July 10, 2004
I drove to Nairobi late yesterday and this afternoon accompanied Dr. Kay Behrensmeyer and the students of the Geology Field Course to Olorgesailie. It's their first night in the field. During the coming weeks, I'll let you know what they're learning about the geology of Olorgesailie.
Before all that begins, I was eager to check the three excavations we started at the beginning of the season. Particularly interesting are Site 15 and CL1-1 because stone tools and fossil bones are appearing in both excavations.
At Site 15, the excavators have uncovered a long depression or channel that runs diagonally across the excavation. You can see it in the photograph below: starting at the upper right, it goes under Benard Mukilya and ends in the lower left. The edges of this channel are interrupted by circular depressions, about the size of dinner plates and about 10 centimeters in depth. After a lot of care in excavating the different colored sediment that filled these depressions, we could see that these were footprints. Probably from large hippos and elephants that walked along, creating the channel as they went. This means, in fact, that the channel was not carved out by flowing water, as in a stream or little river. Instead, this dip in the sediments is a game trail made by the trampling of animal feet about 990,000 years ago.
How did the excavators find this game trail? They relied on the different types of sediment they find in a site. When the sediment is evenly distributed over a landscape, it creates flat layers (strata). At Site 15, outside of the channel the sediment is grey, compact, and very hard. Just above this layer is another that is light brown in color, less compact, and somewhat crumbly with many bits of ancient plant roots. The channel was created by erosion of the grey colored layer. If you look at it in cross section, you see a shallow ditch cut into this grey layer. The light brown layer filled in this ditch.
Remember I described yesterday that we excavate in 5 centimeter layers, called spits. In the case of this depression, we decided to dig the spits only from the light sediment within the channel, leaving the sides and bottom of the channel intact. After we record the footprints and the direction of the game trail, we'll also remove the sediments on the sides. By separating the channel/footprint fill (the light brown) from the eroded channel itself (the grey), we're able to detect a neat bit of animal behavior that went on at this site.
The excavators digging at Site CL1-1 have also discovered a channel. This one, though, has a different story. At CL1-1, the channel was formed by moving water, and is nearly as wide as the trench itself. The channel was eroded into the diatomite and then filled with sand. In the sand are many, many handaxes. The photograph shows the axis of the channel, at least as far as we've uncovered it in the excavation. The handaxes are at the bottom. The sediment difference can only be seen very close up, and it's easily felt by rubbing the rough sand and the smooth diatomite between the fingers.
Water in a stream carries small particles of sand, which bounce and tumble along the bottom and form little ridges called crossbeds. The height of these crossbeds is directly proportional to the depth of the water that carries them. As we expand the excavation, we'll measure the crossbeds. If CL1-1 is like other handaxe sites at Olorgesailie, the ridges may be pretty small, which would tell us that the stream was not very deep or powerful.
The really rich handaxe sites at Olorgesailie are associated with ancient stream channels. Some people have guessed that the flowing water must have moved and collected the handaxes from far away. In fact, that's what I originally thought when I started investigating these sites. But the geologic evidence says otherwise. The crossbeds show us that the stream was only about knee deep. They were almost certainly empty of water during the dry season, except for small pools of water like we see in seasonal streams and rivers in the Rift Valley today. The old streams at Olorgesailie weren't powerful enough to move large rocks like handaxes, and the edges of the handaxes themselves are not rounded, as we'd expect if these rocks had been moved a lot by water.
The balance of evidence, then, is that the hominins who made these tools actually brought them to where we find them in the sandy stream beds. It's interesting the only channels where we find dense collections of handaxes are those very close to volcanic ridges that rise up toward Mt. Olorgesailie and the highlands. My best guess is that the toolmakers visited the dry-season pools in the stream beds. Before going back into the highlands, where there's all the rock they could possibly have needed, the hominins left their stone tools behind. They could have continued using those handaxes during their next visits to the stream beds over the following months. But when the next rainy season arrived, water and sediment would have flowed again in the channels, covering and burying the handaxes.