【填空答案】
topographical: adj. concerned with topography地形學的
prairie: n.a treeless grassy plain［C］大草原，牧場
steppe: n.extensive plain without trees (associated with eastern Russia and Siberia)［C］大草原，干草原
defining adj.any process serving to define the shape of something 典型的
level: adj. having a surface without slope, tilt in which no part is higher or lower than another平坦的
terrain: n.a piece of ground having specific characteristics or military potential 地形
sediment: n.matter that has been deposited by some natural process［C］沉淀物
glacier: n. a slowly moving mass of ice［C］冰川
smooth out: v. free from obstructions消除，使平滑
moist: n. slightly wet［C］潮濕
intervene: v. get involved, so as to alter or hinder an action, or through force or threat offorce介入，干預
windy: adj.abounding in or exposed to the wind or breezes 多風的
intermediate: adj. around the middle of a scale of evaluation of physical measures中間的
【聽力原文】
Look at our topographical map andyou'll see that the middle third of the North American continent --- from the Rocky Mountains almost to the Mississippi River --- is pretty flat. This is the Great Plains. This kind of area is some times called a prairie, sometimes asteppe. That's [ess-tee-ee-pee-pee-ee]. The defining features are level terrain, dry climate, and an absence of trees.The Great Plains are actually the former bed of shallow inland sea. Over millions of years, sediment left by glaciers, water, and wind smoothed out the dry sea bed.As I said, the Great Plaints are bordered on the west by the Rocky Mountains. And it's really the Rockies that are responsible for the formation of the grasslands. The mountains are so high that they block the heavy moist air traveling eastward form the Pacific Ocean. Lighter, drier air passes over the mountains. Until people intervened with irrigation and farms, only grass could grow on the dry, windyplain. In fact, we can divide the Great Plains into three zones. In the west,where it's driest and windiest, the grass is very short. In the eastern zone,there's more rain and grass grows as high as 360 centimeters. In the middle third, there's a mix of grass species that grow to an intermediate height.
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