问题 问答题 简答题

试说明冲积扇沉积的一般特征。

答案

参考答案:

①由山谷口向盆地方向呈放射状散开,平面上呈锥形、朵状或扇形;纵剖面上呈下凹透镜状或楔形,横剖面为上凸透镜状。

②可单个出现,但多数是许多冲积扇彼此相连和重叠,形成沿山麓分布的带状或裙边状的冲积扇群。

③河道从冲积扇顶端呈放射状向下坡方向延伸、河道逐渐变浅,在交切点处河道与冲积扇表面相交,再向下游河流沉积物在冲积扇表面形成沉积朵状体,为直流河或辫状河。

④半径从小于100m到大于150km,通常平均小于10km,厚度变化从几米到8km。

⑤为陆上沉积体系中最粗、分选最差的近源沉积,向下游并入细粒低坡度河流中。

阅读理解

阅读理解.

     The dinosaurs once lived happily on the earth. But 65 million years ago, they suddenly died out. What

happened to them?

     Scientists think that a change in temperature may be to blame. They have come up with lots of ideas

to explain the end of the dinosaurs. 

     The most famous one is the minor planet (小行星) idea. Let's go back to that day 65 million years ago

to see what happened. Everything was OK before a large minor planet suddenly hit the earth. What a

terrible hit! It caused big fires, storms, acid rain (酸雨), and even earthquakes and volcano eruptions. The

hit produced a lot of dust, which flew up into the air and blocked the sun for months. The earth became

very cold without sunlight. The plants died first. Then the grass eating animals died of hunger. Then the

meat eating animals could not find food, either. They began to eat each other, but after a while they died

out, too. The dinosaurs, either plant-eaters or meat-eaters, did not live through the disaster. What a sad

story !

     In April, a British scientist thought of a new idea about the death of the dinosaurs.

     He thinks that after the minor planet hit the earth, the temperature change left more dinosaur

boys than girls! Without enough female dinosaurs to hatch the eggs, the dinosaur population became

smaller and smaller.

1.                  may be the main reason for the dinosaurs dying out according to scientists.

A. Big fires

B. Earthquakes

C. Acid rain

D. Temperature change

2. The passage mentioned                  ideas that the scientists came up with about the end of the dinosaurs.

A. two                        

B. three

C. four                        

D. five

3. The underlined expression "volcano eruptions" probably is                   .

A.the name of a minor plane

B. one kind of dinosaurs

C. one kind of disaster

D. the name of an earthquake

4. The earth became cold because                    .

A. the sun refused to rise

B. the dust kept the sunlight away

C. there's too much rain

D. there are too many dinosaurs

5. This passage may be found in                    .

A. a science fiction      

B. a story book

C. a dictionary          

D. a science magazine

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