sketching, I heard several of their voices drifting down from the height of the valley.

“… those people?”

“Maybe they’re from the college? Is there a van?”

“A red one.”

“Yeah, I bet they’re from the college.”

There were some words that didn’t carry distinctly.

“Naw, don’t bug the archaeologists. They’re learning.”

I could only shake my head at that last comment. I get mistaken for an archaeologist with surprising frequency. Surprising to me, at least; having flirted with archaeology before settling on geology, I find the subjects quite distinct in my mind. This is apparently not true of any number of people with whom I’ve made small talk. They ask me where I go to school, what I study. “I’m interested in paleontology,” I tell them; and “Oh,” they say, “what did you think about this archaeology thing I read about? Have you dug up any cool ruins?” They get even more confused when I explain that I’m actually in the Geology department. “Shouldn’t you be in anthropology?” they ask.

Where does this confusion come from? Do people actually not know the difference between archaeology and geology, or can they just not distinguish the two subjects in the field? Or is the difference, which seems so obvious to me, actually slight enough that it doesn’t matter to the world at large?

Geologists work with rocks; archaeologists work in dirt. Geologists use hammers; archaeologists use trowels. Paleontologists study ancient creatures and their environments; archaeologists study ancient humans and their cultures. Geologists, by and large, depend on nature and industry to expose their study sites; archaeologists get to dig their own holes.

Except that sometimes paleontologists work in unconsolidated sediments – dirt – and pick around bones with trowels. Sometimes archaeologists dig can only dig through hardpan with pickaxes. Paleontologists study isotopes in bones and teeth just like archaeologists do, and archaeologists can be interested in how the environment affects human cultures and movement. Sometimes paleontologists apply their methods to the shells of the last few thousand years – shells that were important to the surrounding human cultures.

Really, the two subjects have enough in common that I sometimes consider archaeology to be only the intersection of paleontology and anthropology. (My archaeologist friends might not necessarily agree with this opinion.) Homo sapiens, after all, is just one creature among many on this planet. Should we hold ourselves separate just because we have invented language and culture?

It’s true that archaeologists sometimes have the benefit of written records, but in the end we’re in this task together: sitting under sky, contemplating the ground, and waiting for the Earth to speak.

3 Comments leave one →
  1. syfr permalink
    May 5, 2012 10:36 pm

    Indiana Jones is an archaeologist. There is no comparable geologist.

  2. May 6, 2012 12:48 am

    Yes, I agree that the lack of a charismatic face for geology is part of the problem. I think it’s harder to find geological motivations for antagonists. All the geology movies are bad …

  3. thegeologyguy permalink
    May 19, 2012 6:37 am

    Loved this post, especially as I’ve just done my structural geology exam! Definitely one of my favourite areas of geology!

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