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Day 30 (July 25, 2011): Back to the Butchery

July 25, 2011

You may recall that we have another excavation going on this season besides the Middle Stone Age site called BOK-2. This other excavation, called Site 15, is one made famous by the fossil elephant skeleton found there along with thousands of stone tools. It’s from the handaxe times, which at Olorgesailie extended from 1.2 million to about 490,000 years old.  A newly published article from a site at Lake Turkana, in northern Kenya, shows that handaxe technology (known as the Acheulean) got its start almost 1.8 million years ago.

Five men sitting, excavating in a light brown and grey sediment
Continuing excavation at Site 15.
The digging at Site 15 has gone very well – that is, ‘slow but sure.’ Let me give you an update about the questions that have cropped up as our team of seven has dug downward a few centimeters at a time to the target level.

First, though, a brief note:  It seems to me I haven’t yet explained how we name our sites.  We’ve divided up the Olorgesailie region into large localities based on the presence of rivers, geological faults, and other physical features.  Site 15 is in Locality A, which is the area of land between the Site Museum (Locality C) and a major fault just east of our camp.  Geologically, Site 15 is located in the lowest stratigraphic unit, Member 1, of the Olorgesailie Formation. So, the official name of the elephant butchery site is A1-15:  Locality A, Member 1, and ‘15’ refers to the series of excavations we’ve done in that particular locality and geological unit.

Using this same formula, ‘BOK-2’ means that that particular excavation can be found in Locality B (the area south of the Ol Keju Nyiro River), and it’s within the OK Member (Olkesiteti Member) of the Oltulelei Formation – the youngest major set of geological beds in the region, dating roughly between 340,000 and 50,000 years old.  A1-15, by contrast, is in the oldest geological set of beds (the Olorgesailie Formation, which ends around 490,000 years ago). The elephant site itself is about 990,000 years old.

OK, so that’s how we name sites.  As for A15-1, we usually just call it Site 15, since this excavation was one of our first, begun in 1986!  For one thing, it takes a long time to carefully excavate an elephant out of the ground. For another, we’ve found a lot of other interesting things at the site in previous years.  We began the season by wishing to test if our new digs at Site 15 (called Extension 17) might reveal fossil zebra bones indicating another butchery area.  Previously, we’ve found the elephant butchery, an antelope butchery, and a first zebra butchery area at the site. Last year, our excavations ended with finding a nice set of zebra teeth right at the edge of Extension 16.  I strongly suspected that another zebra butchery was waiting for us to uncover in this year’s work at the site.  It turns out I was wrong.

a light grey and brown uneven suface floor of an excavation with exposed cobble, fossil bones and stone tools
A good amount of fossils (but not many zebra bones) and stone tools, along with unmodified cobbles, have been found in this year’s Site 15 excavation.
This year’s digging has revealed a few more zebra teeth but no concentration of other zebra bones.  Sure, we’ve found some fragments of leg bones, some of which probably come from a zebra.  But inspection of the bones as they’ve been lifted from the ground offers no clear evidence of grooves made by stone tools on the bone surfaces (butchery slicing marks) or percussion marks, which occur when a hammerstone is used to crack the bones open to get at the fatty internal marrow.  So, no additional zebra butchery has been found.

Last year, we also had other clues to follow.  In Extension 16, we found a few anvil stones, again right at the edge of the excavation.  These anvils had indentations on top, filled by sediment, and at other sites these kinds of anvil stones indicate that early humans used pounding stones to prepare plants for eating by crushing them on top of the anvils.

But this year is different:  Over the past couple weeks, we’ve found no further evidence of anvils in the zone next to the ones from last year. Instead, the excavators have discovered a concentration of large cobbles that show no signs of having been altered by pounding or any other way.  I’m scratching my head about how they got there.

Sometimes, rounded cobbles can be moved and clustered together by a river or stream, which you’ve probably seen before if you’ve looked at a modern-day river.  But the little depressions, game trails, and channels we’ve found at Site 15 are way too small to have delivered these large cobbles to Site 15 via flowing water.  The closest sources for rocks of this type, which were exposed and available in the ancient environment, were located several hundred meters away from our current excavation.  Did the early hominins tote these heavy rocks to the site?  If so, why the big effort? There’s even one really big rock, which measures 36 centimeters long and 19 cm wide.  It sits about a meter away from where we unearthed the anvil stones last year.

a group of cobbles sit in the front corner of the excavation floor of Site 15.
What are these cobbles, in the front corner of the excavation, doing here?
The excavators are slowing digging down toward the base of the excavation, toward the underlying lake bed that had dried up and was eroded before grass and other plants began to cover the surface and create the landscape over which early humans and other animals roamed.  Site 15… what will we find next?  And what clues can help us figure out the cluster of cobbles?