Individual saltating granules form transversely perpendicular to the wind's direction forming small ripples. The isolated hills form one obstacle dune each but the broad hills in chains create several obstacle dunes.
They appeal as linear, more or less symmetrical sand ridges, much higher in proportion to width than the transverse type of dune. The star sand dunes are pyramid-shaped. Since they are wider than they are long they can travel very quickly. The wind can blow material onto the dune from any side.
The speed of the barchans may range from 1-100 meters per year. There must be enough wind to transport the granules.
Last updated: User Permissions. Look for classic barchan dunes directly across from the main Dunes Parking Area. An example of a reversing sand dune is in the Algodones, California. The tips of the parabolic dune point towards the upwind direction.
Barchan dunes above can become aligned together along a plane perpendicular to the wind. Ridge crests are sharp in profile and show minor crenulations lengthwise.
These are generally formed on bare, surface mostly near the settlements. Sand grains are lifted up into the airstreams. The prevailing winds deposited the sand on the bare surfaces and they continued to deposit the sand in the lee of the crest till the angle of repose for loose sand was formed and the sand was moved down wind and created the steep face.
Crescentic dune s are shaped like crescent s, or the shape of a wide letter C. The degree of extend of sand dunes as calculated from the Survey of India toposheels in given as under: As the sand supply increases, barchan dunes begin to connect with others forming barchanoid ridges.
The orientation and the morphology of the dunes are directly related to the nature of the winds which have a free play on the vast structures of sands.