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2004 Field Season: Day 6

June 28, 2004

The excavations that were started last week are continuing without problems, so I thought I'd spend a few minutes explaining the geology of the region, as it's somewhat complex but very important for understanding what we're doing here. In the 1940s, Louis and Mary Leakey stumbled upon a large collection of stone handaxes that were eroding out of a hill. They got permission from the local Maasai to cordon off an excavation area, in which they worked for the next few years. This small area now houses the Olorgesailie Site Museum, and is where the Leakeys in the 1940s and 50s, and Glynn Issac in the 60s and 70s did all their excavation.

Geological map of Olorgesailie.
Map of the geological formations in Olorgesailie. The red dot is camp.

As Lynn and Amanda learned yesterday, the entire Olorgesailie Formation is made of about thirteen units, called Members. The members are labeled 1 at the lowest and oldest member, through 14 at the top and youngest member. The original Member 6 was later found to be part of Member 7, so rather than renumbering all of the later members, we decided to simply drop the name Member 6. Because of several local faults, you can see nine of these members right within the small area set aside for the museum, but out on the rest of the Olorgesailie Basin you can see wider stretches of all thirteen of the members. Each member is defined by a certain pattern or composition of the soil, and from this pattern we can tell a lot about the history of the basin. For example, Member 10 is made up of many small pebbles of pumice. Pumice is a familiar rock to many people - they may use it in their bath to remove calluses, or may have learned in science class that it floats on water. It is a volcanic stone, caused when a volcanic eruption throws molten rock into the air where it gets lots of little air holes, which cause it to be such a light rock. We can see that the pumice layers in Member 10 can reach about 3 meters in depth, and is spread over the entire basin, which means that about 9 feet of this very hot volcanic stone fell within the span of a few years, blanketing the landscape. It wouldn't have been a very nice time to live in the area.

In contrast to Member 10, Member 5 is defined by a thick layer of diatomite with some carbonate. Diatomite is made of the shells of diatoms, which are tiny, single celled organisms that live in water. You may have used toothpaste with diatoms in it, because their shells are made of silica, a very hard compound made of silicon, which is useful for polishing surfaces. When the diatoms that lived in the lake that used to fill the Olorgesailie Basin died, their silica shells fell to the bottom of the lake. After many, many years, thick layers of these silica shells accumulated, and when the lake above them dried and the water was pushed out between the diatoms by the weight of other sediments above them, diatomite formed. Carbonate is formed in diatomite when land-dwelling plants modify the diatomite, creating a soil that has a lot of carbon in it. Because the diatomite and carbonate layers alternate in Member 5, we can get the picture that the lake that used to fill the Olorgesailie Basin went through a period of drying up and returning several times.

By slowly picking apart the clues left for us in the soil, we can get an idea of what kind of landscape you might have seen all across the basin at any point in history. Part of what we're attempting here at Olorgesailie is to unify all of our excavations by looking at the wider paleoenvironmental information. It's important to know what geological setting our artifacts and fossils are in so we can know more how the creatures that left those remains lived. The hominin, for example, was found on the boundary between Members 5 and 6, the same layer in which the stone tools were found near the Site Museum. Also, as you can see in the photo, the hominin was found just to one side of a large lava hump that led from the lowlands of the basin up onto the highlands of Mt. Olorgesailie.

Hominin site, looking toward Mt. Olorgesailie.