The Qurta rock art – a series of carved images of birds, animals and some human figures – is at least 15,000 years old, according to an analysis of grains of sediment blown onto the rocks from the Nile river banks.
The carvings were discovered in 2005 on sandstone cliffs bordering the east side of the Egyptian Upper Nile Valley flood plain. The detective work then began of trying to determine the age of the carvings.
image (above): Detail of a rock art panel at the Qurta II site, showing two superb drawings of wild bovids (Bos primigenius or aurochs) with forward pointing horns. The double belly line of the right specimen is typical of the Qurta II bovids (© RMAH, Brussels)
Using the technique of optically stimulated luminescence (OSL) dating, Dimitri Vandenberghe and colleagues at Ghent University in Belgium have pinpointed the date when sediment was blown onto the carvings.
The result means that the rock art is at least 15,000 years old, and probably older, because the carvings show signs of having already weathered before they became covered in sediment.
Image (right): General view of the Qurta I site from the south. The scaffolding in the centre indicates the location of the main rock art panel (© RMAH, Brussels)
The study confirms the Qurta rock art to be the first known Pleistocene art in North Africa. Indeed, the carvings could be as old as 17,000 to 19,000 years, according to a report in December 2011 issue of the Quarterly Review of Archaeology, Antiquity (see “First evidence of Pleistocene rock art in North Africa: securing the age of the Qurta petroglyphs (Egypt) through OSL dating”. Antiquity vol 85, no 330, pages 1184-93).
Vandenberghe’s approach was to measure the ability of the sedimentary mineral grains to luminescence – or emit light. This property is both useful and challenging at the same time. It is useful because luminescence – which is due to the release of energy by minerals such as quartz builds up over time in the dark. The challenge, however, is that the luminescence clock is set to zero if a sample is exposed once again to sunlight. For archaeologists, this means that only samples of sediment that are still buried are suitable for analysis.
Fortunately, in 2008, geoarchaeologist Morgan De Dapper at Ghent University discovered that there were still some rock carvings at Qurta covered by sediment, and trapped beneath sandstone debris. He invited Vandenberghe to take samples for analysis by OSL, to determine when the sediment was deposited, thus recognising that this could yield a meaningful minimum age for the rock art.
Image (left): Sediment trapped between a rock art surface and a boulder of Nubian sandstone; a sample for luminescence dating was taken from this spot / Dimitri Vandenberghe
“We were lucky that this coarse debris had ensured that the sediment remained in place for many thousands of years,” said Vandenberghe.
The lead archaeologist Dirk Huyge, Curator of Prehistoric and Early Dynastic Egypt Royal Museums of Art and History in Brussels, delayed excavation so that Vandenberghe could take samples from the deepest layer of sediment that was still in contact with the rock art.
He then analysed the sediment in a dark room to show that it was around 15,000 years ago that the particles were last exposed to sunlight and blown by the wind onto the rock.
OSL is already widely used in earth science, and is becoming increasingly popular in archaeology as a useful alternative and complementary method to radiocarbon dating, which is limited to organic matters, requires calibration, and covers a more limited timespan. (For further background and case studies see the English Heritage “Luminescence Dating” guide (2008)).
“Most of what they encounter is sediment, and this is the only method that allows you to obtain an absolute age for sediment,” said Vandenberghe. He sees this type of study becoming increasingly important as archaeologists look to understand the geography around a site – such as when water may have been an important local feature, for example.
Image (right): optically stimulated luminescence dating equipment / Dimitri Vandenberghe
Writen by Julie Clayton