Description
These dunes are straight to irregularly sinuous, elongate, sand ridges of loose, well-sorted, very fine to medium sand. The straight varieties are often called "sand ridges," and the sinuous varieties are often called "seifs." The lengths of individual dunes, which are much greater than the widths, can range from a few meters to many kilometers. Because of the temporal alternation of the slip faces on the ridge from one side to the other, both flanks can be steep, and loose sand will be found on the top of the ridge as well as on both sides near the top. Surfaces on the lower flanks of both sides of the ridge are wind compacted. These temporal alternations, which are caused by changes in wind direction, range in duration from daily to seasonal, depending on the regional wind regime.
Linear dunes can occur in simple, compound, and complex forms.
Linear dunes cover more desert area than any other type of dune, especially in central Australia, southern Africa, the southwestern Arabian Peninsula, and parts of the Sahara, but they are not as common (in the sense of occurring in almost every desert) as are the crescentic dunes.
Linear dunes have formed in areas now characterized by wide ranges of wind speeds and directions. Most are probably "fossil" dunes formed under more vigorous wind regimes during Pleistocene climatic conditions. Since then, wind regimes have apparently become less intense, although wind directions are apparently similar. Thus, where linear dunes are active today, they are commonly being modified into compound or complex features by the addition of secondary dunes. Nonetheless, the long axes of linear dunes are aligned generally within 15° of the prevailing wind or with the resultant drift direction of the local winds. The sinuosity and alternate slip faces develop because crosswinds change direction and alternately shepherd the sand to each side of the dune axes.
The wind regimes of linear dunes usually have a wider directional variability than those associated with parabolic dunes. or dunes in the crescentic group (barchans and transverse dunes), despite the strong directionality of their ground pattern. On small-scale imagery, such as Landsat, the appearance of fields of narrow, simple-to-compound linear dunes can resemble that of large-scale, barren, bedrock yardangs in grooved terrain. Other visual clues besides shape are needed to distinguish between them, such as vegetation, slip faces, color, and local geology.
The grain size of these loose, well-sorted, very fine to medium sands is about 0.06 to 0.5 mm. Linear dunes can be continuous for many tens and even hundreds of kilometers, and they can form parallel barriers to cross-country movement except down the interdunal passages, which are parallel to the long axes of the dunes (see photo). Interdunal passages are frequently called corridors or "streets". Their surfaces are usually stabilized and firm, although they might contain coppice dunes, playas, or other obstacles. Two adjacent dunes often join. The resulting Y junction almost always opens into the prevailing wind and thus closes in the downwind direction (see arrow in photo).
Compound linear dunes typically have many subsidiary oblique ridges whose junctions with the main ridge also open into the wind. Interdunal corridors are sometimes closed by secondary dunes joining the main ridges. In spite of the blind alleys and the interconnected, mazelike nature of these interdunal corridors, they provide the only practical passage through the dune field for foot and vehicular traffic. Areas of loose sand and active dunes cannot be crossed unless the dunes are very small (a meter or so high) or wet, following rain. Where vegetation is visible on the dune surface, vehicles can probably "bull" their way straight up and over the dune and into the next corridor. Travel up the stable flanks may need to follow an oblique path, not perpendicular to the crest, depending on the steepness and surficial characteristics. These judgments can be made at the site.
Serious problems for navigation include the restricted field of view from the floor of the interdunal corridor, the similarity of the adjoining, repeated dune ridges, and the limited view from the tops of the dune ridges--all that can be seen are the flanks and tops of the immediately adjacent ridges and the tops of more distant ridges.
So far as we are aware, except for parts of northern Africa, southern Africa, and central Australia, maps do not show the necessary surface details to support cross-country navigation. Without air photos or equivalent imagery, passage through such a field will be, at best, a time-consuming trial and error procedure, and perhaps impossible. In such a case, the best procedure is to head upwind to avoid being "boxed in" by two dunes that join. The restrictions on the horizontal field of view within dune fields make possible some level of horizontal and oblique cover and concealment from aircraft. However, there is no cover and concealment from overhead aircraft, or from units within the same interdunal area.
(common names are bold) Longitudinal dunes, seifs, sand-ridge dunes, oblique dunes, sigmoidal dunes, slouk, silk, alab, uruq, whalebacks (some).
None.