托福聽力:三次元大平原

Look at our topographical map and you'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 sometimes called a prairie, sometimes a steppe - that’s s-t-e-p-p-e. The defining features are level terrain, dry climate and 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 Plains 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 from the Pacific Ocean. Lighter, drier air passes over the mountains. Until people intervened with irrigation and farms, only grass could grow on the dry, windy plain. 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.