Friday, September 26, 2008

Examples of Geoarchaeology

I'm occasionally asked, "Exactly what is Geoarchaeology? Here are two examples:

1) A team of archaeologists from Tulane University was at an ancient Mayan site in Belize, sitting in the middle of a swamp about a mile inland from the coast. We know the maya farmed the swamps, as other precolumbian cultures did, down to and including the Aztecs, but they didn't build their city in the middle of one.
So, a geologist was flown in, but only circled the area between the Caribbean coast to the ruins one time before landing at a makeshift airstrip not far from the site. When he arrived at the camp, all eyes and ears were pegged on him for help in answering the question: "Why did they build in the swamp?"
"They didn't," he replied. "They built this city on the coast next to the river. Progradation is ongoing here. The river carries suspended sediment from the highlands and drops it when it enters the sea, forming a delta. This entire area is a delta. The coast is constantly moving out; that's what progradation is. When they built this city, the Maya had no idea it would one day be sitting in the middle of a swamp. Delta deposits can be as much as ninety percent water. Over time, they settle, squeezing the water out to the surface. The swamp follows the coast."
"My god," I thought. "How ignorant I am! How can I ever be a really good archaeologist if I'm not a geologist as well?"
That's one of two reasons I decided to get a second Bachelor's degree in geology. I'll tell you the other one later.

Example number 2:

When I was at Cal State, Stanislaus getting my Masters in hydrogeology, I also took a couple of additional courses related to my archaeology (Latin American Studies) degree from Tulane; one was a study of calcareous algae that I took under the tuteledge of an oceanography professor at the University of California at Santa Cruz; I wanted to better understand the strand line above Lake Titicaca in Bolivia. It was formed at the water's edge when the lake was at a much higher elevation anciently - when the city of Tiahuanacu flourished. The only problem is, that strand line dips hundreds of feet from its northern edge to its former southern edge some 400 miles to the south. It was in fact once an inland sea. Oyster beds are still visible on some of the ruins at former lake elevation.
Lakes don't tilt, but I wanted to understand the calcareous algal deposits as part of solving the archaeological implications of the tilted stand line.
The other course was an Archaeological Field School offered by Cal State. We were studying the Indians of northern California that summer. Our professor pointed out the depresssions in the boulders along the river where Indian women had ground acorns into meal by rolling a stone over the shelled acorns
(acorns were very abundant and substituted for corn in that general area). Over time, bowl-shaped depressions were worn into the rock, giving a metate appearance much like the metates used in Central and South America as well.
One day, we were walking across a dry riverbed and he pointed to a bowl-shaped depression in one of the boulders near the center and at what was the bottom of the river when it was flowing.
"There's another metate depression worn into that boulder." he said.
Some of the students, myself included, thought it odd that women would be grinding acorns in the middle of the river, even if it was intermittently dry, when the entire river was lined with more accessible (and much more convenient) boulders, so I took the time to examine it more closely.
"This is not a metate depression," I said. "It was formed by hydrogeologic processes."
Our professor had gained great respect for me over the summer because I was able to identify which rocks had been heated in the fire and then dropped into the acorn mush - the Indian method of boiling and cooking it. I could distinguish them in the lab by the exfoliation apparent on the surfaces. Still, somewhat embarrassed, he asked me how I could tell it wasn't formed by women grinding acorns.
"Sometimes," I explained, "a small stone becomes lodged within a slight depression in a boulder or slab of rock and due to the current, it swirls and spins continuously, eventually smoothing and deepening the depression. That's what caused the depression in this boulder at the base of the streambed. In fact," I added, stooping and picking up the rock responsible, "this is our culprit."

I hope the foregoing illustrations will help bloggers understand how uniquely helpful geology is to archaeology, much as astronomy is in archaeoastronomy.