Sand Dunes: A Phenomenon Of Wind

• A Brief Introduction To Sand Dunes
  
• The Amazing Origin Of Sand Dunes
  
• Calcareous Sands Of Tropical Beaches
  
• Booming Dunes That Make Weird Sounds
  
• Where To See Booming Dunes In The U.S.
  
• Plant & Animals Living On Sand Dunes
  
• Sand Dune Lizards And Sand Snakes
  
• The Bizarre Sand Food And Sand Plant
  
• More Images Of Sand Food & Sand Plant
  
• The Amazing Antlion And Sand Wasp
  
• Go To WAYNE'S WORD Antlion Article
  
• The Preservation Of Sand Dunes
  
• References About Dunes & Their Inhabitants

• See An Aerial View Of The Algodones Dunes
  
• See A Massive Dune At Sand Mountain, Nevada
  
• See Microscopic Comparison Of Sand Grains
  
• See Colorful Sand Dune Wildflowers
  
• See A Colorado Desert Fringe-Toed Lizard
  
• See A Colorful Banded Sand Snake
  
• See A Shovel-Nosed Sand Snake
  
• See North America's Most Bizarre Wildflower
  
• See The Amazing Sand Plant In Full Bloom
  
• More Images Of Sand Food & Sand Plant
  
• See The Utterly Amazing Sand Wasp
  
• See A Fierce-Looking Antlion Larva
  
• See A Desert Iguana In A Sand Burrow

The accumulation of windblown sand marks the beginning of one of nature's most interesting and beautiful phenomena. Sand dunes occur throughout the world, from coastal and lakeshore plains to arid desert regions. In addition to the remarkable structure and patterns of sand dunes, they also provide habitats for a variety of life which is marvelously adapted to this unique environment. Picturesque dunes against a sky of blue or a full moon, with perfectly contoured shadows of ripples and undulating crests, have always been a favorite subject of photographers. Dunes have also been the subject of many desert movies, and have historically been a formidable barrier to vehicular and rail travel. Depending upon one's particular situation, they can be one of the most incredibly beautiful, thrilling, eerie, treacherous or just plain inhospitable places on earth.

The origin of sand dunes is very complex, but there are three essential prerequisites: (1) An abundant supply of loose sand in a region generally devoid of vegetation (such as an ancient lake bed or river delta); (2) a wind energy source sufficient to move the sand grains; and (3) a topography whereby the sand particles lose their momentum and settle out. Any number of objects, such as shrubs, rocks or fence posts can obstruct the wind force causing sand to pile up in drifts and ultimately large dunes. There are even reports of ant hills forming the nucleus upon which sand dunes are built. The direction and velocity of winds, in addition to the local supply of sand, result in a variety of dune shapes and sizes. The wind moves individual grains along the inclined windward surface until they reach the crest and cascade down the steep leeward side or "slip face," piling up at the base and slowly encroaching on new territory. Some California dunes with crests only 30 feet high may advance 50 feet a year, posing a serious threat to nearby farms and roads. If the wind direction is fairly uniform over the years, the dunes gradually shift in the direction of the prevailing wind. Vegetation may stabilize a dune, thus preventing its movement with the prevailing wind. Along the Oregon coast, entire forests may cover sand dune areas. Sometimes severe storms or other disturbances can destroy the forest canopy allowing sand from nearby dunes to move into the disturbed area. In fact, Mr. Wolffia has stood at the crest of a shifting dune where the tip of a sitka spruce (Picea sitchensis) was protruding from the sand.

The structure and mineral composition of sand grains depends on the geology of the mountains that have been eroded away by wind and water. Although most dunes are composed of quartz and feldspar grains, the brilliant snow-white dunes of White Sands, New Mexico are composed of gypsum. Spectacular black sand beaches of the tropical South Pacific islands are made of fine volcanic particles. Gleaming white sands of tropical coral beaches and atolls are composed of a glistening, microscopic assortment of reef animals and algae, including wave-worn fragments of brightly-colored corals, minute one-celled foraminiferans, fragments of sea shells, star-shaped sponge spicules and fragments of jointed plates from a common calcareous green alga (Halimeda) that grows among submarine seagrass meadows and coral reefs.

Evidence of abrasion on sandblasted surfaces of telephone poles and posts reveals that sand grains seldom travel more than a few feet above the ground. Myriads of sand grains bouncing and rolling up the windward surface of a dune often form a series of ridges and troughs called wind ripples. Bouncing sand grains tend to land on the windward side of each ripple, thus producing a low ridge. Without getting too complicated, the spacing of ripples is related to the average distance grains jump. This in turn, is related to the wind velocity and size of the grains. Wind ripples are often very spectacular and photogenic, especially when the thousands of tiny ridges catch the shadows of early morning or late afternoon.

Many people associate deserts with vast areas of drifting sand, as portrayed by a number of Hollywood films depicting the French Foreign Legion, battles of World War II and other dramas. In fact, less than 20 percent of the earth's total desert area is covered with sand, and sand dunes only account for about two percent of the surface of North American deserts. One of the largest dune systems in the United States is the Algodones Dunes. It extends southeasterly for more than 40 miles (64 km), from north of Glamis in Imperial County, California to the southwestern corner of Arizona and into Sonora, Mexico. In California, the dunes range from two to six miles in width, with crests rising 200 to 300 feet (91 m) above the surrounding landscape. Other large dunes occur in Death Valley and Eureka Valley, in the Mojave Desert near Kelso, and along the California coast just south of Pismo Beach. The Eureka dunes rise to nearly 700 feet (200 m) and the Great Sand Dunes in Colorado rise to nearly 800 feet (240 m).

For centuries, explorers and naturalists throughout the world have described strange sounds emanating from sand dunes. Strange squeaking sounds are also produced by beach sand by simply compressing the sand with your foot or poking it with a rod. Some of the earliest references about "acoustical" dunes are found in Chinese and Mideastern chronicles dating back more than 1500 years. Marco Polo described weird sounds on a journey through the Gobi Desert, and Charles Darwin mentioned it while traveling through Chile. The sounds have been variously described as singing, whistling, squeaking, roaring and booming. Some accounts compare the sounds with distant kettle drums, artillery fire, thunder, low-flying propeller aircraft, bass violins, pipe organs and humming telegraph wires. Low frequency sounds are produced when closely packed sand grains slide over each other, such as an avalanche down the slip face of a dune. The stationary sand underneath apparently acts as a giant sounding board or amplifier to produce the enormous volume of sound. The sand must be very dry for sound production, and under a microscope the grains appear more rounded and finely polished compared with ordinary (silent) sand. Astronomers and geologists have speculated that this remarkable phenomenon may be common in the windy and nearly waterless sand dunes on Mars.

Acoustical "booming" dunes are rather widespread on earth, including the Sahara Desert, Middle East, South Africa, Chile, Baja California and the Hawaiian Islands. California has at least two documented areas with booming dunes, the massive Kelso Dunes of San Bernardino County and the scenic Eureka Dunes of Inyo County.

One of the best places to observe booming dunes in the western United States is Sand Mountain, about 16 miles southeast of Fallon, Nevada. A short dirt road north of Highway 50 leads to the base of the massive white dunes. Sand Mountain is composed of two "seif" (sword-shaped) dunes whose summits stand about 390 feet (120 m) above the desert floor. To really appreciate this acoustical phenomenon you must climb to the crest of a dune and then slide down the steep slip face. Going down with an avalanche of sand is sort of like riding down an escalator, ankle deep in sand. As the sand begins to vibrate the sound becomes quite loud, like a low-flying B-29 bomber or squadron of World War II vintage fighter planes.

There are several interesting legends about the mysterious moaning of Sand Mountain. According to Mary Holliday (Nevada Official Bicentennial Book, page 137), a large sea dinosaur or plesiosaur once lived and frolicked with its mate in ancient Lake Lahontan. Strong winds piled the lakebed sediments into what is now called Sand Mountain, completely burying the dinosaur under hundreds of feet of sand. Today the dinosaur moans for its mate and the deep blue waters of Lake Lahontan.

Many animals and plants have adapted to life in a sea of drifting sand. During years with favorable winter rains carpets of pink sand verbena (Abronia villosa), white dune evening-primrose (Oenothera deltoides) and yellow sunflower (Geraea canescens) may extend for miles, and the air is filled with the sweet aroma of fragrant blossoms. Deep-rooted shrubs, such as mesquite (Prosopis glandulosa), creosote bush (Larrea tridentata) and desert buckwheat (Eriogonum deserticola) provide vital shade and food for a host of animals, from insects and reptiles to birds and mammals. Often entire rodent condominiums are constructed beneath the protective cover of dune shrubs.

Two remarkable dune residents, the fringe-toed lizard (Uma notata) and shovel-nosed sand snake (Chionactis occipitalis), are well adapted for rapid burrowing through sand, a phenomenon known as "sand swimming." At depths of only 12 inches (30 cm), the temperature may be 50 Fahrenheit degrees cooler. Fringe-toed lizards are often seen scurrying over sand dunes during mild spring days and even in the hot summer. Their toes are fringed with elongate, pointed scales which provide traction in the sand (like extra-grip tread). They can run with amazing speed across steep dunes and then stop suddenly and wriggle out of sight into the cooler layers of sand. Its special eyelids and countersunk lower jaw keeps out sand grains. The shovel-nosed sand snake and banded sand snake (Chilomeniscus cinctus) are also well-adapted for burrowing through sand with their streamlined head and smooth scalation which minimize friction. The way they glide swiftly through the sand is reminiscent of the giant "sand worms" in the sci-fi movie "Dune," and the terrifying sand creatures in the movie "Tremors." Although it is harmless, the colorful banded sand snake is sometimes mistaken for the poisonous coral snake; however, the coral snake has a black-tipped head and red bands that completely encircle its body, bordered by yellow or white bands.

One of the most interesting of all dune plants, and certainly one of the most bizarre wildflowers in North America is "sand food" (Pholisma sonorae), formerly placed in the genus Ammobroma. This amazing parasitic flowering plant grows in the Algodones Dunes of southeastern California and adjacent Arizona, and in the sand dunes of El Gran Desierto in Sonora, Mexico (north of Bahia Adair in the Gulf of California). The southernmost extent appears to be the region around Bahia Adair on the Sea of Cortez coast of Sonora, Mexico. Within this area, the plants grow on sand dunes produced by wind transport of sand from the beaches of ancient Lake Cahuilla and the Colorado River delta. In an article for the California Native Plant Society (Fremontia Vol. 7, 1980), Professor Armstrong (Mr. Wolffia) reported that sand food only grows on the Algodones Dunes. He thought that the dunes of El Gran Desierto were part of the Algodones dune chain. Although he was in error reporting that sand food only grows on the Algodones Dunes, professor Armstrong was well aware of the occurrence of this species at Bahia Adair. Another unusual species of sand food (Pholisma culiacana) is endemic to rocky, subtropical thorn scrub 500 miles (800 km) south in Sinaloa, Mexico. According to Gary Nabhan (Desert Plants Vol. 2, 1980), the disjunct distribution of these two species may be explained by plate tectonics. Floras west of the San Andreas fault and Gulf rift, in southern California and Baja California, have been displaced northward at least 500 km since the Miocene epoch (about 30 million years ago). Assuming that P. culiacana is more like the tropical progenitor of both species, P. sonorae could have diverged into a more xeric (drought resistant) species west of the San Andreas fault as peninsular California moved northward. Present day populations of P. sonorae east of the fault could have migrated there by shifting sand dunes in the region.

Sand food is a root parasite with a thick, scaly stem that may extend 6 feet (2 m) or more into the dune where it attaches to the roots of nearby shrubs deep in the soft sand. The entire plant lives below the surface of the sand, with only the flower head pushing above sand during early spring. The scaly stem is without chlorophyll and is nonphotosynthetic, and all of its vital organic nutrients (amino acids and carbohydrates) come from nearby host shrubs. An old, dried flower head with a long, subterranean scaly stem superficially resembles a sand dollar attached to a long piece of seaweed (see photo image at left).

One of sand food's common host shrubs on the Algodones Dunes is dune buckwheat (Eriogonum deserticola). [The related P. culiacana has completely different host shrubs in the thorn scrub of Sinaloa, Mexico.] This endemic buckwheat is a large shrub with picturesque, twisted trunks and long, deep roots that are often exposed by shifting sand. Sand food also parasitizes the roots of two mat-like dune shrubs in the Borage Family, Coldenia plicata and C. palmeri. Other host shrubs include dyeweed (Psorothamnus emoryi), bur-sage (Ambrosia dumosa) and arrowweed (Pluchea sericea). The latter shrub is more typical of marsh areas, and in the early 1930s, Franklin A. Thackery discovered 106 sand food plants arising from a single arrowweed plant near an irrigation canal in Imperial County, California (north of Calipatria). The host plant weighed just over one pound, while the 106 sand food plants weighed 46 pounds. In this remarkable case, the parasite outweighed its host by more than 3600 percent. Of course, most of the sand food's combined weight was water stored in its fleshy tissues. Thackery concluded that the parasite was not overtaxing its host, and was absorbing water on its own to supplement that which was provided by the host. Because sand food is so brittle, Thackery had to suspend each of the 106 stems with separate strings in order to take his photograph. His remarkable photograph in Desert Magazine (Vol. 16, 1953) looks like a sand food puppet show.

As in all flowering plants, flowers and seed-bearing fruits provide the vital genetic link between successive generations. The flower heads (inflorescences) of sand food develop on the surface of the sand where they can be visited by pollinators, and resemble fuzzy mushrooms or gray powder puffs bearing numerous tiny lavender flowers. Individual flowers are surrounded by a hairy calyx, and the masses of hairy calyces give the heads a sand-colored, woolly texture and appearance which undoubtedly reflects light and heat on the sun-baked dunes. The heads may reach 5 inches (12 cm) in diameter and the flowers often appear in a circular pattern. Sand food flowers are apparently insect-pollinated, and according to R.L. Dressler and Job Kuijt (Madrono Vol. 19, 1968), the flowers of A. culiacana have a faint, sweet odor and are visited by flies, beetles and small butterflies. Sand food plants on the Algodones Dunes are sometimes infested with small ants and mealybugs. The ants may be after the small seeds, or perhaps the sweet nectar secretion from the mealybugs known as "honeydew." Following pollination, each flower gives rise to a small dry fruit (capsule) composed of 12 to 20 minute seeds arranged in a circle like wedges of cheese. Depending on favorable winter rains and pollination, each flower head may produce hundreds of seeds.

The scaly stems extend more than 6 feet (2 m) into the sand where they are attached to the roots of dune buckwheat, or another host shrub. It is difficult to trace the origin of these strange plants because the sand caves in as fast as you can shovel it out. Native Indians, including the Sand Papagos and Cocopas, ate the fleshy stems of sand food, either raw or roasted over a campfire. The stems were also dried in the sun and ground on a metate with mesquite beans, forming a flour called "pinole." The flavor of raw stems is pleasantly sweet, with a texture similar to a crisp, juicy radish; however, this is a rare desert plant and should not be harvested by hungry nature lovers. According to the authority of desert plants, Dr. Edmund C. Jaeger, when roasted the stems resemble the flavor of well-browned yams (red sweet potatoes). The closely-related root parasite, called sand plant or pholisma (Pholisma arenarium), has a fleshy, scaly, subterranean stem and growth habit similar to sand food. It produces a peculiar egg-shaped cluster of lavender flowers in sandy areas of the Colorado Desert and coastal sand dunes in California. Unlike sand food, its raw stems are not particularly flavorful or palatable--at least in this author's opinion.

How the seedlings of these unusual root parasites are able to find the host root buried deep in the sand is truly remarkable. It is discussed by Job Kuijt in The Biology of Parasitic Flowering Plants, 1969. According to Dr. Kuijt, the sand plant (Pholisma arenarium) sends out "pilot roots" two feet (0.6 m) below the surface of the sand. When they reach the vicinity of a host shrub, the pilot roots send out special "haustorial roots" which connect and penetrate the host root. The haustorial connection (haustorium) absorbs carbohydrates and amino acids manufactured by the photosynthetic host shrub. Other factors may also be involved in finding the host roots. The small seeds may move downward through the sand or may be buried by continually shifting sand dunes which are subsequently colonized by new host vegetation. They may also be carried deep into the sand by harvester ants and by rodents (kangaroo rats) that burrow into the dunes under host shrubs.

Any discussion of denizens of drifting sand would be incomplete without mentioning two very unusual insects, the antlion and the sand wasp. The antlion resembles a grotesque small scale version of the sand creature in the Star Trek II film, "The Wrath of Khan." It builds a circular crater in the sand and then waits patiently under the sand at the bottom of the pit for a hapless passerby. The steep crater walls make escape by small crawling insects virtually impossible. Struggling victims are literally pulled into the sand and sucked dry by the hollow fangs (jaws) of the antlion. If antlions were 6 feet (2 m) long, they would be a formidable desert sci-fi monster that could easily grab humans.

Sand wasps (Bembix species) dig tunnels in soft sand where they live in elaborate "condominiums." They are readily distinguished from other wasp species by their elongate, triangular labrum (lip). Sand wasps differ from spider wasps, mud daubers and many other digging wasps that provide their larvae with a single cache of food that must last throughout the larva's development. Sand wasps continually catch insects, such as flies, and bring them home to their burrows. They are often seen hovering over dunes in search of small insects to feed their young. Although they are not social insects like hornets, yellow jackets and honey bees, they do nest in the same vicinity, and tend to develop a primitive type of colony.

Many people ask why sand dunes should be preserved; "they are just piles of sand devoid of plants and animals." Nothing could be further from the truth. There are hundreds of dune species throughout the southwestern United States and Mexico, some of which are considered rare and endangered by the United States Fish and Wildlife Service. This is because their limited and specialized habitats are threatened by urbanization and various motorized off-road vehicles. In addition, some sand dunes have unique habitats that occur in no other dune chains. For this reason, the Bureau of Land Management has closed a large section of the Glamis Dunes north of Highway 78 in Imperial County, California to all motorized vehicles. This area has been designated a National Natural Landmark to preserve the remarkable wildlife and geology of this region. At least one section of the magnificent Algodones Dunes will remain in its pristine form for educational activities and field research; for photographers, naturalists, scientists and anyone who enjoys accessible, picturesque dunes.

Of all the earth's natural phenomena, sand dunes are one of the most beautiful and fascinating. The complex geological factors resulting in the formation of dunes and their subsequent colonization by plants and animals are absolutely amazing. Sandstone formations, formed by ancient dunes, often reveal many mysteries about the geologic history and weather patterns of a region. To stand before an enormous, gleaming white sand dune and realize that all of this was once an ancient lake bed or coastal plain is quite astonishing. The incredible roaring sounds of distant dunes is an unforgettable experience, particularly during the quiet hours of darkness and daybreak. Starting with the wind and tumbling particles of sand and culminating in picturesque drifts of rippled sand with an entire, dynamic, living community of plants and animals; this is one of nature's most remarkable cycles, and it is truly a phenomenon of wind.