July 31, 2011
We’re close to the end of the field season – just a few more days left – and we’ve come to the end of one of our excavations, Site 15. If you read my dispatch on July 25th, you may recall that as we reached the base of the artifact layer, we couldn’t quite figure out what the cluster of large cobbles in one area of the dig was all about. Why were the cobbles where they were – concentrated in one spot?
The water that rained on, or flowed across, the site as it was forming was probably pretty slow and inadequate to move the cobbles into the position we found them. We can tell that from the beds of sediment we unearthed at Site 15. Yes, we did find sand mixed in with the finer silt at the site, but the cobbles are far larger and would have taken a huge surge of water to introduce them to this spot on the ancient landscape. We don’t see any evidence of such a surge. Yes, large cobbles can roll downhill due to gravity, but there’s no evidence of a cliff or ridge of rock anywhere in the vicinity of the site – today or in the past. So, what gives?
Let me explain our latest thinking based on the photographs of the site, as I took them today when we concluded the digs at Site 15.
The photograph to the left, documents the accumulation of cobbles, including the largest one by far at the site (measuring 36 centimeters long), that was found at the site. The cluster occurs immediately next to a most interesting feature: Do you see the narrow runnel (or furrow) and depression in the sediment in the upper right corner of the photo? Well, we compared these features with other depressions we’ve excavated in previous years at Site 15.
The depressions at this site were typically made as large animals stepped into drying mud. Yet this particular depression was not made in this way. Instead, its steep sides and furrow connecting it to the area of the cobbles seem to have been dug out on purpose. We looked for a longtime, but no animal trails or footprints are found around this main depression. The large cobbles positioned right at the outlet of the depression means that the cobbles would have created a little pond of water in the depression whenever it rained hard or continuously.
Could this have been a way of catching water? If so, shifting the cobbles a little would have allowed anything from a trickle or stream of water to flow into the depression from the higher surface around the depression. And once the water level got to about 20 centimeters deep, repositioning the cobbles right up against the end of the furrow would have stopped water up to 40 cm deep from trickling out of the depression.
The location of this area within about a meter of an area where we found anvils with evidence of their use to pound or grind up plants is really intriguing. Could the little water pond have offered a way to wash off tubers or other plants as they were processed on the anvils? After many discussions and head-scratching, we think this is a feasible interpretation – though a difficult one to prove for certain.
The depression and runnel at the end of the excavation, with all the cobbles at the end of the runnel removed (from the upper right corner). Sediments gradually filled in the depression, but for some considerable amount of time, the depression would have held water blocked by the cobbles at the end of the narrow runnel
After many years of excavation, Site 15 continues to offer its intriguing mysteries and potential insights into early hominin behavior. In previous years, we have found three butchery sites of animals ranging in size from a huge elephant to a small antelope. We have found some of the first evidence of using anvil stones and hammerstones to pound or crush plants that must have been found at the site. And now we have tantalizing clues about how the handaxe-makers may have captured rain water and concentrated it in a small pond.
All of this took place during a dry time that must have presented real survival challenges. The lake had withdrawn or dried up entirely. Most of the landscape was apparently covered by grass. And the dry season of each year must have been a difficult time to live. Site 15 was a relatively wet oasis in this arid landscape. A few hundred years before, the lake was large, indicative of a moist period when food and water were abundant. A thousand years later, the lake returned to an even larger size – more water and food abundance, I imagine. But at the time when a small group of early hominins, or perhaps several groups in turn, came to this place, they likely came to grips with difficult times – likely cooperating, bringing large rocks to catch water, obtaining more elephant meat than any one individual or the entire group could eat in that moment, at least.
It’s moments like this, time after time, over many millennia that comprise the small chapters in the immense story of human origins.