August 10, 2004
It's the last week of the field season. We'll be breaking camp on Saturday, the 14th, and there's much to do to complete our work by the end of Friday afternoon. One of my goals every season is to look around and figure out a plan for our next excavation season. From the outset this year, I've wanted to find a day to walk across the Ol Keju Nyiro (the dry river near the base of Mt. Olorgesailie) to search the sediments in the southern part of Locality B. I was glad that the Geology Field Course students got over there with Kay, and they did a bit of fossil surveying. I wasn't able to go with them that day, but they focused on exactly the sediments that interest me - the lower part of Member 11 of the Olorgesailie Formation, dated around 650,000 years old.
The reason I'm interested in this time period is that, a few years ago, we dug about a dozen excavations in lower Member 11. This was in Locality A, the central area of the Olorgesailie basin. We began to uncover a really nice collection of fossilized animal bones, associated with Acheulean stones tools. The animal bones began to fill a gap in our picture of how fossil mammal species changed over time, especially as the environment was altered by climatic and earthquake activity. I knew that Locality B also had stone tools and fossils lying out on the surface from this same time period. So I was excited to hear from Kay that the geology students carefully noted the presence of many more fossils of turtles, crocodiles, and hippopotamuses in Locality B. We found hardly any of these animals in our previous digs in Locality A.
Could it be that Locality B was wetter, perhaps a lake or wetland at the time, and thus more attractive to water-loving animals like turtles and hippos? Did these fossil animals come from the same layers, and the same time period, as the zebras and antelopes we unearthed in Locality A? Some of the participants in the Field Course thought so, but they didn't have enough time to return to Locality B and look into the question thoroughly.
So the plan for this morning was for Muteti and me to trek over to Locality B. Within a few minutes of arriving, we saw one reason why the geology students (and Kay) were excited about Locality B - lots of stone tools. In the photograph, you can see Muteti holding one of the handaxes lying out on the surface. It's definitely a big one!
We carefully traced the trail of fragmented fossils and stone chips up and down the slopes and gullies. We discovered one layer in particular, a brown sandy stratum, which seems to be the source of many of the fossils and artifacts. Most of the surface finds can be found at or below the level of this layer but not above it. We also saw bits of fossils and flakes sticking out of the sandy layer.
It's true that the layers, especially the paleosols and some layers of silt, resemble the lower Member 11 strata in Locality A, about 3 kilometers away. But here in Locality B, there are more sandy layers. Also, the very bottom of Member 11 here is a grey tuff that fills channels in the top of the underlying Member 10. That's unlike Locality A.
We saw a really impressive number of fossil animal fragments and stone tools. So Muteti and I began to form a plan for excavating in this part of Locality B next season. We'll be able to compare the stone tools to those left by the hominins in Locality A. And finding fossil bones will help round out our picture of the lower Member 11 fauna. I already have an idea, then, of how many geologic trenches and excavations we'll start with next season.
Muteti and I kept on walking, well past the survey area where the geology students worked. Finally, I saw a series of sediments that were exactly the same as the lower Member 11 strata in Locality A. I only had another 30 minutes to explore before needing to return to our digs in other places. But in that time, I saw that the area richest in fossils and stone tools is a place where the sediments of lower Member 11 like those in Locality A had been cut and eroded away by a decent-sized river. This channel was then filled in with silts and sands - and the food and water in this channel had apparently attracted hominins, at least at certain times of year. This seems to me the best explanation of why there are hippo, crocodile, and turtle bones in the fossil record of this area. It was a river, not a lake, during a time that was slightly younger than the fossil-rich sediments of Locality A.
At least for now that seems like the best explanation. We're sure to find out more next year as we focus our digs in this area.