9．How the Grand Canyon （大峡谷） was created remains one of the geology's greatest mysteries．Some evidence suggests that the process was a gradual one in which the Colorado River （which runs through the canyon） slowly cut deeper and deeper into the ground over millions of years．But volcanic rock samples taken from the canyon now suggest that the canyon was down-cut instead．
Down-cutting is when a flood of water rushes over a landscape （地形） with enough force to cut deeply into the ground and leaves behind a canyon．Such a flood is usually released when a natural or man-made dam （堤坝） bursts．
Robert Webb，a research geologist，says natural dams seem to have formed and broken across the Colorado River several times during the last million years．The dams were built when lava（熔岩） from the eruptions of nearby volcanoes flowed into the river．The lava hardened into hard rocks and blocked the river，causing it to back up and form a lake．Each time the lake grew so huge that it broke the rock dam，releasing a flash flood that furthered the down-cutting process and deepened the canyon．
Down-cutting is not just an earthly event．Satellite photos sent back from Mars suggest that the process has happened there，too，say many other researchers．
The photos，taken by the Mars Global Surveyor，indicate that an enormous lake existed on Mars 3.5billion years ago．The lake spilled into a large nearby hole．One edge of the hole broke，releasing a flash flood that quickly carved out a grand canyon．
The existence of down-cutting on Mars is just one more piece of evidence that the cold，dry planet was once warm and wet．
24．In the past，deep canyons were believed to have formedB．
A．as a result of a sudden break of volcanoe
B．due to river flows over millions of years
C．owing to the burst of artificial dams
D．thanks to our ancestors'creative work
25．According to the passage，which of the following statements is TURE？B
A．Several volcanoes broke out on Mars directly creating grand canyons．
B．Several great lakes existed on the upper parts of the Colorado River．
C．People built high dams on the upper parts of the Colorado River．
D．The Colorado River crossed the Grand Canyon to form down-cutting．
26．From the last three paragraphs we learn thatD．
A．there are great lakes on Mars
B．there are active volcanoes on Mars
C．there might be cities on Mars
D．there might be life on Mars
27．The passage can be titled asD．
A．Lakes on Earth and Mars
B．The Cause of Lake Formation
C．The Force of Dam Breaking
D．Canyons-Results of Flood Cutting．
分析 和以往人们的想法不同的是，这篇文章的作者认为大峡谷的形成是洪水下切侵蚀造成的，主要是上游的湖泊泛滥强大的力量造成了峡谷，文章还介绍在火星也有这种现象．
解答 24．B．细节理解题：从文章第一段的句子：Some evidence suggests that the process was a gradual one in which the Colorado River （which runs through the canyon） slowly cut deeper and deeper into the ground over millions of years．可知过去深深的峡谷据相信是由于多年的河流造成的，故选B．
25．B．细节理解题：从文章第三段的句子：The lava hardened into hard rocks and blocked the river，causing it to back up and form a lake．可知在Colorado 河的上游有几个大湖，故选B．
26．D．细节理解题：从文章最后一段的句子：The existence of down-cutting on Mars is just one more piece of evidence that the cold，dry planet was once warm and wet．可知在火星上可能有生命，故选D．
27．D．标题确定题：从文章的内容和第二段的句子：Down-cutting is when a flood of water rushes over a landscape （地形） with enough force to cut deeply into the ground and leaves behind a canyon．可知这篇文章讲的是峡谷是由洪水下切侵蚀造成的，故选D．
点评 本文为人文地理类阅读的科普文，此类文章一般语言简洁，阅读时一般要求考生全面掌握文章中提供的信息，并运用这些信息去解决问题．因此对题干的理解尤为重要．阅读要注意文中具体细节的把握与理解，弄清作者所传达的实际信息及表达的具体内容．阅读时采取速读与精读相结合的方法，力求快速精确地查出试题所设置的关键内容．
