2，300萬(wàn)年后的美國(guó)
The United States in about 23 Million Years
The ordinary raindrop is a mighty earth mover with sufficient strength to cut rock.When rainwater collects on the surface of the ground, some of it evaporates and some of it sinks into the earth.The remainder begins to flow downhill, commencing its lengthy journey from brook to stream to lake, or to a river that will carry it to the sea.
As water flows along the ground, it picks up sand, pebbles, even boulders.It uses them to gnaw at the sides and bottom of its channel, gradually loosening more earth.
By this process enormous amounts of mud and rock are moved from the land to the sea.Each year the Mississippi Rivercarries 730 million of solid matter into the Gulf of Mexico.
This constant hauling of land into the sea is lowering the United States' average height above sea level at a rate of about one foot every 9, 000 years.If erosion continues at the same rate, the United States will be worn completely down to sea level in about 23 million years.
2,300萬(wàn)年后的美國(guó)
普通的雨珠力能移地，其力之大足以切割巖石。當(dāng)雨水在地面集結(jié)時(shí)，一部分蒸發(fā)掉，一部分滲入地下，剩下的部分則向低處流動(dòng)，開(kāi)始其漫長(zhǎng)的旅程，由小溪注入小川，再注入湖泊或江河，然后由江河送入大海。
雨水沿著地面流動(dòng)時(shí)，夾帶了泥沙、卵石，甚至大圓石。雨水利用它們嚙噬著水道的兩邊和底部，逐漸使更多的土壤松動(dòng)。
通過(guò)這一過(guò)程，巨量的泥沙和石塊從陸地移入海洋。每年，密西西比河要把7.3億噸的固體物質(zhì)送入墨西哥灣。
這種不斷的移地填海，正在使美國(guó)海拔以上的平均高度，以每9，000年減少1英尺的速度下降。如果土壤流失以同樣的速度持續(xù)下去，那么，在大約2，300萬(wàn)年以后，美國(guó)就會(huì)完全被磨蝕而下降到海平面一般高了。
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