July 19, 2004

I bet you realize by now that geology plays a tremendous role in all of our studies here. I like to tell my students (and fellow researchers), much of the exciting evidence of human evolution comes from the ground - so we better know our rocks and sediments.

The geology field school visited Locality G today, and spent several hours exploring the differences between the New Formation in that area and the Olorgesailie Formation. As you may recall, Locality G is the low-lying area of the Olorgesailie region to the southwest, and it contains Middle Stone Age sites currently under excavation. These MSA sites are found in the New Formation. There's a lot of work to be done, starting next week, to figure out the age of the New Formation. The Olorgesailie Formation is the older series of sediments spread over a much wider area, and that's where we find the Acheulean handaxes.

The Geology Field School at Locality G

While the students were in Locality G, John spoke with them about the kinds of questions geologists can help archeologists answer.

One of the first questions, which we already discussed here, concerns the age of a site. Knowing the age of a site can help an archeologist place newly discovered stone tools and animal bones in time - which finds are older and what ones are younger. Backed up by measurements in the geologic record, we can estimate pretty precisely when certain changes in stone technology occurred. By knowing the age of animal fossils, we can answer whether the animal species at Olorgesailie were stable over a long time or whether they changed frequently.

Here at Olorgesailie, dating is usually a matter of collecting the right kind of volcanic material and analyzing it by single-crystal argon-argon dating. In other regions of the world where there are no volcanic rocks, other methods have to be used to date archeological sites more than about 45,000 years old (which is about the limit of carbon-14 dating).

A second question a geologist can answer concerns the type of sediments and the depositional environment in which artifacts are found. It's really important to know exactly what the local environment was when the artifacts were deposited and buried in the ground. By looking at the sediments, a geologist can tell if the artifacts were left on the edge of a lake, in a stream channel, or sitting on top of a soil. This helps an archeologist understand whether artifacts are arranged exactly as the hominins left them, or whether they've been moved or disturbed or winnowed (meaning that the smallest pieces have been washed away). We have to know this about the artifacts before we can use the arrangement of artifacts to make any hypothesis about hominin behavior. The sediments can also help estimate how much time a particular site took to accumulate, which helps the archeologist decide how to interpret the artifacts.

Outside of the sites, geologists can also help find new places to excavate, since they often spend time covering the landscape, following the various strata. In fact, this is how the Sandwich Site was found - Kay was following the volcanic tuffs around the landscape when she spotted a few artifacts eroding out between two tuffs. She told John, Alison, and me about it, and we decided to dig there!