Geoarchaeology: Bringing Together the Sciences of Geology and Archaeology

by Anne Boyd, National Forest Geologist

One hot sunny afternoon in 1986, I was walking along t Indiana side of the Ohio River when suddenly my eye caught the glimmer of a shiny piece of riches of water. I reached into the water to pick it up and was startled to find that it was an arrowhead, a relic from the Woodland Indians that inhabited the area thousands of years ago. After I calmed down a bit I began to look around and I asked myself, “How could this arrowhead have gotten into the Ohio River, and where did it come form?” I looked up and down the narrow beach but all I could see was a steep riverbank, and layers of gravel seams paralleling the river’s edge. Certainly no Indians lived on this narrow strip of beach, so the arrowhead must have washed in from above.

I climbed the steep eroding bank and on my way up I saw another piece of flint sticking out of the muddy sediments. It was then that I discovered a layer of soil which was distinct from the rest of the soils on the bank. This was where the arrowhead had come from, and this was where more cultural materials certainly were to be found. I looked back down onto the beach and once again saw the gravel seams lining the shore. With an excited leap, I raced down to the gravel seams and inspected them closely. Sure enough, here were bits of flint and discarded artifacts mixed in with sand and river gravels. The bigger artifacts were found with the larger gravels, while thin flakes were mixed in with the sands and smaller sediments. Obviously the powerful Ohio River had done a good job of sorting the artifacts after they were eroded out of the river bank.

I did not think too much about this at the time, but reflecting back to this experience, L can see that this is probably the first time I used my knowledge of the geological sciences to help understand the archaeology and site formation processes of this strip of land along the Ohio River. Using the principles of geology to help reconstruct past human behavior is the essence of geoarchaeology. Though the naming of the discipline is fairly new, archaeologists have long used geology to help learn more about prehistory. Before radiocarbon dating techniques were developed, archaeologists relied upon stratigraphy or the layering of soils to determine relative ages of cultural deposits. Borrowing radiocarbon dating from geologists swept the science of archaeology into a new realm; now absolute ages could be determined and the long prehistory of humans in the Americans and proto-humans from Africa and elsewhere became known.

But dating of cultural materials (from carbon-containing materials such as wood) is not the only way that geology and archaeology interact to bring about a better understanding of prehistory. We like to think that landscapes are fairly stable in recent human times (as opposed to geologic time), but this is simply not so. Rivers change their paths, floods bury valley floors, volcanoes erupt and spew ash and lava over the landscape, earthquake faults displace the terrain, landslides reduce mountains to hills, and even snow or rain can erode sediments and redeposit them someplace else. A geoarchaeologist looks at the sediments surrounding the cultural deposit and tries to reconstruct where they came from and tries to determine what the prehistoric landscape was like. Finding an artifact without knowing where it came from or finding an archaeological site without knowing what it was like when occupied is akin to telling a joke without delivering the punch line. Out of context, the artifact is nearly worthless.

Geoarchaeologists are also concerned with rock types used to manufacture tools and the locations of these source areas. By determining the rock type, it is often possible to find out where the Indians went to procure the raw materials for making tools. By tracing these tools across the landscape, it is possible to reconstruct past migrations of prehistoric groups and observe trade patterns between different cultures. Volcanic materials are especially suited to this task, as these erupted materials contain unique combinations of elements such as Rubidium and Strontium which “fingerprint” the eruption. For example, obsidian from the Mono Craters contains a different proportion of these elements than does obsidian from the Napa Valley. So if we were to find an arrowhead made of obsidian in Pollock Pines, we could run some tests and find out where it came from. If it came from the west, we can assume that there was a trade network or cultural ties between the Sierra Indians and the Napa area Indians. The El Dorado National Forest is a good place to use geologic principles to better understand its prehistory. Volcanic materials were commonly used as tools, and geomorphic processes have been very active in the last several thousand years. The terrain varies widely from the low foothills and entrenched river canyons to the steep rugged slopes of Desolation or Mokelumne Wilderness. Understanding the geomorphic processes that have acted in these areas over time can help the archaeologist reconstruct site formation processes and assist in better understanding what past vegetative communities were like and what types of resources were available to these people.