六级冲刺练习阅读（137）
<p> <span word="Erosion">Erosion</span></p><p> <span word="Erosion">Erosion</span> goes on continuously and it should be the concern of all of us. There is evidence of this process of erosion all about us. Both the small gullies in the side of a nearly hill and the immense Grand Canyon of the Colorado River and other gorges represent results of erosion. A muddied river after a heavy rain and a dust storm in a parched area of farmland both are carrying particles of soils from one place to another. Likewise, a creeping glacier in a mountain valley, the pounding waves of the ocean, and an underground river in a limestone cavern are all agents of erosion.</p><p> <span word="Erosion">Erosion</span> levels the earths surface. Essentially, erosion is the natural process that ceaselessly tries to level the earths surface. If it were not for other forces also constantly at work, the mountains and hills would eventually disappear and all of the land would be under water. Fortunately, however, many areas of the earths surface are being raised by volcanic action and by balancing movements of the earths crust. For example, great pressures exerted by the water and sediments on the ocean floor along many shorelines result in forces that cause the land area to be uplifted. Thus erosion never completely succeeds in leveling the earths surface.</p><p> <span word="Gravity">Gravity</span> <span word="is">is</span> <span word="influential">influential</span>. <span word="The">The</span> <span word="force">force</span> <span word="of">of</span> <span word="gravity">gravity</span> <span word="is">is</span> <span word="responsible">responsible</span> <span word="for">for</span> <span word="both">both</span> <span word="the">the</span> <span word="falling">falling</span> <span word="of">of</span> <span word="rain">rain</span> <span word="from">from</span> <span word="the">the</span> <span word="clouds">clouds</span> <span word="and">and</span> <span word="for">for</span> <span word="the">the</span> <span word="water">water</span> <span word="running">running</span> <span word="off">off</span> <span word="the">the</span> <span word="land">land</span> <span word="as">as</span> <span word="it">it</span> <span word="always">always</span> <span word="seeks">seeks</span> <span word="a">a</span> <span word="lower">lower</span> <span word="level">level</span>. <span word="Likewise">Likewise</span>, <span word="the">the</span> <span word="great">great</span> <span word="moving">moving</span> <span word="currents">currents</span> <span word="of">of</span> <span word="air">air</span> <span word="that">that</span> <span word="we">we</span> <span word="call">call</span> <span word="winds">winds</span> <span word="are">are</span> <span word="the">the</span> <span word="result">result</span> <span word="of">of</span> <span word="unequal">unequal</span> <span word="heating">heating</span> <span word="of">of</span> <span word="the">the</span> <span word="earths">earths</span> <span word="surface">surface</span> <span word="and">and</span> <span word="the">the</span> <span word="convection">convection</span> <span word="currents">currents</span> <span word="that">that</span> <span word="are">are</span> <span word="set">set</span> <span word="up">up</span> <span word="in">in</span> <span word="the">the</span> <span word="atmosphere">atmosphere</span>. <span word="Here">Here</span> <span word="again">again</span> <span word="the">the</span> <span word="force">force</span> <span word="of">of</span> <span word="gravity">gravity</span> <span word="is">is</span> <span word="responsible">responsible</span> <span word="for">for</span> <span word="the">the</span> <span word="downward">downward</span> <span word="movement">movement</span> <span word="of">of</span> <span word="the">the</span> <span word="cooler">cooler</span>, <span word="denser">denser</span> <span word="air">air</span> <span word="toward">toward</span> <span word="the">the</span> <span word="earth">earth</span> <span word="which">which</span> <span word="starts">starts</span> <span word="the">the</span> <span word="air">air</span> <span word="moving">moving</span>.</p>
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