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Day 15 (July 10, 2011): Geology Walk, part I
July 10, 2011
Investigating human evolution is, at a basic level, about time. It’s about what took place as the signature features of our species emerged and accumulated over time. These features include everything from walking upright to language. Sometimes, when people ask me what I do for a living, I say something like, ‘Well, I’m a time traveler.’ Rather than having a time machine, I walk and dig in the layers of sediment that contain an archive of earlier times. That archive includes the traces of tools, bones, behavior, and environment – that is, the kinds of things we study at Olorgesailie.
So today, my goal was to take our new visitors from China on a walk through time, across the outcrops that comprise about 700,000 years of the period represented in the sediment layers at Olorgesailie.
We began by examining maps of the Olorgesailie region and discussing the history of research . We also discussed the basic geology. The sediments of the Olorgesailie Formation are composed of 14 parts, called members, with the oldest part (Member 1) at the bottom of the sequence and the youngest (Member 14) at the top.
We began with the oldest, Member 1, which runs from 1.2 million years old to about 990,000 years old. It’s defined by a series of diatomite and soil layers. Diatomite is a fine, white sediment composed of the remains of microscopic lake creatures called diatoms – single-celled algae composed of silica. These diatoms accumulate at the bottom of lakes, creating compressed layers of diatomite. At first, it’s hard to imagine that such a dry place today as Olorgesailie was once moist, but just by looking at the sediment, we can tell that in the past Olorgesailie was home to a lake.
As we walked uphill, we could see that these diatomite layers are interspersed with layers of ancient soil. The soil layers are apparent from the clay and root markings that developed as vegetation grew on dry ground when the lake contracted. Ash layers from past volcanic eruptions were also obvious, and these layers contain the chemical elements necessary to date the sediments.
Further up the hillside, in Member 2, a deep lake followed that dry period of soil formation. That deep lake (bright white due to the purity of the diatomite) was then followed by a layer of hard carbonate, which shows that mud cracks had formed as the lake shore began to dry. The carbonate even had microscopic crystals of salt adhering to it – signs of a harsh drought as the lake dried up completely.
Our hearty, sun-drenched team made our way across geological faults, where the layers of sediment had become displaced – evidence of multiple earthquakes in the past. Gradually, we made our way up through Members 3, composed of light gray sediment, indicating a time when erupted volcanic ash had fallen from the sky into lake and darkened the diatomite. Then Member 4, when the lake contracted and gravel filled up shallow channels across the landscape. Member 5 was composed of soil and lake sediments, and included bits of volcanic pumice dated to around 974,000 years old. Only about a dozen steps later we quickly moved into Member 7, where we could see the sands that, a kilometer away, contained thousands of stone handaxes. This sand and the lake sediments above it were capped by a series of soils that had formed over a period of about 100,000 years. Ah, a period of environmental stability after all that action of climate change, volcanic eruptions, and earthquakes!
Every place where we spotted a volcanic ash, Dr. He Huaiyu stopped to inspect it closely. She’s the one in the group who’s an expert in dating volcanic materials. She was very interested to see examples of East African volcanic ashes. We also saw an odd red-colored layer, and when I explained the evidence and my thoughts about how the red sediment formed, my colleagues debated the interpretation. It was a great discussion, fun and full of interesting questions and ideas.