2. 外语
  3. Standing on the rim of the Grand Canyon(大峡谷)， gazing across this giant would in the Earth’s surface, a visitor might assume that

Standing on the rim of the Grand Canyon(大峡谷)， gazing across this giant would in the Earth’s surface, a visitor might assume that

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问题     Standing on the rim of the Grand Canyon(大峡谷)， gazing across this giant would in the Earth’s surface, a visitor might assume that the canyon had been caused by some ancient convulsing (震动). In fact, the events that produced the canyon, far from being sudden and catastrophic, simply add up to the slow and orderly process of erosion.
    Many millions of years ago, the Colorado Plateau in the Grand Canyon area contained 1000 more feet of rock than it does today and was relatively level. The additional material consisted of some 14 layered formations of rock. In the Grand Canyon region these layers were largely worn away over the course of millions of years.
    Approximately 65 million years ago the plateau’s flat surface in the Grand Canyon area bulged upward from internal pressure, geologists refer to this bulging actions upwading(弯 曲上升); it was followed by a general elevation of the whole Colorado Plateau, a process that is still going on. As the plateau gradually rose, shallow rivers that winded across it began to run more swiftly and cut more definite courses. One of these rivers, located east of the upward, was the ancestor of the Colorado. Another river system, called the Hualapai, flowing west of the upward, extended itself eastward by cutting back into the upward, it eventually connected with the ancient Colorado and captured its waters. The new river then began to carve out the 277-mile-long trench that eventually became the Grand Canyon. Geologists estimate that this initial cutting action began no earlier than 10 million years ago.
    Since then, the Canyon forming has been cumulative. To the corrosive force of the river itself have been added other factors. Heat and cold, rain and snow, along with the varying resistance of the rocks, increase the opportunities for erosions. the Canyon walls crumble, the river acquires a cutting tool. Tons of debris, rainfall running off the high plateau creates feeder streams that carve side canyons. Pushing slowly backward into the plateau, the side canyons expose new rocks, and the pattern of erosion continues.
The passage mainly discusses______.
选项 A、patterns of erosion in different mountain ranges
B、forces that made the Grand Canyon
C、the increasing pollution of the Colorado River
D、the sudden appearance of the Grand Canyon
答案B
解析 第一段指出大峡谷的形成是由于缓慢有序的侵蚀形成的；第二段又说几百万年前大峡谷一带还是平面的，后来被逐渐侵蚀；第三段说由于地面降起，便有了高原河流，也有了著名的大峡谷；最后一段又进一步说明了大峡谷的进一步形成是由于力的作用。因而本篇讨论的是形成大峡谷的各种力。故选[B]。
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