Sand Dunes

Throughout the world, there are places where the wind piles sand into strange and wonderful hills and ridges called dunes. All sand dunes are somewhat similar they all need sand, water, space to form, and they are all constantly changing. However, each sand dune is different in its own way. Each sand dunes' shape is determined according to how much sand and what kind of sand is available to be blown into dunes, and also the direction of the wind is essential.

Alot of the sand you observe today was once part of a rock. Rocks may seem indestructible, but they are not. You can crush a rock down into fine sand particles. Nature has several ways of changing rock into sand.

Sand dunes are found in the coastal and inland deserts. A sand dune can be as small as an anthill or as tall as a skyscraper. All sand dunes are formed the same way, every single one is a pile of sand built up by the wind. Once the sand has been picked up by the wind, it will go wherever the wind carries it. Even though most sand dunes are made of the same material and formed in the same way, they vary widely in appearance. For example, there are four major types of sand dunes:parabolic, barchan, transverse, and longitudinal.

Types of Sand Dunes

Parabolic

These are U-shaped or V-shaped stack of well-sorted sand. These types of dunes occur in dry and barren areas, i.e., where some rain is common and near-surface moisture is retained in the lower parts of the dune and the soils underneath. Parabolic dunes are always associated with vegetation--grasses, shrubs, seaweed and occasional trees. Parabolic dunes extend downwind from blowouts in sand sheets only partly anchored by vegetation. Parabolic dunes can also originate from beach sands and extend inland into vegetated areas in coastal zones and on shores of large lakes. Small sand mounds commonly form around bushes, such as mesquite and is called coppice dunes. They are frequently found on sand sheets and around larger parabolic dunes. Most parabolic dunes do not grow to heights greater than a few tens of meters except at their forward portions, where sand piles up as surrounding vegetation halts its advance.

Barchanoid

These dunes are crescent-shaped individual mounds. Barchanoid dunes are formed from the upwind slope, which is usually less than 15ƒ, is wind packed, and firm. The slip face, or lee slope, is composed of unstable, loose sand at its customary angle of repose of about 32ƒ. Two arms, also called horns, extend from the main body of the dune mound point downwind. Sizes of individual simple barchans range from a meter or so to perhaps a hundred meters from horn to horn.

Transverse

These dunes are accumulations of loose, well-sorted, very fine to medium sand in ridges that have a gentle stoss slope (usually less than 15ƒ) and a steep (32ƒ) slip face on the lee slope. The long axes of the ridges are perpendicular to the wind direction. These ridges are relatively straight or only slightly curved, and they look much like linear dunes. However, they are somewhat different. First, the two flanks of a transverse ridge have different angles of slope. The gentler upwind slope is composed of firmly packed sand and the steeper lee slope is soft and loose sand. Second, transverse dunes migrate lately, toward the next dune ridge, instead of longitudinally down the long axis of the ridge. In other words the low ridges or "thresholds" of sand may extend from the lee slope of one ridge across the interdune area and connect with the next ridge downwind. Transverse dunes can become very large (segments as wide as 3 km from horn to horn) and commonly develop compound and complex forms.

Longitudinal

These dunes are usually straight to abnormally sinuous dunes. The straight varieties are often called "sand ridges" and the sinuous varieties are called "seifs". The lengths of individual dunes can range from a few meters to many kilometers. The smaller longitudinal dunes can be 3 to 4 meters high. The strength of the wind pushes the sand and forms long ridges that are parallel to the wind that pushes it. Since the slip faces have a temporary alternation the flanks can be steep which causes loose sand to be found on the top, sides, and near the top of the ridges. The temporary alternations that occur are caused by changes in the direction of the wind. They can range from daily to seasonal according to the regional wind regime.

Life In Sand Dunes

Most people may think that just because a sand dune is a pile of sand that there can't be any form of life living in there, but they're wrong. At a closer glance, you realize that there is both animal and plant life on a dune. A variety of animals have adapted to survive in this unusual habitat. Plants such as poison ivy, wild lupine, and honeysuckle may grow along a dune's surface, while cottontail rabbits and grasshoppers often live in dead trees buried underneath a dune.

Animal Life In a Dune

Animals that live in sand dunes know that the sand just beneath the surface is cooler. That's why they remain underground during the hottest hours of the day. One of the factors that dune animals must face is to know how to conserve water. The kangaroo rat is a great model of water conservation. From the seeds and vegetation the kangaroo rat eats it gets the water it needs. Its urine contains so little water that it turns into solid when it comes in contact with the air. The kangaroo rat has natural defenses that help protect itself while living in the dune. Such as its long tail, which it uses to keep its balance as it escapes from enemies such as the sidewinder rattlesnake and the kit fox. Also the fur around the kangaroo rat's toes turn its feet into "sandshoes" which prevent it from sinking into the sand.

Most dunes has spadefoots, which is a type of toad. They spend 10 months of the year underground only coming to the surface only during the brief summer rains. When the water wets the land and falls into the ground , alot of spadefoots suddenly appears. For a couple of days they can be found in every pool and puddle created by the rain.
During this time the toads mate and the female toads lay their eggs. After a two week time period the eggs have hatched and the olliwogs have developed into adults capable of living on dry land.