这篇雅思阅读背景材料主要讨论的是Dust Bowl也就是风沙中心的问题，也是美国中部大平原南部地区的代号。这个地区为什么会有这样的一个代号呢，有什么特点呢?我们一起来看看吧。

　　Dust Bowl, common name applied to a large area in the southern part of the Great Plains region of the United States, much of which suffered extensively from wind erosion during the 1930s. The area included parts of Kansas, Oklahoma, Texas, New Mexico, and Colorado. In its original state, the region was covered with hardy grasses that held the fine-grained soil in place in spite of the long recurrent droughts and occasional torrential rains characteristic of the area. A large number of homesteaders settled in the region in the 30 years before World War I, planting wheat and row crops and raising cattle. Both of these land uses left the soil exposed to the danger of erosion by the winds that constantly sweep over the gently rolling land. Beginning in the early 1930s, the region suffered a period of severe droughts, and the soil began to blow away. The organic matter, clay, and silt in the soil were carried great distances by the winds, in some cases darkening the sky as far as the Atlantic coast, and sand and heavier materials drifted against houses, fences, and barns. In many places 8 to 10 cm (3 to 4 in) of topsoil were blown away. Many thousands of families, their farms ruined, migrated westward; about a third of the remaining families had to accept government relief.

　　Beginning in 1935 intensive efforts were made by both federal and state governments to develop adequate programs for soil conservation and for rehabilitation of the Dust Bowl. The measures taken have included seeding large areas in grass; 3-year rotation of wheat and sorghum and of lying fallow; the introduction of contour plowing, terracing, and strip planting; and, in areas of greater rainfall, the planting of long “shelter belts” of trees to break the force of the wind. Dry spells in the 1950s, '60s, and late '70s were responsible for recurrences of dust storms in the region.

　　In the 1930s and 1940s, the region also received two waves of major migrations: that of farm families from the southern Great Plains migrating west to escape the Dust Bowl, and that of African Americans moving out of the American South. During World War II (1939-1945) the need for labor, especially in ship and aircraft production, boosted the population even more. The population of Los Angeles County jumped from 3 million to 4.7 million between 1940 and 1950.

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