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- Herausgeber: John Wiley & Sons
- Kategorie: Wissenschaft und neue Technologien
- Sprache: Englisch
The wild beauty of deserts has always been a source of fascination the world over. Mankind and Deserts 3 the third and final volume focuses on wind, frequently present in all deserts, either hot or cold. Wind plays a major role in aridity and landscapes bear numerous forms due to its action, erosion, transportation and surface formations, some discreet and others spectacular, such as vast expanses of towering yardangs. Aeolian dynamics lead to dune formation, simple or associated with sand ridges or ergs, as in the Sahara. Mankind has attempted, to varying degrees of success, to cope with sand accumulation; ignoring aeolian dynamics has led many development projects to failure. This is developed by Yann Callot, a Professor at Lyon University who studied aeolian dynamics in the Sahara. Traditional societies have adapted to live in deserts, establishing vibrant civilizations with original ways of living, managing water resources and creating routes for trade, especially for salt. In a changing environment, useful lessons can be drawn from the genius of mankind s adaptation to such diverse and fragile environments. This is explained by Marc Cote, who was a Professor at Constantine (Algeria) and Aix-en-Provence Universities. From ancient, almost mythical, exploration to modern scientific studies, deserts have come to be better known yet still hold great appeal. This book traces the history of their knowledge while providing a basis for understanding their features and the tools needed for their protection, in an ever-changing world.
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Table of Contents
Cover
Title Page
Copyright
Foreword
Introduction: Aeolian Dynamics and Processes
I.1 Conditions and environments in which wind acts
I.2 Aeolian processes
I.3 References
1 Aeolian Landforms in Deserts
1.1. Forms and surface states
1.2. Ablation forms
1.3. Accumulation formations
1.4. Aeolian systems
1.5 Ergs, the most complex aeolian systems
1.6 Conclusion: a new paradigm to explain the organization and orientation of active dunes
1.7. Martian dunes, still mobile…
1.8. References
2 Humans and Winds in Deserts
2.1 Traditional societies and the wind
2.2. The battle against sand encroachment, or the Barrel of the Danaids: the Zouerate railroads (Mauritania)
2.3 The Dust Bowl, the first ecological disaster of the 20th century in a developed country
2.4. The western Algerian Green Barrier, or an error in management
2.5. References
3 Living in Deserts
3.1. Humans and the desert
3.2. Bedouin civilizations
3.3. Hydraulic civilization
3.4. A salt civilization?
3.5. Urban civilizations in the desert
3.6. Conclusion
3.7. References
List of Authors
Index
End User License Agreement
List of Illustrations
Introduction
Figure I.1. A sandy spit that curves around relief in the SE of the Tibesti rang...
Figure I.2. Spontaneous Markouba steppes protecting the Nouadhibou – Zouerate ra...
Figure I.3. Distribution of areas with wind deposits around the globe (as per Th...
Figure I.4. SEM image of a wind-blown sand grain (in the Taghit region of Algeri...
Figure I.5. Relation between the ablation velocity, fall velocity and granulomet...
Figure I.6. Dust falling over Aleg (Mauritania) in April 2001. Colors not modifi...
Figure I.7. How the roughness of the substrate influences saltation (as per Bagn...
Figure I.8.
Blowing sand in the Tassili of Hoggar, between
Tamanrasset and In Gu...
Figure I.9. The mechanism of formation of a deflation pavement: (A) the initial ...
Figure I.10. A vehicle that has “fallen” into a fech fech outcrop of the Mekhera...
Figure I.11. Satellite image of the area where the “river of sand” begins in Cap...
Figure I.12. Views of the soil in the area where the “sandy river” of Cape Juby ...
Chapter 1
Figure 1.1. Reg. For a color version of this figure, see www.iste.co.uk/bourrie/...
Figure 1.2. A serir close to Doush (south of the Kharga depression, Western Dese...
Figure 1.3. Discontinuous rust layer in the Kharga depression (Western Desert, E...
Figure 1.4. Dreikanter in Ténéré (Niger) (photos F.M. Callot). For a color versi...
Figure 1.5. Corrasion lines on a sandstone outcrop in Nubia, in the Kharga depre...
Figure 1.6. Corrasion grooves on a limestone block that is about 1.5 m high, loc...
Figure 1.7. Árbol de Piedra, a mushroom rock that is 7 m in height in the high-a...
Figure 1.8. Yardang in a homogeneous coherent rock in a region with monodirectio...
Figure 1.9. Line of yardangs along an Appalachian outcrop of hard rock in the Se...
Figure 1.10. Yardangs in the Doush region (Kharga depression, Western Desert, Eg...
Figure 1.11. Overview of the Ridge-Corridor system in the SE of the Tibesti (sou...
Figure 1.12.
The central region of
kaluts in the Lut desert (Iran) (source: Goog...
Figure 1.13. Ridge-Corridor Systems (RCS) in the Lut desert (Iran) (source: Goog...
Figure 1.14. El Hobeur hydro-aeolian depression, to the west of the Oued Namous ...
Figure 1.15. Area with sand nappages in presaharan Tunisia. In a case like this,...
Figure 1.16. Relationships between the topography, wind direction and aeolian sa...
Figure 1.17. Ripple marks on the two faces of a sif (north of the Grand Erg Occi...
Figure 1.18. Megaripples. For a color version of this figure, see www.iste.co.uk...
Figure 1.19.
Diagram and cross-section of a nabkha
Figure 1.20.
Schematic cross-section of a rebdou
Figure 1.21. Rebdous at the foot of Issaouanne n’Tiffernine (Algeria). The struc...
Figure 1.22. Zibars in the Erg of Fachi-Bilma (Chad-Niger). For a color version ...
Figure 1.23. Stratification within a sif (according to Tsoar (1982)). Note the n...
Figure 1.24. The chief components of the barchan (plane and cross-section, accor...
Figure 1.25. Crumbling slope of a barchan, near Tarfaya (Morocco). The gently sl...
Figure 1.26. The transition from a shield, a shape with no defined edges, to a b...
Figure 1.27. Barchan formations to the east of (NW Mauritania) with all the inte...
Figure 1.28. Barchan, called “Dune Coursin”, close to Boulanouar (NW Mauritania)...
Figure 1.29. Barchans with wings in opposite directions 8 km south of Tarfaya (M...
Figure 1.30.
Orientation of silks (source: Google Earth)
Figure 1.31. The simplest kind of ghourd (Algeria, Grand Erg Occidental, NW of T...
Figure 1.32. Ghourd with four edges converging on the summit, or top, in the Nam...
Figure 1.33. Large, complex ghourds at the meridional end of the Issaouane n’Tif...
Figure 1.34. The ksar of Taghit, at the foot of the ghourds on the western borde...
Figure 1.35.
Two types of Aklés in Algeria (source: Google Earth)
Figure 1.36. The lunette at the Sebkha d’Oran (sketch) constructed leeward of th...
Figure 1.37. Sand arrows leeward of the Fachi “cliff” on the meridional border o...
Figure 1.38. Two dextrous elbs about a 100 km to the NW of Dirkou (Niger) (sourc...
Figure 1.39.
Draas in Algeria (source: Google Earth)
Figure 1.40. A sahane surrounded by ghourds, silks and aklés in Edeyen d’Oubari ...
Figure 1.41. Feidj in the Erg of Fachi-Bilma (Niger), almost entirely covered in...
Figure 1.42. Renewed erosion in dunes in the Ogolian erg (Mali). These fixed dun...
Figure 1.43. Sand ridges in the Simpson desert 140 km west of Birdsville (source...
Figure 1.44. Sand ridge on the eastern border of the Simpson Desert, 40 km from ...
Figure 1.45. Arrangements of dunes in the northeastern part of the Grand Erg Occ...
Figure 1.46. Ground view and cross-section of the large dunes of the erg. For a ...
Figure 1.47. Northern part of the erg in the Registan, clearly limited by the wa...
Figure 1.48. Arrangement of dunes in the southern part of the central section of...
Figure 1.49. The central and septentrional part of the Grand Erg Occidental (sou...
Figure 1.50. Structure of the Grand Erg Occidental (as per Callot 2008, modified...
Figure 1.51. Relationship between stationary oscillations and relief features al...
Figure 1.52. Simulation of the differences in the creation of the formations in ...
Figure 1.53. Dunes leeward of a Martian plateau in the Hellespont region (latitu...
Figure 1.54.
Detailed views of Martian dunes
Figure 1.55. Mars dune blocking a hill, photo from the Curiosity Rover (source: ...
Chapter 2
Figure 2.1. Fixation of dunes on the left bank of the Niger, east of Timbuktu (p...
Figure 2.2.
Cross-section of an afreg
Figure 2.3.
Afregs protecting the Akabli oasis (Algeria). The first dune is over
...
Figure 2.4.
Afregs in the Douz region (southern Tunisia). The dune is about
15 m...
Figure 2.5.
Location of the Nouadhibou–Zouerate railroad. The zones marked
1
and
...
Figure 2.6. On the Nouadhibou–Zouerate railroad, workers constantly clear one pa...
Figure 2.7. Techniques for destroying a barchan that threatens the Nouadhibou–Zo...
Figure 2.8. Extent and thickness of loess in the United States with respect to t...
Figure 2.9. Two “identical” photographs of Garden City (Kansas) on Black Sunday ...
Figure 2.10. Extension of the Dust Bowl and the migrations associated with the p...
Figure 2.11. A farmer watching a dust storm in Cimarron county, Oklahoma (photo ...
Figure 2.12. A farm buried under aeolian deposits in Dallas (South Dakota) in 19...
Figure 2.13. Septentrional Algeria. The Green Barrier was chiefly established at...
Figure 2.14. Precipitations in western Algeria and Morocco, as per Dubief 1959–1...
Figure 2.15. The lands of the Green Barrier, near the city of El Bayadh, in a se...
Figure 2.16. The Green barrier in the Naama region, Algeria (High Plains in the ...
Figure 2.17. Aeolian formations around the Kheneg Menhar (meridional dustfall fr...
Figure 2.18. Aeolian formations around Kheneg Menhar (the meridional skirt falli...
Figure 2.19. Google Earth image of the coalescent barchans, SW of the Djebel Men...
Figure 2.20. Directions of the migration of sands and the barchans of Kheneg Men...
Figure 2.21. Wind roses showing the potential migration of sands at the Aïn Séfr...
Chapter 3
Figure 3.1.
The Arid Diagonal within the ancient world
Figure 3.2. Area covered by the desert and area where Islam spread and was estab...
Figure 3.3.
Transhumance in the mountains of Afghanistan
Figure 3.4.
Transhumance in the Saharan Atlas
Figure 3.5. Pastoralism over small distances. The area traveled by the Ouled Sid...
Figure 3.6. Nomad territories in Syria. Spaces where transhumance is practiced o...
Figure 3.7. Nomad territories in the Malian Sahel, in South Gourma, in the loop ...
Figure 3.8. Nomad territories in southern Tunisia. The territories have been div...
Figure 3.9. Tribal borders and political frontiers. Territories of the four larg...
Figure 3.10. Traditional water-lifting systems: Archimedes screw, shaduf, noria ...
Figure 3.11. The foggara technique. Technique for underwater drainage of a water...
Figure 3.12. An example of the Saharan hydraulic systems. Gourara, Touat, Tidike...
Figure 3.13. An example of a Saharan hydraulic system: the oases in southern Mor...
Figure 3.14. Hydraulic systems in the Arid Diagonal. Hydraulic civilizations wer...
Figure 3.15. The hydraulic system in Mesopotamia. These plains owe their rich so...
Figure 3.16. The hydraulic system of the Indus Valley. “Today, the developed Ind...
Figure 3.17. Some fundamental limits in the deserts of Arid Diagonal. Limits of ...
Figure 3.18. State infrastructure and individual land-use. Perimeter of the agri...
Figure 3.19. Saharan architecture. Some examples of a ksour, large compact burgs...
Figure 3.20. The Silk Route, a route that connected countries from the Levant to...
Figure 3.21. The Gold Routes. The caravan trails between the northern and southe...
Figure 3.22. A desert city, an Islamic city, Timbuktu. A traditional African cit...
Figure 3.23. Supplying water in the desert. A belt of seawater desalination unit...
List of Tables
Chapter 1
Table 1.1. Computation of the transport coefficients based on the original table...
Guide
Cover
Table of Contents
Title Page
Copyright
Foreword
Introduction: Aeolian Dynamics and Processes
Begin Reading
List of Authors
Index
End User License Agreement
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Series Editor
Françoise Gaill
Mankind and Deserts 3
Wind in Deserts and Civilizations
Edited by
Fernand Joly
Guilhem Bourrié
First published 2021 in Great Britain and the United States by ISTE Ltd and John Wiley & Sons, Inc.
Apart from any fair dealing for the purposes of research or private study, or criticism or review, as permitted under the Copyright, Designs and Patents Act 1988, this publication may only be reproduced, stored or transmitted, in any form or by any means, with the prior permission in writing of the publishers, or in the case of reprographic reproduction in accordance with the terms and licenses issued by the CLA. Enquiries concerning reproduction outside these terms should be sent to the publishers at the undermentioned address:
ISTE Ltd
27-37 St George’s Road
London SW19 4EU
UK
www.iste.co.uk
John Wiley & Sons, Inc.
111 River Street
Hoboken, NJ 07030
USA
www.wiley.com
© ISTE Ltd 2021
The rights of Fernand Joly and Guilhem Bourrié to be identified as the authors of this work have been asserted by them in accordance with the Copyright, Designs and Patents Act 1988.
Library of Congress Control Number: 2021930490
British Library Cataloguing-in-Publication Data A CIP record for this book is available from the British Library ISBN 978-1-78630-632-6
Foreword
Mankind and deserts
Fernand Joly1 departed from this world before he was able to complete this book, through which he had hoped to share his experiences of and passion for deserts.
“Yet another book on deserts!” some might think; another book to add to the numerous publications dedicated to these alien and fascinating worlds.
This book, however, is different from earlier books, as can be seen from its title “Mankind and Deserts”. It is based on the singular relationships that are formed between humans and the world of the desert – relationships that are unique because they can be traced back to the very origins of humanity. Indeed, it is from the arid Horn of Africa (East Africa) that large migrations began and it is along the deserts, if not within the deserts themselves, that we find the major cradles of burgeoning historical civilizations. This inhospitable world is also associated with great spiritual leaders such as Moses, Jesus, Mohammed and the Buddha, while serving as the backdrop for adventurers and empire builders from Alexander the Great to Genghis Khan, or from the Incans to the Conquistadors in the Andes and Mexico.
What is this universe that is so barren and yet so mesmerizing?
“All about a word” was how Fernand Joly introduced his book: “What is a desert?” The ambiguity in this word results from the fact that it has been used in different senses across literature and throughout history. For a geographer-writer such as Fernand Joly, the one fact that stood out was that there was no one desert; instead there were multiple deserts, diverse and varied, ranging from Death Valley to the Kalahari, from the Namib to the Atacama or the Gobi desert. Each of these is a unique landscape, whose uniqueness was born out of its position with respect to the general atmospheric circulation, its geographic location with respect to the sea and to its relief features. And yet, transcending all differences, there is one constraint that binds them all together: aridity, defined as a natural physical state characterized by persistent dryness with the corollary of extremely scarce water resources. Both these concepts, aridity and water, are at the heart of the following chapters. Aridity (Chapter 3) because it “transcends time and takes over space” and water (Volume 2, Chapters 1–3) because it is the essential resource for all life, especially in the desert. Aridity is distinct from “drought”, which is simply a “period with insufficient rainfall”. Water is seen through the lens of how it appears on land: “wild water”, which flows over slopes in an un-channeled manner (Volume 2, Chapter 1), under the impact of violent but brief downpours, and “concentrated waters”, i.e. waters “concentrated” into a channel, fed by distant precipitation upstream of the borders of the desert. As can be seen, there is in fact a true hydrography of the arid world. Satellite images, among other sources, offer us clear and accurate reproductions of these systems: fossil hydrographic networks, the legacy of ancient humid periods, a map of intermittent water bodies (Volume 2, Chapter 2): playas and sabkhas, permanent lakes with fluctuating shorelines, such as Lake Chad or Lake Eyre, or large allogenous rivers (Volume 2, Chapter 3) that are born outside the desert but travel through the desert, sustaining life, such as the Colorado, the Niger and the Nile, “the first and most remarkable of rivers in the arid world”.
The role played by salts in hot deserts is rarely discussed in a systematic manner. Guilhem Bourrié, geochemist and soil scientist at INRAE, has analyzed the origins and nature of these salts and demonstrated how important these salt deposits in the desert are for humans, whether they live off agriculture, livestock or, indeed, the salt trade (Volume 2, Chapter 4).
Chapter 1 of Volume 3, drafted by Joly, was edited after his demise by Yann Callot, a professor at the University Lyon 2 who is a specialist in ergs and dunes. This chapter examines the importance of wind in the desert. Wind, sometimes considered to be more emblematic of a desert than even dryness, counts among the earliest dynamics on Earth, an element that humans have not always been able to control. Indeed, this lack of understanding of wind has sometimes had disastrous consequences for certain projects (see the Green Dam in Algeria).
The final chapter in Volume 3, “Living in the Desert”, was taken up by Marc Côte, Professor Emeritus at the University of Provence, who worked as a professor for 20 years at the University of Constantine. He has drawn on his deep knowledge of the land and the people of the Saharan region to present what he calls “The Desert Civilization”.
Finally, it must be noted, with great regret, that, since 2010, “geopolitical turbulences have tended to change the fundamentals, to burn away knowledge and to prevent researchers from keeping in touch with this part of space and humanity.”
Acknowledgments
Most of the illustrations were refined by Éliane Leterrier.
Yvette DEWOLF
Honorary Professor at the University Paris VII, Denis Diderot
Paris
January 2021
A man in the desert: Fernand Joly, Tademaït, Sahara, Algeria
1
Professor at the University Paris VII, Denis Diderot, who spent 15 years at the Moroccan Institute of Science in Rabat.
Introduction: Aeolian Dynamics and Processes
I was in a world of stark, pure sand dunes stretching away to infinity on either side in strict geometric forms, somewhat affected by the winds but only inasmuch as the same shapes were re-formed (from that point all is calm).
Jean Giono, “Noé”, in OEuvres romanesques complètes tome III, R. Ricatte (ed.), Gallimard, Paris, 1974, p. 839.
The action of the wind is perceived to be so omnipresent and preponderant in arid regions that it is still sometimes considered to be even more emblematic of deserts than dryness. Is this why there are so many errors and false information around wind and its best-known manifestation, the dunes? The uniqueness of wind-formed shapes and wind action is the most likely reason for this fascination, leading to some researchers (sometimes quite influential ones) succumbing to the mirage of the wind and overestimating its actions. As we will see, however, the facts are more complex: striking effects are not necessarily synonymous with effectiveness.
Aeolian processes (“aeolian” comes from Aeolus, the Greek god of the wind) are among the most original geomorphological processes in the world. The reason behind this is simple: the circulation of material through the air partially frees the wind, especially over a medium scale, from topographicalconstraints (Figure I.1). The wind thus enjoys relative freedom vis-à-vis gravity and, therefore, vis-à-vis pre-existing slopes and relief. For example, along with glaciers, it is the only entity capable of moving material upwards, as certain developers have learned the hard way!
Figure I.1.A sandy spit that curves around relief in the SE of the Tibesti range, in a region with almost unidirectional wind (image from Google Earth). For a color version of this figure, see www.iste.co.uk/bourrie/deserts3.zip
Given the unique feature of wind actions, we will proceed as follows: we will first more closely examine the conditions and processes that make it possible to form and, sometimes, remake wind formations or aeolian landforms; we will then take a differentiated approach based on the environment and associated formations. Finally, we will conclude with a few representative examples of the interactions (often disastrous!) between the winds and humans.
I.1. Conditions and environments in which wind acts
I.1.1. Where is the wind effective?
The wind is undeniably unique: it blows across the entire surface of the earth, with varying intensity that is dependent on a number of criteria. These differences can be observed on a global scale. For example, the vast stretches of the Southern oceans are swept by atmospheric depressions that are specific to certain Southern latitudes: the Roaring Forties and Furious Fifties, which are much sought after in sailing races. On the continental level, there are calmer regions, such as central Siberia. The great Canadian north, which is less cold than Siberia, is sometimes considered to have a more difficult winter as it has more violent winds. There may also be marked differences at a local level. A remarkable example of this is the rhodanian corridor: when the northern mistral blows, the Rhone valley, oriented N-S, or the plateaus that ring it, may be battered by an intolerable mistral, while tributary valleys (such as those with rivers that descend from the Cévennes), which are oriented W-E, experience an almost summery micro-climate.
The omnipresent nature of the wind may lead us to conclude that aeolian landforms exist on all continents. However, this is not the case in reality except over very small areas. Indeed, a few conditions must be met in order for wind dynamics to be effective:
1) Strong and frequent winds are a favorable condition, but not essential. If all other conditions are satisfied, even weak winds can create large aeolian landforms. However, there is a threshold velocity for aeolian erosion below which the wind cannot displace any material. The displacement threshold for sandy materials can vary according to local conditions and, notably, depending on the size of the material being transported. However, it most often lies between 4.5 and 6 m/s, at a height of 10 m. This will be examined in greater detail when we study the quantities of sand that can be displaced by the wind.
2) However, this first condition is not sufficient. In many regions, the wind by itself is not very effective on firm land except during exceptional storms, such as the storms that hit France in December 1999 or Cyclone Xynthia in 2010. It is the substances themselves, which are transported within very precise granulometric limits, that carry out significant morphogenic actions for accumulation and ablation. However, the wind must find available material to transport. Materials that can be picked up and carried are sand, dust (see below), and also volcanic ash, as is sometime seen in Iceland.
3) The previous condition presupposes the existence of at least some portion of bare ground. It thus implies the absence, or at least discontinuity, of plant cover, which usually binds materials and blocks wind action through the roughness and cohesion that it introduces. The reason for this is the strong relationship between green cover and the availability of material. Vegetation causes the soil surface to develop a kind of “roughness” that is so effective at disrupting and preventing displacement by wind that when there is any plant cover over 30%, the wind becomes completely ineffective (Marticorena and Bergametti 1995). The effectiveness of this green cover can be seen along the Britanny coast where tempests simply ruffle through the tips of the grassy, prairie covering. The difference is all the more marked if this prairie abuts a cultivated plot of land with bare, exposed soil!
There are any number of situations that fall between covered soil and partially or totally bare soils, and which govern the extent to which aeolian activity occurs. Hot and cold deserts are regions where aeolian activity prevails. A study was carried out in conditions of pronounced aridity, in north-west Mauritania, to examine the fight against sand accumulation on the railroad tracks linking the Zouerate iron mines with the Nouadhibou port and to measure how effective green cover was. It was seen that over one particular zone, measuring a few hundred meters, sand never accumulated on the railroad tracks through wind action. This was an area covered by a few acacias that are too scattered to be effective, and a steppe that is not very dense and almost mono-specific, with a tall, perennial grass that is locally called Markouba (Panicum turgidum) (Figure I.2). A series of 20 wind-speed measurements in this Markouba zone gave an average of 9.1m/s at a height of 2 m and 1.5m/s at a height of 30 cm; just behind a green knoll, the wind was almost zero, between 0.4 and 0.1m/s.
These conditions may be present-day conditions or may have existed in the recent geological past, during the last quaternary climatic variations, or much further in the past: Paleozoic aeolian sediments are of interest for their hydrocarbon potential (Glennie 1970; Alsharhan et al. 1998). It is often useful, therefore, to distinguish between actual, functional formations (such as active dunes) and ancient formations (such as loess or certain covering sands).
I.1.2. The environments where wind action occurs
The necessary conditions, listed above, limit wind action to areas where they are present and where several causes may combine.
I.1.2.1. “Natural” regions
This term denotes regions where natural conditions are the principal factor allowing wind action. This does not exclude human interventions as a factor that limits or, alas!, exacerbates aeolien action. These regions are as follows:
1) areas in littoral zones where beaches combine the absence of vegetation with an inflow of sand that may be large;
2) areas around volcanoes where there may be abundant ashes and where unique aeolien phenomena may occur, linked to the shockwaves associated with the explosion;
3) regions where harshness of climate either prevents or slows vegetation growth. These are where aeolian landforms are most often seen.
Figure I.2.Spontaneous Markouba steppes protecting the Nouadhibou – Zouerate railroad, which can be made out in the background. Even such a loose formation is remarkably effective at preventing aeolian action. For a color version of this figure, see www.iste.co.uk/bourrie/deserts3.zip
In this last case, the limiting factors may have two origins:
– cold glacial and periglacial climates. The formations could be inherited, like European loess, or present-day formations, further north, in areas where plant cover does not protect the soil;
– arid and semi-arid environments in hot or temperate regions, which are the subject of study in this book, with very little precipitation and high evaporation. Such regions are found in several parts of the world (
Figure I.3
): Australia, with its interior deserts, the North American West and the arid diagonal through South America; the western part of southern Africa and, above all, the vast arid arc from the Atlantic Sahara to West Africa.
Figure I.3.Distribution of areas with wind deposits around the globe (as per Thomas 1997, modified). Some areas are exaggerated so they are visible on the scale of this map
This arc covers the whole of the Sahara and its extensions into East Africa. This then continues, interrupted only by the Red Sea and Persian Gulf, into
