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2004 Field Season: Day 20
July 12, 2004
Mt. Olorgesailie is a huge part of our experience here. We wake up every morning and see it from our tents. It overlooks our camp and helps orient us wherever we walk. I've already described the things we find in the basin below, but what's up on the mountain?
The answer is: rocks! I mentioned earlier that the mountains within the rift are almost entirely volcanic, formed when the super hot magma within the earth erupted to the surface and cooled. There are many different kinds of volcanic rocks. We talked about pumice and ash, but there's also obsidian, which is a kind of volcanic glass. Basalt is a dark, dense rock that erupted repeatedly from Mt. Olorgesailie and nearby fissures. And there are several other kinds of volcanic rock. The different kinds vary chemically and in the size of the little crystals you can see. The basalt rocks have a lot of black crystals, called pyroxenes, which are rich in magnesium, and some shiny crystals, called feldspars, which are rich in calcium. But basalt doesn't have much silica (a compound of the elements silicon and oxygen); in fact. it only has about 50%. Obsidian is similar chemically but with much more than 50% silica and smaller proportions of the other minerals.
The rocks on Mt. Olorgesailie and in the surrounding highlands are mostly made of basalt, phonolite and other volcanic rocks called trachyte and nephelinite. You can see in the photograph where all the different kinds of rocks are found. The symbols stand for all the different kinds of rocks. What's so interesting about knowing the rocks on the mountain is that we can compare the rocks to those the early hominins used to make stone tools. It turns out that almost 99% of all the stone tools are made from rocks from Mt. Olorgesailie and the surrounding mountains. The other rocks they used, mainly obsidian and quartzite, came from further away. The closest place the early humans could find obsidian is 16 to 18 kilometers away, and they could find quartzite about 46 kilometers from here. Very rarely, they used a rock called chert, but we're not sure where they got that type of rock. All of the stone tools made from obsidian, quartzite, and chert are really small and heavily used.
What does this mean about the behavior of the hominins who lived here?
We think that these early humans were nomadic - they moved around perhaps from one season to the next. (Remember, here in East Africa the seasons are rainy and dry, not so much cold and warm like in the temperate zones to the north and south). When they came to the Olorgesailie region, they were in a place full of rocks that were easy to get. But how about the stones from distant sources? You might think that they traded with other groups of hominins for obsidian, quartz and chert. But the fact that these stones are so rare - and that they are in such a worn and used-up condition - makes it unlikely that the hominins traded for these pieces. Instead, we think the nomadic groups moved across a wider region, from one basin to the next, taking with them a few local stone tools from one place to wherever they ended up. By the time they got to Olorgesailie, it must have seemed like an oasis of rock - all the rocks they could possibly want. A good thing, too, since the rocks they brought from distant places were pretty much used up by the time they got here.