[单项选择] The Grand Canyon Standing on the rim of the Grand Canyon(大峡谷), gazing across this giant wound in the Earth’’s surface, a visitor might assume that the canyon had been caused by some ancient convulsion(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 1,000 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 upwarding (弯曲上升) ; it was followed by a general elevation of the whole Colorado Plateau, a process that is stil
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

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