单选题Directions: Read the following text. Answer
the questions on each text by choosing A, B, C or D. Standing on the rim of the Grand Canyon, gazing across
this giant wound in the Earth's surface, a visitor might assume that the canyon
had been caused by some ancient convulsion (剧烈震动). In fact, the events that
produced the canyon, far from being sudden and cataclysmic (剧变的), simply add up
to the slow and orderly process of erosion. Many millions of
years ago, the Colorado Plateau (高原) in the Grand Canyon area contained 10,000
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 action as upwarping; 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 meandered (蜿蜒而流) across
it began to run more swiftly and cut more definite courses. One of these rivers,
located east of the upwarp, was the ancestor of the Colorado. Another river
system called the Hualapai, flowing west of the upwarp, extended itself eastward
by cutting back into the upwarp; 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 has been added other factors. Heat and cold,
rain and snow, along with the varying resistance of the rocks, increase the
opportunities for erosion. The canyon 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.
单选题
What does the passage mainly discuss?
A. Patterns of erosion in different mountain ranges.
B. Forces that made the Grand Canyon.
C. The increasing pollution of the Colorado River.