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TPO54托福听力lecture4听力文本
Title: New evidence of River Hypothesis
Listen to part of a lecture in a geology class.
Professor: About 30 years ago, a geologist named Edward Cotter, that’s C-O-T-T-E-R, published a paper that contained a very interesting hypothesis.
He was studying ancient rivers in the North American mountain chain. And he noticed that about 450 million years ago, rivers started to behave differently. Before then, rivers were wide, shallow and straight. But after that time, they became deeper and had move curves. They became increasingly meandering, and that’s actually how rivers behaved till these days.
So why might this change have happened?[a1]
Student: Maybe there was some kind of a climate shift?
Professor: Well, lots of climate shifts have happened since then.
Student: Was the change worldwide? Or just in that geographical area?
Professor: Well Cotter speculated that rivers changed worldwide, but he couldn’t prove it. Because he only had evidence from the one North American mountain chain. But his studies gave him an idea about why rivers started to change. He hypothesized it had to do with the spread of plant life on earth.[a2]
Student: So there was no plant life before 450 million years ago?
Professor: Very little according to fossil records. Anyway, geologists were intrigued by this hypothesis which claims that as plants evolved and spread, they had an effect on terrain and rivers. In the past 30 years, more studies have been done. And now we have a lot of data about river systems from around 450 million years ago from all over the world.
In a recent study, a couple of researchers gathered together the existing data and combine them with their own new field data to get a comprehensive picture of the situation.
Their study was specifically designed to identify changes in the shapes of rivers during the time period when vegetation was evolving.
And when the researchers compared the data about river shapes with data they have collected about plant life from the same period, the data seemed to prove Cotter’s hypothesis.[a3]
Student: OK, but how did plant life affect rivers?
Professor: Well, in order to answer that question, we need to look at the geological evidence.
You see, as rivers flow, they leave layers of sediment behind that eventually fossilized. The content, thickness and shape of these fossilized layers and rocks gave us information about how rivers flowed. The earliest records from 500 million years ago show that the sediment in river deposits was largely composed of quartz grain of sand and gravel. That tells us that rivers weren’t defined, they were very shallow and wide, almost like floods.[a4]
But around the time of the rise of plant life, the content of these sediment layers began to change. The quartz grains became much finer. And we see evidence of mud. This suggests that plants promoted the preservation of mud when they sent their roots into the ground. The roots helped to reinforce the ground, which in turn allowed for the creation of river banks.
And we also see evidence of a process called lateral accretion.
Lateral accretion happens when water flows around the curve or bend in the riverbed. Now the speed of the flow on the outside of the bend is fastest, and slowest on the inside of the bend. This sets up what’s called the secondary flow across the river bottom. The fast flowing water on the outside of the bend digs out material from the riverbank and pushes these material laterally across the bottom, and it gets deposited on the other side of the river, on the inner side of the bend.
So, when we see in the sediment layers, evidence of lateral accretion, the erosion on one side and deposit on the other, that’s an indicator that meandering rivers existed.[a5]
And according to the study, strong evidence of lateral accretion appears in the geological record. At the same time, there is also evidence of plants with underground root systems. This suggests that plants promoted the development of modern rivers by creating stable banks, which resulted in the flow of water in single meandering channels.
Student: So it looks like the researchers were able to prove that hypothesis.
Professor: Well, there is no denying that the study presents a very strong case. But some questions about this hypothesis remain. For example, it’s well-known that on other planets, like Mars, there is clear evidence of meandering rivers. But it’s there any evidence of vegetation on Mars?[a6] I think not.
[a1]Q12 出题点
[a2]Q13 出题点
[a3]Q14 出题点
[a4]Q15 出题点
[a5]Q16 出题点
[a6]Q17 出题点
TPO54托福听力lecture4题目解析
12、What does the professor mainly discuss?
A. A hypothesis that rivers formed before the rise of plant life
B. A study of the effects that rivers have on soil formation
C. A debate surrounding two opposing hypotheses
D. New evidence in support of a previously proposed hypothesis
答案：D
解析：本题为内容主旨题，题目问教授主要讨论什么?由出题点可知，以前Cotter提出河流的改变是世界性的，但是Cotter并没有证明这个理论，纵观全文，文章接下来的部分都是针对Cotter的这个观点提出的新的论据，则D选项正确。
A选项的意思为：河流形成于植物圈形成之前的假设，这是一个事实，并不是一个假设，且不是文章主旨，则A项不正确;
B选项的意思为:河流对于土壤形成的影响的研究，河流确实对土壤的形成有影响，但是这只是文章中的一个小细节，并不是教授主要想要说明的问题;
C选项的意思为：两个对立假设的争论，文章中并没有提到两个对立的观点，更没有说关于两个观点的争论，故排除。
13、What limitation of Cotter's research does the professor mention?
A. Cotter's research findings could not be replicated.
B. Cotter’s research was based on one geographical area.
C. Cotter's research did not take into account lateral accretion.
D. Cotter's research did not account for changes in climate.
答案：B
解析：本题为细节题，题目问教授提出Cotter的研究的局限性是什么?由出题点可知 ，Cotter的研究证据仅仅来源于北美山脉，而河流的变迁则是一个全球性的问题，则Cotter的研究的局限性是它仅仅在一个区域，则B选项正确。
A选项的意思为Cotter的研究结果不能被复制，显然是不对的，因为Cotter的发现是一个全球性的问题，并不是不可以被复制，是可以适用于其他的相关的研究的;
C选项的意思为Cotter的研究没有考虑侧向堆积的影响，文章中的“lateral accretion”是教授提出的支持Cotter的观点的一个论据，故排除;
D选项的意思为:Cotter的研究没有考虑气候的改变，气候的改变是教授提出的一个支持Cotter论点的一个论据，所以不能说Cotter的观点没有考虑到气候的改变这个因素。
14、What did researchers try to confirm about rivers in a recent study?
A. That rivers now have the same shape worldwide
B. That rivers were responsible for the spread of plant life
C. That a change in the shape of rivers is linked to the spread of plant life
D. That most rivers emerged during a specific geological time period
答案：C
解析：本题为细节题，题目问在最近的调查研究中，学者们想要证明什么?由出题点可知，研究者们把河流形状的数据与他们收集到的同时期的植物生命的数据相比较，来证明科特的假设，所以C选项正确。
A选项的意思为：现在世界上的河流具有相同的形状，显然是不对的，一方面是不符合逻辑的，另外一方面文章中也没有提及;
B选项的意思为:河流对植被的发展起着一定的作用，文章中虽然提到过这个问题，但是并不是学者们的研究中想要证明的;
D选项的意思为:大部分的河流出现在特定的地理时间内，文章中并没有提到过这个问题，故排除。
15、Why does the professor describe the composition of the sediment of the earliest rivers?
A. To identify the evidence used to determine how the rivers flowed
B. To explain why the rivers could easily dry out
C. To suggest that the sediment was once fine sand
D. To suggest that the geological record might be flawed
答案：A
解析：本题为组织结构题，题目问教授为什么要描述最早的河流的沉积层的形成?由出题点可知，教授是为了找到河流是怎样流动的证据，则A选项正确。
B选项的意思为:为了解释为什么河流容易干涸，原文未提及，排除;
C选项的意思为：为了表明沉积层原来是由沙子组成的，原文未提及，排除;
D选项的意思为:为了表明地理记录可能有缺陷，沉积层的组成和地理记录有缺陷没有任何关系，且原文未提及缺陷，排除。
16、According to the professor, what is an effect of lateral accretion on a meandering river?
A. Bends in the river become gradually straighter.
B. Material from one riverbank is deposited on the other.
C. The intensity of the river’s flow increases.
D. The growth of plants on the outer riverbank is enhanced.
答案：B
解析：本题为细节题，题目问，由教授可知，侧向堆积对蜿蜒的河流有什么影响?由出题点可知，快速的水流将河岸的一边的泥土冲击到另一边，形成了弯曲的河流，则B选项正确。
A选项的意思为：弯曲的河流在逐渐的变直，原文未提及，故排除;
C选项的意思为：水流的强度逐渐增大，文章没说侧向堆积能加大水流强度，只提到河湾外部的水流速度是最快的，故排除;
D选项的意思为:河岸外部的植物的生长被加强了，原文未提及，故排除。
17、重听题：Why does the professor say this:
A. To find out what the students know about Mars
B. To question certain geological studies of other planets
C. To express a reservation he has about the hypothesis
D. To indicate that he is going to change the topic
答案：C
解析：本题为功能题，需要重听一段内容后再作答。重听部分，教授明确提到这个假设还存在一些问题，所提他应该是持怀疑态度的。再结合重听部分下文中，教授说的“I think not”，可以进一步确定对于这个假设，教授的态度是有所保留的，故正确答案为C。
A选项的意思是：看看学生是不是了解火星，错误，教授不是这个意思。
B选项的意思是：质疑某个关于其他星球的地理学研究，错误，这里确实质疑了，但是质疑的内容是前面的假设，没提到其他星球的地理学研究，故排除。
D选项的意思是：来说明他要转移话题了，但原文没有这层意思，排除。
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