Everything in archaeological investigations requires patience – and nothing more so than dating. While we waited for Geoff Duller to arrive we had to work quickly, but we knew also we had to make sure we were digging in areas that would be most relevant to dating the sands and their archaeological record. No time for mistakes.
But there was another frustration with those enigmatic red sands – they were very hard. Yes, hard. As we came down on the final metre of sand, above bedrock, the pick only dented the surface. It took almost two days of slow toil to reach the base. Dog tired and working to a tight timetable – Geoff was arriving soon – we pushed on.
The sound of a pick striking rock brought a collective sense of relief. The remaining sand was removed with the tip of a trowel and small chisel to avoid breaking any artefacts and to work around crevices of the coral-like bedrock floor. The reward was four small fresh flakes and a core. Doesn’t sound like much except they are crucial evidence of a human presence very early in the formation of the sands.
Dating these rock-hard sands and the bedrock itself would be Geoff’s task and in the final week he was kept busy sampling all the pits we had excavated. Back at the lab, it will take time to extract the crucial information his samples contain. It is painstaking, cutting edge work.
After days out in the field, Geoff and I work discussed (over a beer or two) the conditions that led to the formation of the sands. But that’s another riddle for another time.
“And how old is it?”
The Zambia National Broadcasting Corporation film crew had been spending quite a bit of time with us and took the opportunity to film Geoff sampling the sands and bedrock. On his first morning, he was filmed at the Maramba trenches, and late that afternoon we were given privileged access to the Field Museum for more sampling and filming.
The Museum is a building constructed over a 2m deep stepped excavation trench left by Desmond Clark. Each step down through time shows the artefacts found – it’s a visual record of the Stone Age record just next to the entrance to the modern falls. I’d been itching to get into that trench since first laying eyes on it in the 1990s, but it’s off limits to the public. This time I was returning as a researcher with clear justification for taking the stairway down to the Early Stone Age passing the Sangoan en route. With Geoff’s expertise we would provide the first dates for the sequence, contributing to the current renovation of the Museum.
We climbed over the high railings and gingerly tip toed (not easy in size 13 boots) down the sediment steps. They were firmer than I expected but the edges were crumbling. The crew knew the routine and in a minute or so Geoff was covered in thick black plastic and the edges sealed to prevent light entering and ruining the samples.
After the sampling was done the plastic was lifted and Geoff measured the background radiation of the sediment surrounding the samples for about an hour each. It was early evening by the time we finished, and we drove home in the dark.
The technique he is using is called ‘optically stimulated luminescence’ (OSL) and Geoff is a pioneer in the field, continuously working to improve the accuracy and time range of the technique. And this trip he had a new trick up his magician’s sleeve, the dating of individual cobbles.
I’ll spare you the finer details of how OSL works, but it’s based on the fact that some crystalline minerals, such as quartz sand absorb natural background radiation from the soil. Over time they become charged with electrons, like a battery. Exposure to sunlight clears the electrons and when the sand is reburied it starts to absorb radiation again. Wind-blown sand and river sand are good candidates for exposure to sunlight and reburial. And, now cobbles too. The trapped electrons are released in the lab as the samples are heated carefully and the subtle light emitted is measured. The brighter the light the older the signal.
It’s a far from routine technique, involving many measurements and calculations including a measure of uncertainty. The results take time to appear – at least a year if you’re on the waiting end, or in the case of one previous collaboration with Geoff the results dribbled out after almost nine years. But we’re still friends 😉
OSL dating has transformed our understanding of the chronology of the Stone Age. Its age range far exceeds that of radiocarbon dating and it doesn’t require the preservation of organic material as does radiocarbon. And sands and gravels are common sediments, especially near sources of water, as in the case of the Victoria Falls region.
We ‘buried’ Geoff under plastic to collect samples from the Maramba river site and in the pits of red sand in the national park. He also hammered foot-long tubes of plastic into the sand scarp, collecting a sequence ranging from near the top to the bottom. (Tubes are a quick way of collecting sands – and it’s dark inside the tube.) The top and bottom dates will be very interesting as they’ll give us a fix on when the sands formed. I have a feeling we might not have to wait a full year for the results as Geoff is equally keen to get results. He flew home with the top and bottom samples in his carry-on baggage and they are already in the lab being processed.
And you might think that was it – but no. The deep pits had to be backfilled, backbreaking, dusty work – especially when I have what might be tick bite fever, might be malaria, might ‘just’ be food poisoning – but it had to be done. The excavation pits posed a hazard to the wonderful wildlife and – the not at all wonderful but inevitable blight of African wildlife conservation, poachers.
By the way, the featured image at the head of the post is the Lookout Tree, an ancient Baobab form which you can see Victoria Falls. It is not, as often believed, the spot where Livingstone saw the falls for the first time. Geoff is on the staircase for scale.