Before Victoria Falls formed, the Zambezi River was very much like we see it today above the falls – wide and shallow. But the old Zambezi was not where the Zambezi flows today. It has left remnants of its former presence 12-15m above the modern river in the National Park, and the gravels it left there also contain artefacts. If they can be dated then we will have a maximum age for the cutting of the falls (why the falls formed is a contentious issue among geographers).
Zambia has none of the volcanic debris that facilitates dating of the archaeologically famous sites in East Africa such as Olduvai Gorge, so the potential to date the exposure of gravel to sunlight using optically stimulated luminescence (see last post for idiot’s guide to OSL) is a real bonus to the project. The technique has been invaluable in places like Zambia where there are few other reliable options.
We dug a small pit for Geoff and even though the gravels were shallow (30cm) he was willing to try his magic on these.
The scouts who accompanied us could not believe the story of a once higher Zambezi. Wry smiles and shaking heads accompanied our explanations.
There are also gravels left high and dry downstream of the modern falls on a series of flat top platforms that mark the position of previous falls. They form a six mile long zig-zag leading to the present falls. The contrast couldn’t be greater between the tranquil Zambezi above the falls and roiling torrent below. The earliest and furthest from the falls is at the confluence with the Songwe River where there is a spectacular gorge.
It’s a beauty spot, but also a National Monument for its abundant Stone Age remains. The gravels contain raw materials, such as glass-like chalcedony, that are perfect for making stone tools. It’s a rare resource in an otherwise barren landscape for tool-makers.
We took Geoff there because of the gravels and a patch of sand that might be sitting on the gravels. Early in the season we had visited Songwe and found pits in the sand-cover where it was being collected as building material. A quick scrape with a trowel in these pits revealed Middle Stone Age (MSA) artefacts. Not the big stuff we found at Maramba, or on the sand scarp, but still part of the record of humans in this landscape and worth dating.
We spent an hour or so tracing the Songwe gravels and noting how thin the spread is and wondering how it might be dated using OSL. Bedrock was just beneath the surface – no chance here for Geoff’s technique.
Next season we’ll have Andy Hein (University of Edinburgh) with us who specialises in dating how long surfaces have been exposed to the sun, just the opposite of Geoff’s magic. We gave up on the gravels and returned to the sand pit and its known MSA. After clearing the surface of the pit floor a 1m square was laid out and excavation began. It wasn’t long before very sharp flakes started appearing, some smaller than the nail of your little toe – a sign we were in a relatively undisturbed deposit where even the smallest manufacturing debris remained. That’s rare and this place is on the growing ‘to do’ list for next year.
But there was another surprise in store – the gravels of the Zambezi. They formed the bottom of our square and there was no doubting these were river gravels given the rounding of the stones. There were also artefacts in the gravels, probably Early Stone Age based on size, also rolled from river action. Whatever they turn out to be in terms of archaeological labels, they are very different from the fresh material on top. Out came the thick black plastic and Geoff was buried again. He collected samples from the overlying sands and two cobbles from the gravels.
We were rapidly running out of daylight and still there was gravel to be excavated. It will have to wait until next year. And maybe a year from now we’ll have some OSL dates to share with you.
And so, field work all but at an end, what more is there to say?
Well, just few practical matters still to sort out …