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Schematic of coastal parabolic dunes. U-shaped mounds of sand with convex noses trailed by elongated arms are parabolic dunes. These dunes are formed from blowout dunes where the erosion of vegetated sand leads to a U-shaped depression. The elongated arms are held in place by vegetation; the largest arm known on Earth reaches 12 km.
In those systems, other kinds of dunes may be closest to the water. [1] [5] Landscape of Northern California parabolic dunes. A foredune ecosystem begins with the first dune ridge directly behind an active beach. The ridge of a foredune can range in height from a few meters to tens of meters tall.
A depression may then develop until it hits a non-erodible substrate, or morphology limits it. The eroded substances climb the steep slopes of the depression and become deposited on the downwind side of the blowout which can form a dune that covers vegetation and lead to a larger depression; a process that helps create parabolic dunes. [1]
As barchan dunes migrate, smaller dunes outpace larger dunes, catching-up the rear of the larger dune and eventually appear to punch through the large dune to appear on the other side. The process appears superficially similar to waves of light, sound, or water that pass directly through each other, but the detailed mechanism is very different.
For example, there are ergs or fields of linear dunes, of crescentic dunes, of star dunes, and of parabolic dunes, and these dune arrays tend to have consistent orientations and sizes. [10] [11] By nature, ergs are very active. Smaller dunes form and migrate along the flanks of the larger dunes and sand ridges.
Wind erosion of soil at the foot of Chimborazo, Ecuador Rock carved by drifting sand below Fortification Rock in Arizona (Photo by Timothy H. O'Sullivan, USGS, 1871). Aeolian processes, also spelled eolian, [1] pertain to wind activity in the study of geology and weather and specifically to the wind's ability to shape the surface of the Earth (or other planets).
The Egyptian “chevrons” are active, wind-generated dunes, but the “chevrons” in the Bahamas are inactive and have been variously interpreted. [3] The most common interpretation of large, chevron-shaped bed forms is that they are a form of parabolic dune, and that most examples are generated by wind action. [4]
With the end of the glacial period, the mobility of the dunes quickly came to a halt as a result of reforestation. The shape of inland dunes varies depending on the prevailing wind direction and strength. Most of them are rather irregular dunes or shifting belts of sand. But there are also occur very well-formed parabolic dunes and longitudinal ...