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2004 Field Season: Day 35

July 27, 2004

One of Al's colleagues at the Berkeley Geochronology Center, Warren Sharp, joined us to help with the dating. Warren does a slightly different kind of dating from Al, but I'll describe that later. He, Al, Kay, John, and Amanda spent today walking up the Ol Keju Nyiro, the name of the river that runs through the Olorgesailie basin. The Ol Keju Nyiro is dry most of the year, including the present dry season. Even though Olorgesailie has been explored for about 60 years, there's always more to learn, especially about the geology. Al, Kay and Warren were looking for places to take new samples to get dates from the youngest strata in the Olorgesailie region. Meanwhile, John and Amanda were looking for new areas that may be interesting to excavate.

Al Deino, Kay Behrensmeyer and Warren Sharp discuss geology

The youngest series of sediments in the Olorgesailie basin is known as the Oltepesi Formation. It's even younger than the New Formation. It has the same kind of sediment deposition process as the New Formation, with rivers that cut large channels that later filled up with sediment. There were also some layers that seemed useful for dating. Warren calls our time together a 'geochronological jubilee.'

John and Amanda had mixed success - they didn't find as many potential sites as they hoped, but they did come across one rather interesting find that raises some questions. As you can see in the photograph, the artifact is the tip of a rather small biface (a stone artifact, like a handaxe, that has been worked on both sides). It seems like it could be a handaxe, but it was found in an area between two tuffs that had previously been dated, the bottom one to 220,000 years and the top one to 49,000 years old. Remember, handaxes are the hallmark of the Acheulean, an early human stone tool technology that was replaced by the Middle Stone Age in Africa by at least 250,000 years ago. There were no sediments dating to the earlier Acheulean age in the area, so where did this peculiar-looking tool come from?

An unusual bifacial stone tool.

The tool has fresh edges, so it doesn't seem like it eroded naturally from the older sediments and deposited again in the younger sediments. One idea is that in recent times a person found this piece in older sediment somewhere else, carried it for a while, and dropped it in the place where John found it. In other words, possibly it was picked up and removed from its proper context - so it's not much help to our scientific study. Another idea is that a person who lived between 220,000 and 49,000 years ago found this tool eroding out of the older sediments, used it for a while, and then dropped it in the younger deposits of his or her time; finally the artifact eroded out of those sediments where John found it. Or another idea is that this particular artifact is part of a previously unknown stone tool technology that occurred in this age range and involved making very fine but somewhat large stone points.

There's only one way to figure out which of these hypotheses is correct, and that is to return to the site and excavate, to look for similar (or very different, or no) stone tools. It may have to wait a few years while John and Alison finish their work in Locality G, but they do plan to return to this area and explore further.