Academic Reading — Test 108

Reading passage

Across parts of central Asia, central Europe and the central United States lie thick blankets of pale, fine-grained sediment known as loess. The word, borrowed from a German dialect term meaning loose or crumbly, describes a soft yellowish material that can stand in near-vertical cliffs yet washes away readily once disturbed. The most famous accumulation is the Loess Plateau of northern China, where deposits reach depths of more than two hundred metres in places and cover an area larger than many countries. These vast plateaus were not laid down by rivers or seas, as many sedimentary landscapes are, but were built grain by grain from dust carried on the wind. Understanding how they formed offers a striking record of the cold, dry climates that gripped the planet during the ice ages. The story begins with glaciers. During the repeated cold periods of the past two and a half million years, enormous ice sheets advanced across the northern continents. As these glaciers crept forward, they ground the underlying bedrock into a fine powder, a process geologists call glacial grinding. When the ice melted at its margins, torrents of meltwater carried this powder away and spread it across wide outwash plains. Left exposed and dry, the fine particles were vulnerable to the wind. Crucially, the grinding produced an abundance of silt, a particle size intermediate between coarse sand and fine clay, and it is silt that makes up the bulk of loess. Sand grains are generally too heavy to travel far through the air, while clay tends to bind together and resist lifting, so silt occupies the ideal range for long-distance transport by wind. The climate of the ice ages was not only cold but exceptionally windy and dry. With so much of the world's water locked up in ice, rainfall declined and vegetation thinned, leaving bare ground across huge regions. Powerful, persistent winds swept across these barren surfaces, lifting silt into the air in great dust storms. The particles could be carried for hundreds, sometimes thousands, of kilometres before settling. When the wind slowed, or when it met an obstacle such as a hill or a belt of grassland, its capacity to hold the dust fell and the silt was deposited. Over tens of thousands of years, layer upon layer accumulated, and the slow but relentless rain of dust gradually raised the surface of the land. Vegetation played a quiet but vital role, for grasses and other plants trapped the falling silt and helped to hold it in place, preventing it from simply blowing onward. The properties of loess reflect this airborne origin. Because each particle was sorted by the wind, the deposits are remarkably uniform in grain size, far more so than sediment dropped by water. The grains are loosely packed, giving loess a high porosity and a tendency to hold together through faint mineral and clay bonds between particles. This explains the curious habit of loess to form steep, stable walls when cut, even though the material is soft. It also makes loess soils extraordinarily fertile, since the fresh mineral dust is rich in nutrients and easily worked, which is why loess regions have long supported dense farming populations. The same looseness, however, brings danger. Loess is highly prone to erosion by running water, and over time rivers and rainfall have carved the Chinese plateau into a maze of deep gullies and ravines. Geologists value loess deposits for another reason: they preserve a detailed archive of past climate. Because dust accumulated steadily during cold, dry phases, thick layers of pale loess mark the ice ages themselves. During the warmer, wetter intervals between glaciations, dust delivery slowed and soils developed on the surface instead, forming darker bands rich in organic matter and weathered minerals. The result is an alternating sequence of loess and ancient soils, stacked one above another like the pages of a book. By measuring the thickness, chemistry and magnetic properties of these layers, scientists can reconstruct how the climate swung between cold and warm states over the past few million years. In this way the loess plateaus serve not merely as fertile farmland but as one of the longest and most continuous records of ice-age climate found anywhere on land.