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 3.5 billion 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 formed _______.

　　A. as a result of a sudden break of volcanoes

【2017届四川省名山县高考英语一轮复习阅读理解训练：1（含答案）】相关文章：

★ 2017届高考英语二轮复习课时训练：20（含解析）

★ 重庆市2014高考英语阅读理解一轮（精品）训练题（8）附答案

★ 【北京专用】2014届高考英语一轮复习方案课时作业：必修3 Unit 5 Canada

★ 浙江省湖州市2014高考英语阅读理解一轮（暑假）精炼（5）含答案

★ 四川省2016届高考英语一轮复习课时作业（40） Module 4 Music Born in America 外研版选修8

★ 2017高考四川省广安市英语阅读理解一轮系列训练：23（含解析）

★ 河南省三门峡市2014高考英语一轮（暑假）基础训练（13）附答案

★ 2017届高考英语二轮复习大题冲关秘籍完形填空之记叙文：创新押题（含解析）

★ 湖南省汝城二中2014届高考英语一轮复习阅读训练 (22)

★ 2014届高考英语一轮复习课时作业(四十四)选修8Module 2《The Renaissance》（外研版）