Academic Reading — Test 151

Reading passage

Few materials beneath our feet have shaped human history as quietly as loess. This pale, fine-grained sediment blankets roughly a tenth of the Earth's land surface, yet most people have never heard its name. Loess is composed mainly of silt particles, which are intermediate in size between coarse sand and microscopic clay. What distinguishes loess from ordinary soil is the manner of its arrival: rather than being washed into place by rivers, it was carried through the air by wind and laid down grain by grain over thousands of years. For this reason, geologists classify it as an aeolian deposit, meaning one transported by the atmosphere. The story of loess begins, in most cases, with ice. During the great glacial periods, advancing sheets of ice ground the underlying bedrock into a flour-like powder. When the glaciers retreated, they released vast quantities of meltwater that spread this rock flour across wide outwash plains. As these plains dried, strong and persistent winds lifted the lightest particles and bore them across the landscape, sometimes for hundreds of kilometres. Heavier grains settled quickly, but silt could remain suspended long enough to travel great distances before drifting gently to rest. Where vegetation slowed the wind, the silt accumulated, building up layer upon layer until thick deposits formed. In parts of China, the resulting accumulation reaches depths of more than two hundred metres, the most extensive such formation on the planet. Not all loess, however, owes its existence to glaciers. In certain regions, the silt originates instead from deserts, where the relentless weathering of exposed rock and the abrasion of wind-blown sand generate fine dust. This desert dust is then transported and deposited in much the same fashion as its glacial counterpart. Whatever its source, the defining characteristic of loess is its remarkable uniformity: the grains are strikingly consistent in size, a direct consequence of the sorting action of the wind, which separates particles according to their weight as it carries them. The agricultural value of loess is difficult to overstate, and it arises from several properties working in concert. Because the deposit is unstratified and loosely packed, it is easily penetrated by plant roots and by the tools of the farmer. Its porous structure allows water to infiltrate readily while still retaining enough moisture for crops to draw upon during dry spells. Crucially, loess is naturally rich in minerals such as calcium carbonate, which help to keep the soil from becoming too acidic and supply nutrients that growing plants require. These qualities together produce land of exceptional fertility, and it is no coincidence that some of the world's most productive farming regions sit upon loess. The grain belts of the central United States, the plains of northern Europe, and the agricultural heartlands of China all rest on deposits of this wind-borne silt. Yet the very features that make loess so fertile also render it vulnerable. The same loose, open structure that welcomes roots and water offers little resistance once the protective cover of vegetation is stripped away. When farmers clear native grasses to plant crops, the exposed surface becomes prone to erosion. Wind can lift the dry topsoil and carry it off, while heavy rainfall can carve deep channels, known as gullies, into a slope within a single season. The Loess Plateau of China provides perhaps the most striking illustration of this danger: centuries of cultivation and the removal of forests left the land scarred and the rivers below choked with sediment. Indeed, the characteristic colour of the Yellow River derives from the immense load of loess it carries downstream. Recognising these risks, farmers and governments have developed a range of techniques to keep loess in place. Planting along the natural contours of a hillside, rather than up and down the slope, slows the flow of water and reduces its power to cut into the soil. Building low earthen banks called terraces transforms a steep gradient into a series of flat steps, each one holding back rainfall and sediment. Maintaining a continuous cover of plants, whether crops or deliberately sown grasses, shields the surface from both wind and rain. Through such measures, much of the damage of earlier centuries has been slowed and, in some places, reversed. The lesson of loess is therefore a double one: it is among the most generous of soils, but its generosity depends entirely on the care with which it is managed. Treated carelessly, it vanishes on the wind from which it came; treated wisely, it can sustain the harvests of nations for generations to come.