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发表于 2010-1-23 23:40:30
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tpo 9 l3
Listen to a part of lecture in geology class.
So, continue we are (continuing our) discussing the 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 (Arabian Peninsula). Today, it is pretty desert (desolate), barren and extremely hot. But there’ve been times in the past when (monsoon) rains (soaked) in Empty Quarter and turn it from a desert into a grass land that was (dotted with) lakes and home two (various) animals. There were actually two periods of rains and lake formation. The first one began about 37,000 years ago, and the second one dates about 10,000 years ago.
Excuse me, professor. But I am confused, why would the lake form the desert, it is just sand after all.
Good question. We know from modern (day) desert lakes like (Lake Eyre) South Australia, that under the right conditions, lakes do form desert. But the Empty Quarter lakes disappear thousands of years ago. They live (left) behind their beds or (basins) as limestone formations that we still can see today. They look like low (lying) white or grey builds, long narrow hills from (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 these studies only looked at 19 formations. And about a thousand (have been) documented. So there is (a lot more work) to be done. According to the study, two factors were important for lake formation in Empty Quarter. First, the rains fell there were (torrential). So it would (have been) impossible for all the water (to soak into) the ground. Second, as you know, sand dune contains other types of particles (beside) the sand including (clay) and silt. Now, when the rain fell, water ran down the sides of the dunes, carrying clay and silt particle with it. And where these particles settled, they formed the pan. A layer water cannot penetrate. Once the pan formed, (further run-off collected) and formed a lake. Now, the older lakes about half formations, the one (started) forming 37,000 years ago, (the) limestone formations we see, they are up to a kilometer long, but only a few meters wide, and they are (scattered along the desert floor) in valleys between the dunes. So the theory is the lake formed here, in the desert floor, in these long narrow (valleys). And we know, because we know similar ancient desert lakes. We know that these lakes does not last very long, from a few month to a few years average. As for more recent lakes, the one from 10,000 years ago, they seemed to be smaller, so may (have) dried up quickly. Another difference, very different today for distinguish between old lake beds and new (newer) ones, is the location of limestone formations. The more recent beds are higher (high up) in the dunes. Why these difference? Well, there are some ideas about that? They have to do with the shapes of the sand dune when lakes were formed. 37,000 years ago, the dunes were probably nicely round at the top. So the water just ran 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 are a kind of (chopped up at the top full of hollows and ridges). And these hollows would have capture the rains right there on the top. Now, in grass land of (Lake) Ecosystem. We except to find (fossils) from a variety of animals. And numerous fossils have been found at least at these particular sides. But where these animals come from? Well, the theory that has been suggested is that they migrate from nearby habitats where they were already leaving (living). Then is (as) the lake dry up, they died out. The study makes a couple of interesting point about fossils, which I hope will be look at in future studies. At older lake sties, their fossils remains from (hippopotamuses, water buffalo) animals spending much of their lives standing in water and also fossils in the (of) cattle. However, at the sites of lake recently, there are only cattle fossils. Addition evidence for geologists that these lakes were probably smaller, shallower, because cattle only use water to drinking, so they survive much less. Interestingly, there are (clams and snails shells); but no fossil of fish. We are not sure why. Maybe there is a problem with the water. Maybe was too salty. That is certainly true of these (other) desert lakes. |
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