TPO9听力文本——Lecture.doc

【托福听力备考】TPO9听力文本——Lecture 3
众所周知，托福TPO材料是备考托福听力最好的材料。相信众多备考托福的同学也一直在练习这套材料，那么在以下内容中我们就为大家带来托福TPO听力练习的文本，希望能为大家的备考带来帮助the dunes, carrying
clay and silt particles with it. And wherever these particles settled, they
formed a pan, a layer that water couldn’t penetrate. Once this pan formed,
further run-off collected, and formed a lake.
Now, the older lakes, about half the formations, the ones that started
forming 37000 years ago, the limestone formations we see, they’re up to a
kilometer long, but only a few meters wide, and they’re scattered along the
desert floor, in valleys between the dunes. So, the theory is, the lakes formed
there on the desert floor, in these long narrow valleys. And we know, because of
what we know about similar ancient desert lakes, we know that the lakes didn’t
last very long, from a few months to a few years on average. As for the more
recent lakes, the ones from 10000 years ago, well, they seemed to have been
smaller, and so may have dried up more quickly.
Another difference, very important today for distinguishing between older
lake beds and newer ones, is the location of the limestone formations. The more
recent beds are high up in the dunes.
Why these differences? Well, there are some ideas about that, and they have
to do with the shapes of the sand dunes, when the lakes were formed.