Reading — 2026 Jan–Apr Recall Set 89

A One of the main problems posed by sand dunes is their encroachment on human habitats. Sand dunes move by different means, all of them aided by the wind. Sand dunes threaten buildings and crops in Africa, the Middle East, and China. Preventing sand dunes from overwhelming cities and agricultural areas has become a priority for the United Nations Environment Program. On the other hand, dune habitats provide niches for highly specialized plants and animals, including numerous rare and endangered species. B Sand is usually composed of hard minerals such as quartz that cannot be broken down into silt or clay. Yellow, brown and reddish shades of sand indicate their presence of iron compounds. Red sand is composed of quartz coated by a layer of iron oxide. White sands are nearly pure gypsum. Sand with a high percentage of silicate can be used in glassmaking. Sandstone is created by sand, mixed with lime, chalk or some other material that acts as a binding agent, that is deposited in layers at the bottom of a sea or other area and pressed together into rock by the great pressure of sediments that are deposited on top of it over thousands or millions of years. C The most common dune form on Earth and on Mars is crescentic. Crescent-shaped mounds are generally wider than they are long. The slipfaces are on the concave sides of the dunes. These dunes form under winds that blow consistently from one direction, and they also are known as barchans or transverse dunes. Some types of crescentic dunes move more quickly over desert surfaces than any other type of dune. A group of dunes moved more than 100 metres per year between 1954 and 1959 in China’s Ningxia Province, and similar speeds have been recorded in the Western Desert of Egypt. The largest crescentic dunes on Earth, with mean crest-to-crest widths of more than 3 kilometres, are in China’s Taklamakan Desert. D Radially symmetrical, star dunes are pyramidal sand mounds with slipfaces on three or more arms that radiate from the high center of the mound. They tend to accumulate in areas with multidirectional wind regimes. Star dunes grow upward rather than laterally. They dominate the Grand Erg Oriental of the Sahara. In other deserts, they occur around the margins of the sand seas, particularly near topographic barriers. In the southeast Badain Jaran Desert of China, the star dunes are up to 500 metres tall and may be the tallest dunes on Earth. Straight or slightly sinuous sand ridges typically much longer than they are wide are known as linear dunes. They may be more than 160 kilometres (99 mi) long. Some linear dunes merge to form Y-shaped compound dunes. Many forms in bidirectional wind regimes. The long axes of these dunes extend in the resultant direction of sand movement. Linear loess hills known as pahas are superficially similar. E Once sand begins to pile up, ripples and dunes can form. Wind continues to move sand up to the top of the pile until the pile is so steep that it collapses under its own weight. The collapsing sand comes to rest when it reaches just the right steepness to keep the dune stable. This angle, usually about 30-34°, is called the angle of repose. Every pile of loose particles has a unique angle of repose, depending upon the properties of the material it’s made of, such as the grain size and roundness. Ripples grow into dunes with the increase of wind and sand input. F The repeating cycle of sand inching up the windward side to the dune crest, then slipping down the dune’s slip face allows the dune to inch forward, migrating in the direction the wind blows. As you might guess, all of this climbing then slipping leaves its mark on the internal structure of the dune. The image on the right shows fossil sand dune structure preserved in the Merced Formation at Fort Funston, Golden Gate National Recreation Area. The sloping lines or laminations you see are the preserved slip faces of a migrating sand dune. This structure is called cross-bedding and can be the result of either wind or water currents. The larger the cross-bedded structure, however, the more likely it is to be formed by wind, rather than water. G Sand dunes can “sing” at a level up to 115 decibels and generate sounds in different notes. The dunes at Sand Mountain in Nevada usually sing in a low C but can also sing in B and C sharp. The La Mar de Dunas in Chile hum in F while those at the Ghord Lahmar in Morocco howl in G sharp. The sounds are produced by avalanches of sand generated by blowing winds. For a while, it was thought that the avalanches caused the entire dune to resonate like a flute or violin but if that were true then different size dunes would produce different notes. In the mid 2000s, American, French and Moroccan scientists visiting sand dunes in Morocco, Chile, China and Oman published a paper in the Physical Review Letters that determined the sounds were produced by collisions between grains of sand that caused the motions of the grains to become synchronized, causing the outer layer of a dune to vibrate like the cone of a loudspeaker, producing sound. The tone of the sounds depended primarily on the size of the grains. H Scientists performed a computer simulation on patterns and dynamics of desert dunes in laboratory. Dune patterns observed in deserts were reproduced. From the initial random state, stars and linear dunes are produced, depending on the variability of the wind direction. The efficiency in sand transport is calculated through the course of development. Scientists found that the sand transport is the most efficient in the linear transverse dune. The efficiency in sand transport always increased through the evolution, and the way it increases was stepwise. They also found that the shadow zone, the region where the sand wastes the chance to move, shrinks through the course of evolution, which greatly helps them build a model to simulate a sand move.

A Just over a kilometer south of Berlin’s Potsdamer Platz, near the left bank of the Landwehr Canal, an extensive, triangular-shaped area of waste ground once separated the neighbourhoods of Kreuzberg to the east and Schöneberg to the west. Known as Gleisdreieck, meaning ‘triangle of rails’, it was formed by the intersection of different railway lines built in the mid-nineteenth century that entered Berlin from the south. Tracks, sheds and warehouses belonging to three old railway stations – Dresdner Bahnhof (1875-1882), Potsdamer Bahnhof (1838-1944) and Anhalter Bahnhof (1839-1952) – are situated on a raised area some twenty hectares in size, at a height of four metres above the surrounding ground level. B As the railway infrastructure gradually ceased to be used, the whole area of Gleisdreieck became increasingly run down and abandoned, to the point of being used as a rubbish tip after 1945. Meanwhile, vegetation took over, turning it into a surprising, natural haven in the middle of built-up areas. The closeness of the wall which divided the city of Berlin between 1961 and 1990 also contributed to the fact that Gleisdreieck was for decades clearly identified as no-man’s-land. Only a station of the same name in Berlin’s underground railway system testified to its existence. Shortly before the fall of the Berlin Wall, however, the German Museum of Technology was established there, attracting large numbers of visitors and giving visibility to the site. More importantly, the unification of the formerly divided city gave the zone more centrality, at which point it became imperative to identify a purpose for it. C After 2006, the State of Berlin put forward the proposal of converting Gleisdreieck into a large urban park that would integrate the different urban zones which converged there. The decades-long isolation of the site, which had formerly presented a problem, now represented an opportunity for joining the southern area of Potsdamer Platz with Kreuzberg and Schöneberg. The creation of the park would trigger one of the biggest urban expansions inside Berlin, all within a framework of multiple uses and social characteristics. It was necessary to stimulate the development of sixteen new hectares of local amenities that would be capable of integrating different generations and social groups in a sustainable way, and in harmony with nature. Following intense discussion with local proprietors and residents, the need to reconcile these goals with the conservation of railway heritage also emerged. D Once the area had been subjected to a process of undergrowth clearance and decontamination, it was then organized around a combination of existing and added elements. The project as a whole was planned around a large central meadow, intersected from east to west by a concrete footpath, and from north to south by a pair of railway lines. Once a month, a train slowly travels along these lines from its parking shed to the German Museum of Technology. The concrete footpath, which is a continuation of one of the main Kreuzberg boulevards, starts in the east, bridges the four-metre difference in ground level by means of a stairway, and suddenly ends in the west on reaching the underground lines. E North of the meadow, there is a large concrete slab with rounded edges. Being south-facing, this functions as a big sunny terrace, full of benches complete with footrests. In the south, the meadow overlooks Yorckstrasse, an underpass crossed by more than fifteen former railway bridges. To the east of the meadow there is quite a dense forest of pre-existing maples, oaks and birches, as well as newly planted trees of the same species. In this area, a couple of large metal frames each hold two swings. The edges of the park are finished with a collection of distinctive, functional spaces, for example a nursery, sports fields, concave surfaces for skateboards, stages for dancing, community gardens, or simply areas covered in gravel obtained from the site itself. F Although initially the regeneration of Gleisdreieck Park was the subject of disagreement between those who were in favour of safeguarding the railway heritage and those who wanted to regenerate the adjacent neighbourhoods, it was opened to the public in September 2011. The heart of Berlin now has a new green lung, in which the atmosphere of various small, very different corners fits neatly into a large-scale, wide-ranging and robust general order. This has been possible precisely because the intervention was not limited to conserving industrial remains in order to promote railway history. And as a reminder of the six decades of human absence, during which nature took over, the park has been able to conserve to some extent the spirit of the non-place that preceded it.

A. While it may not be possible to completely age-proof our brains, a brave new world of anti-aging research shows that our gray matter may be far more flexible than we thought. So no one, no matter how old, has to lose their mind. The brain has often been called the three-pound universe. It’s our most powerful and mysterious organ, the seat of the self, laced with as many billions of neurons as the galaxy has stars. No wonder the mere notion of an aging, failing brain—and the prospect of memory loss, confusion, and the unraveling of our personality—is so terrifying. As Mark Williams, M.D., author of The American Geriatrics Society’s Complete Guide to Aging and Health, says, 'The fear of dementia is stronger than the fear of death itself.' Yet the degeneration of the brain is far from inevitable. 'Its design features are such that it should continue to function for a lifetime,' says Zaven Khachaturian, Ph.D., director of the Alzheimer’s Association’s Ronald and Nancy Reagan Research Institute. 'There’s no reason to expect it to deteriorate with age, even though many of us are living longer lives.' In fact, scientists’ view of the brain’s potential is rapidly changing, according to Stanford University neuroscientist Robert Sapolsky, Ph.D. Thirty-five years ago we thought Alzheimer’s disease was a dramatic version of normal aging. Now we realize it’s a disease with a distinct pathology. In fact, some people simply don’t experience any mental decline, so we’ve begun to study them. Antonio Damasio, M.D., Ph.D., head of the Department of Neurology at the University of Iowa and author of Descartes’ Error, concurs. 'Older people can continue to have extremely rich and healthy mental lives.' B. The seniors were tested in 1988 and again in 1991. Four factors were found to be related to their mental fitness: levels of education and physical activity, lung function, and feelings of self-efficacy. 'Each of these elements alters the way our brain functions,' says Marilyn Albert, Ph.D., of Harvard Medical School, and colleagues from Yale, Duke, and Brandeis Universities and the Mt. Sinai School of Medicine, who hypothesizes that regular exercise may actually stimulate blood flow to the brain and nerve growth, both of which create more densely branched neurons, rendering the neurons stronger and better able to resist disease. Moderate aerobic exercise, including long brisk walks and frequently climbing stairs, will accomplish this. C. Education also seems to enhance brain function. People who have challenged themselves with at least a college education may actually stimulate the neurons in their brains. Moreover, native intelligence may protect our brains. It’s possible that smart people begin life with a greater number of neurons, and therefore have a greater reserve to fall back on if some begin to fail. 'If you have a lot of neurons and keep them busy, you may be able to tolerate more damage to your brain before it shows,' says Peter Davies, M.D., of the Albert Einstein College of Medicine in the Bronx, New York. Early linguistic ability also seems to help our brains later in life. A recent study in the New England Journal of Medicine looked at 93 elderly nuns and examined the autobiographies they had written 60 years earlier, just as they were joining a convent. The nuns whose essays were complex and dense with ideas remained sharp into their eighties and nineties. D. Finally, personality seems to play an important role in protecting our mental prowess. A sense of self-efficacy may protect our brain, buffeting it from the harmful effects of stress. According to Albert, there’s evidence that elevated levels of stress hormones may harm brain cells and cause the hippocampus—a small seahorse-shaped organ that’s a crucial moderator of memory—to atrophy. A sense that we can effectively chart our own course in the world may retard the release of stress hormones and protect us as we age. 'It’s not a matter of whether you experience stress or not,' Albert concludes, 'it’s your attitude toward it.' Reducing stress by meditating on a regular basis may buffer the brain as well. It also increases the activity of the brain’s pineal gland, the source of the antioxidant hormone melatonin, which regulates sleep and may retard the aging process. Studies at the University of Massachusetts Medical Center and the University of Western Ontario found that people who meditated regularly had higher levels of melatonin than those who took 5-milligram supplements. Another study, conducted jointly by Maharishi International University, Harvard University, and the University of Maryland, found that seniors who meditated for three months experienced dramatic improvements in their psychological well-being, compared to their non-meditative peers. E. Animal studies confirm that both mental and physical activity boost brain fitness. At the Beckman Institute for Advanced Science and Technology in Urbana, Illinois, psychologist William Greenough, Ph.D., let some rats play with a profusion of toys. These rodents developed about 25 percent more connections between their neurons than did rats that didn’t get any mentally stimulating recreation. In addition, rats that exercised on a treadmill developed more capillaries in specific parts of their brains than did their sedentary counterparts. This increased the blood flow to their brains. 'Clearly the message is to do as many different things as possible,' Greenough says. F. It’s not just scientists who are catching anti-aging fever. Walk into any health food store, and you’ll find nutritional formulas—with names like Brainstorm and Smart ALEC—that claim to sharpen mental ability. The book Smart Drugs & Nutrients, by Ward Dean, M.D., and John Morgenthaler, was self-published in 1990 and has sold over 120,000 copies worldwide. It has also spawned an underground network of people tweaking their own brain chemistry with nutrients and drugs—the latter sometimes obtained from Europe and Mexico. Sales of ginkgo, an extract from the leaves of the 200-million-year-old ginkgo tree, which has been shown in published studies to increase oxygen in the brain and ameliorate symptoms of Alzheimer’s disease—are up by 22 percent in the last six months alone, according to Paddy Spence, president of SPINS, a San Francisco-based market research firm. Indeed, products that increase and preserve mental performance are a small but emerging segment of the supplements industry, says Linda Gilbert, president of HealthFocus, a company that researches consumer health trends. While neuroscientists like Khachaturian liken the use of these products to the superstition of tossing salt over your shoulder, the public is nevertheless gobbling up nutrients that promise cognitive enhancement.