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Day 26 (July 21, 2011): Deciphering the Dig
July 21, 2011
It was great to return to Olorgesailie and Kampi Safi today after my trip to western Kenya and Lake Victoria. In Nairobi, I was joined to two colleagues who are also members of our team, and they accompanied me back to the Rift Valley. I have known Kay Behrensmeyer for many years, ever since I was getting my start in East Africa as a young graduate student. Kay, who is a colleague of mine at the Smithsonian’s Natural History Museum, leads our geological studies at Olorgesailie, and she is a font of information about sediments, stratigraphy, and bones. I’ve known Naomi Levin since 2002 when she first visited Olorgesailie. Naomi is currently on the faculty at Johns Hopkins University in Baltimore, Maryland, and she specializes in isotope geochemistry, a topic we’ll explore on another day soon. She’s interested in how the chemistry of fossil teeth and of nodules found in ancient soils (or paleosols) can help understand past vegetation and climate. Kay and Naomi are both geologists but with different areas of expertise. They have come to Olorgesailie to investigate several important geological questions.
Upon arriving, we headed over to BOK-2 to show Naomi and Kay the terrific new artifact assemblage, and to get to present some geological quandaries – things we don’t quite yet understand in the dig (Day 25).
The puzzle can be broken down into two questions: 1) Are the stone tools found at the excavation located within an ancient stream channel? And 2) Can we easily tell when we reach the boundary between the BOK-2 sediments and the underlying Olorgesailie Formation, which is much older in time?
The deliberation today required a lot of observation, teamwork, and discussion. We first observed the back wall of the excavation, carefully examining the layers of sediments. We followed a gray sandy layer containing some volcanic ash – and this ash does not contain stone artifacts. We saw that this gray layer started to dip down in the center, forming what could be the bank of a channel. We then discussed the meaning of these layers and gave our opinions of their meaning, often asking new questions.
Soon Kay was able to answer assuredly our first question and confirm that a channel had formed, and the site was sitting in the channel. She added, ‘This channel had a life.’ In other words, the channel can be found outside of the site, and it had a complex history with other paleo-channels observed in the sediments of the surrounding area. All of this is important to the archeological interpretation because the BOK-2 channel wasn’t just a trough that occasionally filled with water; early humans had occupied the area when it was dry.
The next task will be to determine the lower boundary of the channel and how to distinguish it from the rather similar, but much older, sediments below. One surprising question archeologists may face is, when do you know to stop digging? Where can you expect to find the bottom of the artifact-bearing layer? In the case of BOK-2, we think the bottom is the ash layer. It will take Kay and Naomi a few days of in depth geological investigation to be sure.