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2004 Field Season: Day 28
July 20, 2004
I like to think of early humans as the very first geologists. They liked rocks, they hit them, they used the results, and they knew a lot about the properties of different rocks. The geologists on our project take that knowledge many huge steps further, such as figuring out the age of sediments. We also need to name the geologic features we see to communicate with one another.
OK, then, for today, more geology! I'll tell you about the differences between the New Formation, where John is working to uncover Middle Stone Age tools, and the Olorgesailie Formation, where the other excavations are uncovering Acheulean artifacts.
The Olorgesailie Formation is the name of the fourteen members we've talked about in previous dispatches. It was formed during a time when a series of lakes covered the Olorgesailie basin, and we can easily identify the formation because so many of the strata contain the blinding white diatomite, created by the bodies of the lake-living diatoms. The Olorgesailie Formation is the oldest series of sediments in the region, dating from 1.2 million years ago. After about 700,000 years of depositing these sediments, a big gap in the record occurs because of erosion. The landscape was uplifted by earthquakes and the lake drained away probably toward the south, in the vicinity of the present Lake Magadi. This was the first really big erosion period in the basin, and it was a sign of things to come.
The next time sediments were deposited in the basin dates to at least 220,000 years ago. We got this date from some pumice that was surrounded by some very white diatomite sediment. So, originally, we thought that this was just a part of the Olorgesailie Formation that was more recent than Member 14. For a while, we called this Member 15, but we knew we needed to look more closely at the area, since the date was so recent, and we kept finding interesting artifacts in these sediments. Closer geologic study showed us that the dated sediments were not part of the Olorgesailie Formation. What we hadn't seen when we first looked at the sediments was that there was this big erosional surface just above (later than) Member 14 and beneath (prior to) the 220,000-year-old sediments. The erosion created deep and wide channels, which were filled with these later deposits. In contrast to channels cut by water in the Olorgesailie Formation, which are at most one to two meters deep, the channels in these newer sediments were tens of meters deep. The photograph shows you an idea of just how deep these channels were. The dotted line shows the edge of the channel, and the small green blob at the lower right is actually a backpack - you can get an idea of how big this channel is.
The white color above and below the 220,000-year-old sediments represented not only a few remaining swampy areas, which produced new diatomite, but also from the eroding and re-depositing of older diatomites of the Olorgesailie Formation. The thing is that these white layers have a lot of gravel mixed in, brought in and deposited by the more energetic water flowing in the big channels. So we now realize that these diatomites do look different from those laid down by the older lake. Interspersed between the layers of gravelly diatomite are some brown soils and many pumice layers indicating volcanic activity. We can also see faults that mark the times of earthquakes.
All of this geological activity is what caused the rivers and swamps to change where they were, sometimes draining them completely and allowing them to fill with soil, and then later cutting once again deeply to form new channels and gullies.
Once we realized what we had, we needed a way to refer to this series of dramatic cutting and filling cycles that came after the Olorgesailie Formation. But because we're still studying it, we don't want to give it an official name yet, so we call it the New Formation. With all the volcanic activity, earthquakes, and uplift, it would have been a very interesting and dramatic time to live in the Olorgesailie region.