Sunday sand: Bagnold's grains

Classic sand grains of the desert dunes: fine-grained, some rounded, some quite angular, but all smoothed and frosted by, of course, the process of sand-blasting; and all with that veneer of iron mineral colouring. These are typical of virtually all desert dunes, but they come from, what is for me at least, a special place.

A couple of years ago, I wrote a piece about Ralph (pronounced, as is common in the UK, “Rafe”) Bagnold, and called it “The man who figured out how deserts work.” An explorer, soldier and scientist, he was an extraordinary individual, in the late 1920s and early 1930s pioneering long-range motorised desert travel, before laying the foundations of today’s understanding of desert sands, helping turn the tide of the war in North Africa, and then turning his attention to sand transport by water.

Having established the quantitative basis of wind-blown sand transport in the laboratory, in 1938 Bagnold set off on an expedition to revisit one of his old haunts in the Libyan Desert of southwest Egypt and test his lab results in the field. His base camp was set up beneath the cliffs of the vast plateau of the Gilf Kebir, the border of Libya just to the west and that of Sudan not far to the south. He chose an ideal field lab, camping along the flanks of a huge lee, or falling, dune, where the sand blows southward from the Great Sand Sea, and spills over the edge of a row of cliffs to form a long and elongate dune:

 
I was lucky enough, on what can honestly be described as a trip of a lifetime, to join an expedition in the footsteps of Bagnold’s 1938 venture and we visited the site of his base camp. In the top picture you can see the dune streaming off to the south, where, at its tip, it breaks up into smaller dunes (see the Google Earth image above, right). My colleagues are clustered around what are clearly the remains of a shelter that Bagnold’s team built (bottom, left), somewhat the worse for wear some seventy years on, but still not covered in sand – the dune largely passes them by. And the Shell kerosene cans used by the expedition are still lying around. Was this one opened by Ralph Bagnold? It’s from here, a special place, that the grains at the head of this post come. The sand grains that Bagnold slept on and worked with are likely long blown off on their travels to the south, but even so….

Some of Bagnold’s most important field measurements at the Gilf Kebir were wind velocities and the associated sand movement under different conditions. Here are a couple of photos of him at work there (courtesy of Stephen Bagnold), the one at left taken in a slight sandstorm:

 And here, in his own words from his autobiography, he describes such work:

My work necessitated waiting for a sand storm to blow up.….A heavy one blew up within a few days. I was well prepared for it except, alas, I had lost my sand goggles. I spent some very uncomfortable hours sitting in the open, directly exposed to a violent sand blast, trying to keep my eyes open while taking readings from an array of gauges and sand traps. The purpose of eyelashes was very evident. Fortunately, I managed to get some reliable measurements which nicely confirmed my wind-tunnel measurements made in London.

This was Bagnold’s last desert expedition – he had other things to keep him busy - but he wrote of being at home in England:

And then comes some trivial sense-impression—the hot-varnish smell of a car standing in the sun, a cloudless sunset, the finding of sand grains in the pocket of an old coat. Out comes the map again; and the eye hovers over some blank space still father away which nobody has yet reached. Happy calculations follow about petrol and distances—dreams of just one more desert trip.

I know what he meant – after all, where else can you wake up in the morning to a view like this?

Comments

[Blaize]

So, I went to Google maps to look more closely at the area, and wondered what the oval shapes just south of the falling dune are. On the satellite view they look like depressions, but I imagine they are rocks. I guess I just answered my own question. Anyway, I'm looking at the desert! From space! So thanks for posting this stuff about Bagnold.

[Sandglass]

Yes, rocks - isolated outcrops. But there are, in the region, all kinds of circular and oval features, often depressions, that have been the subject of much speculation with respect to meteorite impacts. A number of these features are volcanic craters with raised rims, it's been suggested that some are of hydrothermal origin, but some are definitely of impact origin. Quite recently one of these has been located to the South of the Gilf, close to the Sudan border - see http://www.sciencedaily.com/releases/2010/09/100923081902.htm. This can be seen on Google Earth if you zoom in on 22 degrees 01' 06.34" N, 26 degrees 05' 15.50" E.

Then if you head west from there towards the Libyan border and look around 22 degrees 18' N, 25 degrees 30' E, you'll see some of the volcanic craters with some spectacular associated dike swarms. I have some photos - indeed, the one in the "about" section of the sidebar of this blog was taken there, but I have geological ones too...

I have to say that the Google Earth folk have done an amazing job of stitching together the imagery from this are into an almost seamless whole - a few years ago it was just a patchwork of different resolutions and displays. The trouble is that, once you start poking around, it's difficult to stop!

Thanks for the comment!

[Blaize]

It IS really difficult to stop. I, too, have been very grateful for the more even resolution on Google Earth, although some of the places I look (the mountains of Tajikistan, for example) are still pretty fuzzy. Thanks for the coordinates. I'll be looking at them presently.

[Blaize]

Whoa. The resolution on those volcanic craters in unbelievable. What, if I may ask, is a dike swarm?

[Sandglass]

Oops - sorry about that - fell instantly into geo-jargon!

A volcanic dike forms when molten magma finds it way up to the surface through a more or less vertical crack in the rocks, and, if it reaches the surface, erupts (many of the Icelandic eruptions are like this, often called "fissure eruptions"). When everything has cooled off and solidified, the volcanic rock filling the crack is generally harder than the surrounding rock aand so, when erosion strips away the surface, the dike is left as a kind of topographic wall (Shiprock in New Mexico is a famous example). There's quite a good explanation at http://www.geology.um.maine.edu/geodynamics/AnalogWebsite/UndergradProjects2008/Nate%27s_website/Web%20Page/HTML%20Files/Introduction%20page.html

Dikes rarely occur singly but rather in "swarms", lots of them often radiating out from a volcanic center (again as at Shiprock). A good example of the ones I was referring to in Egypt can be seen if you zoom in around 22 degrees 12' N, 25 degrees 07' East - and tilting the Google Earth view shows the "walls" quite effectively. The odd thing about the volcanic features in this area is that they are a long way from where the continents are splitting apart (the Red Sea) and are situated essentially in the middle of old, stable, crust; but no-one seems to have done much work on them.

[Blaize]

Thank you for the explanation. I did not know the proper term for the Shiprock formation, though I did drive through that area with a geologist last September, and learned a ton.

[Google Map Listing]

The photos were great and attractive; these are one of the best sceneries I've ever seen. I enjoy reading your blog, and it is relevant to the daily life of the people now. I am looking forward to read more thoughts and ideas here in your site.

[Richard Bready]

Glorious photos, thanks. Speaking of Google Earth, did I get this from you, or am I telling you about it, or neither?
http://earthasart.gsfc.nasa.gov/image_index.html

[Sandglass]

Richard - welcome back!

I'm not sure of the answer to your question - I certainly have much enjoyed this great series of images, but exactly what the link itinerary was is lost in the mists of time (never mind my feeble brain).