李東

An ice age is a period when large sheets of ice cover wide areas of the earth. An ice age can last for hundreds to millions of years. Between these periods， interglacial （間冰期的） times occur when one major sheet of ice is present. At this moment in history， the Earth is in an interglacial period because of the Antarctic and Greenland ice sheets.

These cooling periods happen when snow in an area does not completely melt. The bottom layer turns to ice. The snow and ice hold the moisture of the Earth， and the sheets grow.

Many changes occur on the Earth because of an ice age. Glaciers （冰川） push up rocks and wear off hills. This action changes the landscape of the Earth. The weight pushes down the Earth's crust （地殼）. Cold weather plant life is forced to move to climates that are more moderate when the temperature drops near the ice cliffs. The levels of the seas drop. Rivers hollow out deeper paths and lakes and reveal bridges between lands which had been under water previously. When the glaciers melt， new lakes are formed.

Scientists have identified five important ice ages during the history of the Earth. They are the Huronian， Cryogenian， Andean-Saharan， Karoo and Quaternary. What scientists now call “the Ice Age” was at its peak 18，000 years ago. They believe that changes in the tilt of the Earth on its axis （軸） and the shifting of the plates under the Earths' crust have been responsible for the glacial and interglacial periods. They believe that the Earth's plates shift and cause changes to the landmasses around the world. The oceans' currents may change along with the currents in the atmosphere. Volcanic activity may increase.

During the peak of this last ice age， sheets of ice as thick as 12，000 feet covered Scandinavia， Canada， Russia and South America. Temperatures around the world fell between 10-40 degrees Fahrenheit. Sea levels went down 400 feet. What is now known as the Gulf Coast region of the United States was covered with pine forests and prairie grass. Today， this type of vegetation is associated with the northern United States and Canada.

The theory of the Ice age began when some scientists believed that glaciers in the Alps Mountains in Europe had become smaller. A geologist from Switzerland， Louis Agassiz， went against the long-held belief that a great flood had carried away all of the large creatures like the mastodon （乳齒象）. He showed proof of glacier activity and plant life in the glacial sediment.

Milutin Milankovitch from Serbia was a mathematician who wanted to study the temperature of the Earth for the last 600，000 years. In 1941， he published a book called Canon of Insolation and the Ice Age Problem. He used technology to analyze deep-sea ice cores to try to determine the periods of glaciation.

According to scientists， the current interglacial period began 11，700 years ago. At this point， humans developed. Conditions were favorable for agriculture. They used the land bridges to travel to new areas. They created tools including bone needles to sew warm clothing. Mastodons， saber-toothed cats， giant ground sloths and other large creatures had become extinct by the end of the previous glacier.

There is much speculation as to why and how these species disappeared from the Earth. One suggestion is disease. Another is over-hunting by humans. Scientists must continue to study these questions.

Choose the best answer.

1. Which of the following is the number of ice ages identified by scientists？

A. Two. B. Three. C. Five. D. Seven.

2. Which of the following did not occur at the peak of the last ice age？

A. Thick ice covered Scandinavia.

B. Temperatures fell 10-40 degrees.

C. Sea levels went up 500 feet.

D. Sea levels went down 400 feet.

3. Which of the following arguments as to why huge creatures had disappeared did Louis Agassiz prove wrong？

A. A flood occurred.

B. Disease killed them.

C. Violent killer bees attacked them.

D. They drowned.

4. Which of the following wanted to study the temperatures of the Earth for the last 600，000 years？

A. Louis Agassiz.

B. Milutin Milankovitch.

C. Albert Einstein.

D. Arnie Tor.

5. Which of the following states how many years ago the current intergalatial period began？

A. 11，700. B. 12，400.

C. 16，000. D. 20，000.

6. Which of the following statements is True？

A. Agassiz believed that a flood carried away the giant animals.

B. The current interglacial period began 15，000 years ago.

C. Milankovitch was a mathematician.

D. Agassiz was from France.