从雅思阅读复习资料积累词汇Sand Dune

很多雅思考生在阅读了雅思复习资料后，或许更了解了出题思路和解题方法，但不知大有没有做过有心人，发掘雅思阅读复习资料的其它功能呢？其实，通过雅思阅读复习资料也可以有的放矢地扩充雅思词汇量。今天，向大推荐一篇地理类科普性的雅思阅读复习资料文章Sand Duner。阅读本文后，大在丰富知识面之余，雅思词汇量也在不知觉中上涨哦。

雅思阅读复习资料Sand Dune

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;

(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.


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.

雅思阅读复习资料词汇积累：

Dune 沙丘
Arid 干旱的
Picturesque 风景如画的，特的
Formidable 强大的 Eerie 可怕的，奇异的
Treacherous 背叛的
Inhospitable 冷淡的
Prerequisite 先决条件
Devoid 缺乏的
Topography 地形学
Drift 漂流
Velocity 速度