August 9, 2004
We had a really interesting surprise today. At least once or twice each summer, we find something really neat in our digs we didn't know about before.
Today's big job was to carefully remove the hundreds of stones found in excavation Site CL1-1. You may recall that it's the oldest handaxe site known so far at Olorgesailie. The site is nearly 2 meters above a volcanic tuff dated 1.2 million years old (by the argon-argon technique). And about 2 meters beneath a younger tuff that's 992,000 years old. Sandwiched between the two dates, Site CL1-1 is probably a little older than a million years.
As you can see in the photo on the left, we discovered a really dense accumulation of Acheulean handaxes. As the excavators unearthed them, we left the handaxes in place in order to get a good sense of how many there were altogether. We dug an area of about 18 square meters, and we got about 125 handaxes - an average of about 7 handaxes per square meter. That's a really good concentration! The layer they're in is only about 10 to 20 centimeters deep, or about the depth of one of the thick cobble stones that occurred along side the handaxes.
The surprise? As work proceeded today, our attention suddenly shifted from the handaxes to the cobble stones! The handaxe is the "signature piece" of Acheulean toolmaking, which lasted more than one million years. The hominins of this lengthy era in Africa kept on making handaxes and similar pieces, all called large cutting tools. They kept making these types of tool for a tremendously long time. As we carefully measured in and lifted the stones at CL1-1 (using the compass and laser transit), we found that virtually all the handaxes were lying in a flat, stable position. Yet many of the cobble stones had sharply curved or angled bottom surfaces and flat top surfaces. Specimen #104 really caught my eye as I lifted it from its original place. As you see in the second photo, it has a strongly rounded bottom surface (exactly as I'm holding it), AND it has a really indented top, flat surface! (This photo below, by the way, was taken later this afternoon, back at my tent - thus the green canvas backdrop.)
Soon, I began to notice that about every third cobble had this same arrangement in the sediment - angled bottom, flat top. And the flat top surfaces were pitted or indented, unlike any other surface on the rock. I think you can see this pretty clearly in the photo of specimen #104.
What do you think these stones are? To answer this kind of question, as you probably know by now, we start with geology. So here are some clues. The sediment is sandy, and the site occurs in a broad but very shallow, ancient channel that had been eroded by flowing water. Many if not most of the cobbles are from a volcanic outcrop barely 20 meters away. Those stones were probably washed into the channel. But if that were the end of the story, why were there so many rocks resting in an oddly unstable position, with the flat surface on top?
At this early point in our work at CL1-1, I don't think that was the end of the story. Instead, it appears that rocks with sharply angled or curved surfaces, and with flat opposing surfaces, were purposefully selected. Early humans, I think, were doing the selecting. These rocks were then anchored in the ground by jamming their angled surfaces well into the sand, and their top surfaces were then beaten with hammerstones, which we also found at CL1-1.
Our first idea, then, is that these cobbles may be anvil stones, which were useful in cracking open marrow bones, or in cracking nuts or seeds, or in pulverizing tubers before eating them. It's intriguing that tiny microfauna bones of rodent and frog occur in our excavation, but there's essentially nothing from large animals. Could it be that in this site - dug because of all the handaxes - the "star attraction" was actually the use of cobble stones as anvils to process plant food? Our team is really excited about this possibility. It gives us a lot to think about, and a lot of work ahead to see whether it holds up to further scrutiny and testing.