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Author Topic: THE SAHARA  (Read 4718 times)
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« Reply #15 on: May 17, 2009, 09:00:18 am »

Rock Art

The flats of Wadi Erni may have offered good fishing in the broad shallows, where rock art indicates humans may have been joined in the water by hippo.

After a night under a canopy of a million stars, we break camp and split into two groups—one will look for archeological sites and the other, with Mather, for alluvial fans and other datable geological formations.

To most reliably date events like ancient climate change, several indicators are best, to cross-check one against another.

Mather’s passion is the alluvial fans, which happen when water flows erode hillsides into flat valleys and the transported sediment forms large, delta-like structures. If these can be dated, they could indicate when the region last experienced regular heavy rainfalls. Before coming here, she pored over satellite photos to pinpoint fans that looked promising.

We bounce over rocks and soft sand with our GPS units at arm’s length out the window, looking for Mather’s fans. She looks at escarpment after escarpment. “Don’t know if these are alluvial,” she says. “They all look colluvial to me.” (Colluvial means they were built up through rockfall.) “We really need a place where there is some catchment area.”

When she finds the right one, it will be a prize.

“The newest sediments end up on the bottom of the fan and the oldest near the top,” she says. She wants to date the newest, because that’s the last time great volumes of water flowed here.

But she says she needs fans with well-defined layers deep enough to yield a sample that she could
date by optically-stimulated luminescence (OSL).

OSL is expensive—around a thousand dollars per sample—but it provides a pretty accurate measure of when the soil was last exposed to sunlight.
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