Directions: Now answer the questions.
1. What is the lecture mainly about?
A. Reasons that geologists study lake fossils in desert regions
B. A comparison of ancient and present-day lakes in desert environments
C. Geological evidence for the formation of ancient sand dunes
D. A hypothesis for how some ancient desert lakes formed
2. What is the professor’s opinion about the conclusions of the recent study of the limestone formations in the Empty Quarter?
A. They have changed the way geologists study desert environments.
B. They contradict findings about similar desert lakes.
C. They explain the causes of monsoons in the desert.
D. They need to be confirmed by additional studies.
3. According to the professor, what feature of the sand dunes made the formation of the lakes possible?
A. The degree of slope of the sides of the dunes
B. The presence of clay and silt particles in the dunes
C. The position of the dunes relative to the wind and rain
D. The narrowness of the valleys between the dunes
4. How is it possible to determine in which rainy period a lake was formed? Choose 2 answers.
A. By examining the location of the lake bed
B. By measuring the amount of sand covering the lake bed
C. By examining the color of the limestone formation
D. By identifying the types of fossils found in the limestone
5. What does the professor imply about the lack of water buffalo and hippopotamus fossils in the more recent lakes?
A. The level of water in the lakes was not sufficient for these animals.
B. The bottoms of the lakes were too sandy for these animals to stand in.
C. The location of the lakes made them too difficult for these animals to reach.
D. The vegetation near the lakes did not attract these animals.
6. What possible explanation does the professor give for the apparent absence of fish in the most ancient lakes?
A. The presence of predators
B. Lack of appropriate food
C. Lack of suitable water
D. Extreme desert temperatures
Listen to part of a lecture in a geology class.
So, continuing our discussion of desert lakes, now I want to focus on what is 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 37,000 years ago. And the second one dates from about 10,000 years ago.
Excuse me, professor, but I’m confused. Why would lakes form in the desert? It’s just sand, after all.
Good question. We know from modern-day desert lakes. . . like Lake Eyre in 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 gray buttes … 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 this study, two factors were important for lake formation in the Empty Quarter. Um, first, the rains that fell there were torrential. So it would have been impos-sible 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 wher-ever these particles settled, they formed a pan … a layer that water couldn’t penetrate. Once this pan formed, further runoff collected and formed a lake.
Now the older lakes … about half the formations, the ones that started forming 37,000 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 10,000 years ago … Well, they seem to have been smaller and so may have dried up more quickly. … Another difference, very impor-tant 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. 37,000 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 have captured the rain right there on the top.
Now, in a grassland and 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, there’s fossil remains from hippopotamuses, water buffalo… uh, 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’s 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 clam and snail shells, but no fossils of fish. We’re not sure why. Uh, maybe there was a problem with the water. … maybe it was too salty. That’s certainly true of other desert lakes.