Day 51

Passage 51

Glacial Lakes

①

Throughout the earth's history the climate has fluctuated and those times when the temperatures was cooler during a period, somewhere between 2 to 10 million years, are normally referred to as ice age. There have been at least five major ice ages in the past one billion years, the most recent of which, the Pleistocene Ice Age, began about 2 million years ago. Glaciers did not continually cover the earth during this time and there have been interglacial periods where temperatures warmed slightly and the glaciers melted and retreated. The repeated advancing and retreating of a glacier scoured the bedrock, preparing the way for the formation of lakes. By the time glaciers completely melted around 10,000 years ago, numerous glacial lakes were formed when the water, from melted glaciers, filled the hole or space formerly created.

②

It was at the end of this last Ice Age that a finger of the Cordilleran ice sheet crept southward into the Idaho panhandle, forming a large ice dam that blocked the mouth of the Clark Fork River, creating a massive lake 2,000 feet deep and containing more than 500 cubic miles of water. Glacial Lake Missoula stretched eastward for some 200 miles and contained more water than Lake Erie and Lake Ontario combined.

③

Periodically, the ice dam would fail and bring about catastrophic consequences. A large flood of ice-filled and dirt-filled water would burst through the dam, shooting out at a rate that was 10 times the combined flow of all the rivers in the world. This towering mass of water and ice literally shook the ground as it thundered toward the Pacific Ocean, stripping away hundreds of feet of soil and cutting deep canyons into the underlying bedrock. With flood speeds approaching 65 miles per hour, the lake would have drained in as little as 48 hours.

④

Over time the Cordilleran ice sheet blocked the Clark Fork River again and again, recreating Glacial Lake Missoula. Over approximately 2,500 years, the ice dam, lake and flooding sequence was repeated dozens of times, leaving a lasting mark on the landscape. Grand Coulee, Dry Falls and Palouse Falls were all created by these flood waters, as were the Missoula and Spokane ground-water resources, the fertile Willamette Valley and Quincy Basin and numerous wetlands.

⑤

Among the most varied features left by glaciers are ice-contact deposits due to continental glaciation. These landforms develop along the margin or outwash of the glacier, leaving behind ice deposits. Glacial outwash is generated when streams of melt water flow away from the glacier and deposit sediment to form broad outwash plains called sandurs. The floods caused by receding glaciers or the sudden drainage of an ice-dammed lake often rapidly deposit large quantities of sediment onto the sandur surface. During this process, kettle holes are often formed by the melting of a detached mass of glacial ice that has broken off a glacier and becomes wholly or partly buried, leaving a depression in a glacial outwash drift. In most cases, kettle holes eventually fill with water from the surface or underground rivers or streams and become kettle lakes.

⑥

Generally speaking, kettle lakes are the result of continental glaciation and are found in the northern areas of North America and Europe. In contrast to this kind of lake, alpine glacial lakes, including paternoster lakes and tarns, are formed by alpine glaciations and are mostly found in mountainous areas. Paternoster lakes are a chain of lakes that may develop in a valley with glacial steps, each step often having a lake. They are named so because of their resemblance with the beads on a rosary. Examples of paternoster lakes occur in the Sierra Nevada Mountains in California and Glacier National Park in Montana. A tarn is a lake that forms when water along with additional snowmelt seeps down into the depression of a cirque, the bowl or amphitheater-shaped valley head characteristic of all alpine glacial landforms. The head and sidewalls of a cirque may be nearly vertical, while the floor is often carved into a shallow basin. Excellent examples of tarn lakes are found in North Cascades and Rocky Mountain National Parks.

——2012年4月14日北美考试机经

The word “depression” in Paragraph 5 is closest in meaning to____.

A. deposit

B. recession

C. hole

D. hill

核心词汇：

词汇练习：

阅读下列句子，用所给单词的正确形式填空。

strip bead retreat stretch deposit periodically glacier catastrophic canyon recede depression fluctuate alpine resemblance seep massive drainage

1. The numbers of deer have _____ markedly since the entry of Europeans into Puget Sound country.(TPO-4:Deer Populations of the Puget Sound)

2. For example, during past ice ages, _____ advanced down through North America and Europe and gradually cut off parts of populations from one another.(TPO-31:Speciation in Geographically Isolated Populations)

3. These soils were evidence that the glaciers _____ as the climate warmed. (TPO-19:Discovering the Ice Ages)

4. Although a small proportion of the total population, this perhaps had a _____ local impact when a large proportion of the young men were removed from an area. (TPO-19:The Roman Army's Impact on Britain)

5. Wilson suggested that the long chain of volcanoes _____ north west from Hawaii is simply the surface expression of along-lived volcanic source located beneath the tectonic plate in the mantle.(TPO-27:The Formation of Volcanic Islands)

6. It might be that yawning helps to clear out the lungs by _____ lowering the pressure in them.(TPO-18:The Mystery of Yawning)

7. Outflow channels are probably relics of _____ flooding on Mars long ago. (TPO-18:Running Water on Mars)

8. A male deer make sabuck rubby _____ the bark (outer layer) of a small tree with its antlers.(TPO-28:Buck Rubs and Buck Scrapes)

9. In the Mesa Verde area of the ancient North American Southwest, living patterns changed in the thirteenth century, with large numbers of people moving into large communal dwellings called pueblos, often constructed at the edges of _____ , especially on the sides of cliffs.(TPO-28:Moving into Pueblos)

10. But close examination of those fossil _____ now reveals a somewhat different story. (TPO-30:The Pace of Evolutionary Change)

11. Proponents point to features such as the terraced “beaches” shown in one image, which could conceivably have been left behind as a lake or ocean evaporated and the shoreline_____.(TPO-30:Running Water on Mars)

12. Forty-three windmills powered the _____ pumps so that they were able to lease the reclamation to farmers as early as 1612, with the investors receiving annual leasing payments at an interest rate of 17 percent. (TPO-8:Seventeenth-Century Dutch Agriculture)

13. Flats and _____ where water can collect are common features, but they make up only a small part of the landscape.(TPO-12:Water in the Desert)

14. Long-handed Neolithic spoons of yew wood preserved in _____ villages dating to 3000 B.C. have survived; the signs of rubbing on their left side indicate that their users were right-handed.(TPO-12:Which Hand Did They Use)

15. They bear a strong _____ to river systems on Earth, and geologists think that they are dried-up beds of long-gone rivers that once carried rainfall on Mars from the mountains down into the valleys.(TPO-8:Seventeenth-Century Dutch Agriculture)

16. The earliest discovered traces of art are _____ and carvings, and then paintings, from sites dating back to the Upper Paleolithic period.(TPO-4:Cave Art in Europe)

17. Water does not remain immobile in an aquifer but can _____ out at springs or leak into other aquifers.(TPO-12:Water in the Desert)

参考答案：

1. fluctuated 2. glaciers 3. retreated 4. massive 5. stretching 6. periodically 7. catastrophic 8. striping 11. receded 12. drainage 16. beads 17. seep 9. canyons 10. deposits 13. depressions 14. alpine 15. resemblance