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Parabolic dunes, like crescent dunes, occur in areas where very strong winds are mostly unidirectional. Although these dunes are found in areas now characterized by variable wind speeds, the effective winds associated with the growth and migration of both the parabolic and crescent dunes probably are the most consistent in wind direction.
Parabolic dunes are identified by vegetated dune ridges and vegetated deflated plains. Due to variable wind gusts, parabolic dunes are commonly unvegetated in troughs or dune swells where wind tunnels transport currents. Ripple alignment in association with the main dunes can also identify parabolic dunes.
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.
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]
Sufficient vegetation has grown on the valley floor and there is little sand blowing into the main dune field from the valley; however, small parabolic dunes continue to originate in the sand sheet and migrate across grasslands, joining the main dunefield. Some of these migrating dunes become covered by grasses and shrubs and stop migrating.
The Macedonian king and military mastermind famously trekked across the desert in 332 B.C.E. to consult the Oracle of Siwa. While no one knows for sure what transpired between them, Alexander ...
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]