At Olorgesailie, we are surrounded by dirt! The dirt is under every step we take. It blows around and gets us dusty during the day. And the dirt is what we dig - it preserves the prehistoric tools and fossil bones buried under the ground.

Dirt is also called sediment, a word we've used a lot so far in these dispatches. Much of our information about the ancient habitats where early humans lived comes from the study of sediment.

Lynn and Amanda joined me on another geology tour this morning. We focused on the strata (the layers of sediment) that can be seen at our excavation sites AD5/7-1 and CL1-1. You may recall that on a previous day I used the term "member" when talking about the geology of Olorgesailie. Amanda and Lynn wanted me to go over exactly what this means.

OK, ready? Sediments are laid down (deposited) one layer at a time. The layers are built up pretty much one on top of another so that the older ones are beneath the layers that come later. A single layer of sediment is called a stratum. (By the way, the plural is strata, and the study of these layers is called stratigraphy). Each stratum was usually deposited by a single process - like volcanic ash that fell out of the air and into the bottom of the old lake; or, the many diatoms that fell to the bottom of the lake and so created a separate layer of diatomite. In some cases, a stratum may be defined by the way the sediment has been altered. For instance, when vegetation inhabits and grows on a dried up lake bed, the plant roots eventually turn the white diatomite into a brown soil. That's a stratum that can easily be distinguished from the lake strata.

A member is actually made up of many strata. The pattern of strata making up a member must be widely recognizable across the present landscape, and it can encompass several different ways of depositing sediment.

Understanding the layering of members in the basin is very important for anyone who excavates here. Lynn, Amanda, and I talked about why it's important, beginning with how the Olorgesailie region came into being.

As the Rift Valley formed, basalt and other volcanic lavas erupted from nearby volcanoes, like Mount Olorgesailie. The lavas also oozed from fissures in the ground. As these molten lavas cooled and hardened, they formed the bedrock at the very bottom of the Olorgesailie basin. This foundation is not shaped like a smooth bowl, but rather it's full of ridges and valleys.

After the volcanic activity in the area slowed, water collected in local valleys, forming the first lakes. Because water always runs down a slope, these lakes formed in the lowest, most central part of the basin. Diatoms lived in these lakes, and, over time, formed the diatomite that defines Lower Member 1. Rain and wind also eroded the ridges, and the sediments washed or rolled down into the valleys. Eventually, small valleys became filled with sediments, both from diatoms and by sediment eroded from the higher, surrounding area.

At times, during many years of poorer rainfall, the lake became smaller and plants could grow on the exposed lake margin, creating the first soils. The lakes returned during rainy periods, though, and covered a larger area due to the fact that the bottoms of the deepest valleys were already filled with sediments.

Sometimes I like to demonstrate all of this using my hat as a prop, which Amanda captured with her camera. Imagine the top of my hat (in the photograph) is the basin. You can see that it's not a smooth bowl, but has little ridges and valleys. When I pour water into it, the water comes up to a certain height along the side of the depression in the hat. Now, imagine that this first level of water has left behind many layers of diatoms, which we recognize today as the sediment called diatomite. When the lake waters return (adding more water to the hat to represent another lake), the surface of the water is at a higher level than before. The water is spread further horizontally because the sides of the depression in the hat flare. The water also covers more of the valleys and ridges than before. The water in the hat behaves sort of like the lakes did in the Olorgesailie basin. Over time, as the lakes filled, the sediments rose higher and spread further to the sides. The oldest members were originally deeply buried in small areas in the lowest points of the basin. The youngest members covered the top of the basin from brim to brim.

If this were still the state of things here, the Olorgesailie region wouldn't be of any interest to archeologists. Why? Well, all of the fossils and artifacts would still be very deeply buried and no one would know they were there. The Rift Valley continues to change, though, and earthquakes have lifted up the buried landscape and exposed it to erosion. Instead of collecting as a lake, water now runs off the uplifted ground surface and into a river at the base of Mt. Olorgesailie. As a result, water and wind erode big gullies that expose a huge number of layers that have been deposited over the past 1 million years or so.

The first researchers to work at Olorgesailie had to come up with some way of organizing all of these exposed layers. So they numbered the different groups of strata from oldest to youngest - Member 1 through Member 14, which represent the period from about 1.2 million to 500,000 years ago.

You might wonder how in the world we can see those oldest sediments, originally buried so deeply. In fact, the faults (from earthquakes) that crack the sediments can lift some areas very high, and this allows us to see the oldest members. We now see the youngest sediments around the edge of the basin, where they have not been eroded off, and the older ones towards the middle. Where the gullies cut through sediments that have archaeological materials, we can find artifacts and fossils that have eroded out of the hillsides. Recognizing which member we're standing is the first step in knowing about how old the tools and fossils are.

After this detailed examination of the members, we returned to camp for the evening, when Dr. John Yellen arrived in camp. He's an archeologist who's interested in the Middle Stone Age artifacts discovered here in the younger sediments along the edge of the basin. John will be with us for the rest of the season, and he and his wife, Dr. Alison Brooks (who will arrive later), will run several excavations in Locality G, across the river on the southwestern slopes of Mt. Olorgesailie. We'll spend some time tomorrow figuring out exactly which places to excavate there.