新托福考试必备：新托福TPO（1-24）听力原文文本TPO9

　　TPO 9 Lecture 3
　　Geology
　　Lecturer: So, continuing our discussion of desert lakes, now I want to focus on
　　what's known as the "Empty Quarter". The "Empty Quarter" is a huge area of
　　sand that covers about a quarter of the Arabian Peninsula. Today it's pretty
　　desolate, barren and extremely hot. But there've been times in the past when
　　monsoon rains soaked the Empty Quarter and turned it from a desert into
　　grassland that was dotted with lakes and home to various animals. There were
　　actually two periods of rain and lake formation: the first one began about
　　35000 years ago; and the second one dates from about 10000 years ago.

　　Female Student: Excuse me, Professor. But I'm confused. Why would lakes
　　form in the desert? It's just sand, after all.

　　Lecturer: Good question! We know from modern day desert lakes, like Lake
　　Eyre, South Australia, that under the right conditions, lakes do form in the
　　desert. But the Empty Quarter lakes disappeared thousands of years ago.
　　They left behind their beds or basins as limestone formations that we can still
　　see today. They look like low-lying, white or grey builds, long, narrow hills with
　　flat tops, barely a meter high. A recent study of some of the formations
　　presents some new theories about the area's past. Keep in mind though that
　　this study only looked at 19 formations. And about a thousand have been
　　documented. So there's a lot more work to be done.
　　According to the study, two factors were important for lake formation in the
　　Empty Quarter: first the rains that fell there were torrential. So it would've been
　　impossible for all the water to soak into the ground. Second, as you know,
　　sand dunes contain other types of particles, besides sand, including clay and
　　silt. Now, when the rain fell, water ran down the sides of 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 started forming
　　35000 years ago, the limestone formation 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, along
　　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. 37000 years ago, the dunes were probably nicely rounded at the top,
　　so the water just ran right down their sides to the desert floor. But there were
　　thousands of years of wind between the two rainy periods, reshaping the
　　dunes. So, during the second rainy period, the dunes were kind of chopped up
　　at the top, full of hollows and ridges, and these hollows would've captured the
　　rain right there on the top.
　　Now, in grassland of Lake Ecosystem, we'd expect to find fossils from a variety
　　of animals, and numerous fossils have been found at least at these particular
　　sites. But, where did these animals come from? Well, the theory that has been
　　suggested is that they migrated in from nearby habitats where they were
　　already living. Then as the lakes dried up, they died out. The study makes a
　　couple of interesting points about the fossils, which I hope will be looked at in
　　future studies. At older lake sites, their fossil remains from hippopotamuses,
　　water buffalo, animals that spend much of their lives standing in water, and
　　also, fossils of cattle. However, at the sites of the more recent lakes, there’re
　　only cattle fossils, additional evidence for geologists that these lakes were
　　probably smaller, shallower, because cattle only use water for drinking. So
　　they survive on much less. Interestingly, there are clams and snail shells; but,
　　no fossils of fish. We're not sure why. Maybe there is a problem with the water.
　　Maybe it was too salty. That's certainly true of other desert lakes.