Dune cross-bedding in action

Ron Schott recently posted some spectacular outcrop photos of the Navajo Sandstone showing superb dune cross-bedding – aeolian processes at work 200 million years ago. He wrote that “Sure, I’m a hard rock geologist, buy who wouldn’t be moved by sedimentary structures as beautiful as these.” Well, naturally, I certainly was. Below is an example of Ron’s photos, more of which can also be found on his flickr pages.

These images were inspiration to look back at my experience of watching these features form, live action, nature red in tooth and claw. A couple of years ago, on a clear candidate for the trip of a lifetime, I was revelling in the fantastic landscapes of the Western Desert, in the far southwest corner of Egypt, and a very long way from anywhere. We were camped in the midst of the superb dune field shown at the top of this post (I say “camped” but I had long since given up on a tent, much preferring to simply put a sleeping bag on the sand and relish the desert nights and mornings). That morning, as can be seen in the photo, the wind began to stir the sand, first into ephemeral plumes blowing off the crests of the dunes, and then, as it gathered strength, the landscape became kinetic. The notes that I made evolved into the opening of the deserts chapter of my book:

When sand moves under a gathering desert wind, it seems to take on a life of its own, to become a different form of matter—like a gas, like liquid nitrogen spilling and spreading, following the ground surface. Spraying off the crest of a dune, shimmering in the light, veils of sand race and ripple, spread and vanish, their place continually taken by the next gossamer sheet, dancing, playing, celebrating. Are these jinns, the spirits of the desert? The sight is beautiful and hypnotizing in the evening sun, but if the wind gathers speed, beauty rapidly vanishes as the violence and menace of a sandstorm grows. Suddenly, it seems as if the entire mass of desert sand has sprung from the ground to hurtle with the wind. On the surface, everything is moving, even the largest grains, rolling, tumbling, kicking smaller grains into the rushing current. The sky disappears, and the howl of the wind seems amplified by its cargo of sand. The air is filled with flying sand, unbreathable.

Suddenly, the entire crests of the dunes began avalanching down the steep slip faces, and, in somewhat challenging conditions for photography, I tried to record the way in which successive avalanches poured off the crests, the whole sequence of instability taking on the appearance of a curtain being drawn across the dune face. I’ve reproduced two successive photos below in an attempt to illustrate this. Both images are “anchored” at the same point on the lower left, and, if you trace the number of individual slides from left to right, you’ll see the change from one photo to the next. The curtain of avalanches continued to rush across the face of the dune – but I needed to put my camera away, out of the cloud of blowing dust and sand.

I’ve written before about Ralph Bagnold - “The man who figured out how deserts work”. He was, first and foremost, a scientist and an engineer, but occasionally he allowed his lyrical side to show through in his writing. Standing there that morning, transfixed by the drama unfolding in front of me, I was reminded of Bagnold’s description of exactly what I was watching:

Up above the great fine-grained crests of the dunes were on the move. Cornices dissolved as we looked, swaying along the curving surfaces in heavy dark folds, as if the mane of some huge animal was being ruffled and reset in a new direction by the gale.

This is exactly what was happening 200 million years ago in Ron Schott’s Navajo Sandstones: successive avalanches of sand carving away at the pre-existing surfaces of the dunes, piling up new layers at a different angle to the older ones, cross-bedding in action.

Such is the behaviour of granular materials – on all scales. You can replicate cross-bedding formation in the kitchen – as I did, and described in earlier posts, together with the role of the angle of repose. The photo of my experiment below is intentionally scale-less.

 

The Navajo Sandstone is justifiably famous for its dramatic displays of ancient dunes and cross-bedding. But I should point out that, in the UK, we have some quite dramatic examples too. For a long time during the Permian and Triassic, around 270 million years ago, much of the UK was a desert. The dunes are well-preserved in the photo below of the old quarry at Ballochmyle in southwest Scotland.

The Mauchline sandstone, as it was locally known, was for a long time a valued building stone, not only for much of Scotland, but also for export to the US. The peak of production at the quarry was around the turn of the nineteenth century, with dedicated railway transport and more than two hundred people employed. By the early years of the twentieth century brick had replaced stone as the construction material of choice, and the quarries closed by the 1950s. We may have great outcrops of aeolian cross-bedding in the UK, but, sadly, the Ballochmyle quarry is no longer one of them – it’s now filled with household refuse.

[The quarry photo comes from a great resource for UK geology that I have only just discovered. In the past, the British Geological Survey was a poor relation of its US counterpart in terms of web-based public domain resources. But now, their OpenGeoscience site contains, amongst other treasures, a collection of thousands of images – including comprehensive aerial photographic coverage of the recent floods that I described last month.]