In Sand: A Journey Through Science and the Imagination, geologist Michael Welland weaves together the many facets of sand – its science, its art, its music, its metaphorical power. At every scale, from grain to sand pile to vast deserts, sand is an extraordinary substance. Did you know, for instance, that the sand dunes of Morocco hum a G#? In this excerpt from the book, Michael Welland talks about the pioneering work of sand expert Ralph Bagnold.
The journey of a sand grain tumbling in the wind is a complex one, and while many of the aspects of that journey are understood, there is much, again, that is not. The foundation of what we do know, and of the research desert landscapes that continues today, is entirely the result of the pioneering work of one man (of whom we have already heard)—Ralph Bagnold. Today’s academic textbooks on sand transport often include advice along the lines of ‘for inspiration, read Bagnold (1941)’.
Bagnold’s early encounters with sand occurred after he was posted to Egypt in 1926. Shortly after his arrival in Cairo, he watched the first successful excavation of the Great Sphinx: ‘I watched the lion body of the Great Sphinx being slowly exposed from the sand that had buried it. For ages only the giant head had projected above the sand. As of old, gangs of workmen in continuous streams carried sand away in wicker baskets on their heads, supervised by the traditional taskmaster with the traditional whip, while the appointed song leader maintained the rhythm of movement’ (Sand, Wind, and War). It was never an ideal place to construct one of the world’s great monuments. Arguments about the age and meaning of the Sphinx still rage—there are limitations to the wisdom of that which, according to the Sphinx’s riddle, goes on four legs in the morning, on two legs at noon, and on three legs in the evening (the answer being humankind). However, its link with the building of the pyramids is clear, and King Khafre (or Chephren) was the likely builder. The Great Sphinx has spent most of its existence largely covered by the continuously drifting sand, with only its head, blasted and worn by that sand, protruding. The ravages suffered by the head confirm that the sand that buried the body has been its salvation, preserving it from abrasion. In spite of its role over the centuries as an inspiration for archaeologists, poets, travellers, and those who believe it was built by refugees from Atlantis, its life has largely been like that of an iceberg, demurely hiding its bulk beneath the surface.
It was not until early in the nineteenth century that serious attempts at excavating the Great Sphinx were made, but these were defeated by the enormous volumes of sand involved. Further efforts in 1858 and 1885 revealed a good part of the body and some of the surrounding structures, but these attempts were again abandoned. The Great Sphinx had to wait until 1925 and the arrival of the French archaeologist Émile Baraize for its full glory to be revealed. Removal of the vast quantities of sand required eleven years of labour.
In watching the results of natural sand movements on a staggering scale, Bagnold perhaps had an inkling of the way in which his future would be intimately driven, grain by grain, by sand. The insatiable curiosity that had blowing in the wind possessed him since childhood—from an early age he was ‘aware of an urge to see and do things new and unique, to explore the unknown or to explain the inexplicable in natural science’ (Sand, Wind, and War)—would carry him through an extraordinary diversity of accomplishments until his death in 1990 at the age of 94, still in full stride.
Bagnold was one of those larger-than-life characters, but he was also, unusually, deeply modest. In spite of his scientific achievements and accolades, he always regarded himself as an amateur. His obsession was with seeking the truth through well-designed scientific experiment and observation, unsullied by conventional wisdom or tradition. Peacetime soldiering, as Bagnold observed soon after arriving in Egypt, left time for extracurricular activities, and so he set off on adventurous excursions into the desert that, over the next twelve years, until 1938, would become more and more ambitious and extensive, earning him his place among the pioneering explorers of the desert. The Western Desert of Egypt in the 1920s was a little known place outside the great oases; the maps were conspicuously empty, the words ‘limit of sand dunes unknown’ providing the only description to be found in otherwise large areas of blankness. Bagnold regarded these words as a challenge, and whereas journeys by previous explorers had largely taken place on the backs of camels, he set out by car—first the Model T Ford and later the Model A, painstakingly adapted for the desert and the need to be entirely self-sufficient, particularly in water supplies. In determining that the giant dunes of the erg of the Great Sand Sea could be crossed by car, Bagnold made close and meticulous observations of the nature and quality of sand—early work on the physics of granular materials. Bagnold’s method of crossing dunes required careful selection of the right location and then full-frontal assault, flying up the side of the dune in a cloud of sand. He recognized that while the sand might be quite soft, the way in which the grains were packed together created a firm enough foundation for a truck to leave only shallow tracks from its deflated tires. Nevertheless, he recognized equally that the sand was ‘unreliable’ and, depending on its location within the dune system, could contain ‘pools’ of unconsolidated sand into which a vehicle could sink, instantly and deeply.
In battling his way across the Great Sand Sea, Bagnold realized that there were three fundamental questions about sand dunes and sand transport that had perhaps never been asked and certainly had never been answered:
1. What determines the distinct shape of the different kinds of dunes, and how do they retain that shape while moving inexorably across the desert?
2. Why does sand gather itself into dunes at all, rather than being spread evenly over the desert floor? Why is sand self-accumulating?
3. How do individual sand grains interact with each other and with the wind to feed the dunes?
Being of an analytical and inquisitive nature, Bagnold decided that the only approach was to go back to basics, ignore the few theories that had been put forward to that point, and conduct some carefully designed laboratory experiments. He was provided space at Imperial College in London, where he proceeded to design and build a wind tunnel within which he could blow (or suck) sand grains under controlled conditions and carefully collect the quantitative data that he needed. It was an exquisite piece of apparatus, and the results were revolutionary. The experimental data, blowing in the wind combined with the thousands of painstaking analyses of sand-grain sizes that Bagnold had assembled through endless sieving, culminated in The Physics of Blown Sand and Desert Dunes, finally published in 1941 while Bagnold was causing logistical havoc behind enemy lines in North Africa through the activities of his Long Range Desert Group. The book remains a masterpiece of scientific enquiry and analysis—it was used as a reference by NASA for planning Moon and Mars missions.
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