OFFICIAL9 According to the professor, what feature of the sand dunes made the formation of the lakes possible?

[00:00.00]Narrator: Listen to part of a lecture in a geology class.
[00:05.79]MALE PROFESSOR: So, continuing our discussion of desert lakes, now I want to focus on what is known as the Empty Quarter. [00:13.69]The Empty Quarter is a huge area of sand that covers about a quarter of the Arabian Peninsula. [00:21.83]Today it’s pretty desolate … barren and extremely hot.
[00:26.38]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. [00:39.26]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.
[00:51.50]FEMALE STUDENT: Excuse me, professor, [00:54.33]but I’m confused. [00:55.66]Why would lakes form in the desert? [00:57.44]It’s just sand, after all.
[00:59.22]MALE PROFESSOR: Good question. [01:00.47]We know from modern-day desert lakes … like Lake Eyre in South Australia … that under the right conditions, lakes do form in the desert.  [01:10.45]But the Empty Quarter lakes disappeared thousands of years ago. [01:15.57]They left behind their beds, or basins, as limestone formations that we can still see today. [01:23.93]They look like low-lying white or gray buttes … long, narrow hills with flat tops … barely a meter high.
[01:31.98]A recent study of some of the formations presents some new theories about the area's past. [01:38.35]Keep in mind, though, that this study only looked at 19 formations … [01:43.16]and about a thousand have been documented, [01:45.43]so there’s a lot more work to be done.
[01:47.98]According to this study, two factors were important for lake formation in the Empty Quarter. Um, first, the rains that fell there were torrential. [01:58.48]So it would have been impossible for all the water to soak into the ground. [02:02.75]Second, as you know, sand dunes contain other types of particles besides sand … including clay and silt.
[02:11.16]Now, when the rain fell, water ran down the sides of the dunes, carrying clay and silt particles with it. [02:18.73]And wherever these particles settled, they formed a pan … a layer that water couldn't penetrate. [02:25.87]Once this pan formed, further runoff collected and formed a lake.[02:32.35]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.
[02:52.04]So the theory is the lakes formed there … on the desert floor … in these long, narrow valleys. [02:59.25]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.[03:11.43]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 …
[03:22.48]Another difference, very important today for distinguishing between older lake beds and newer ones … is the location of the limestone formations: [03:32.24]the more recent beds are high up in the dunes.[03:35.80]Why these differences? [03:38.46]Well, there are some ideas about that and they have to do with the shapes of the sand dunes when the lakes were formed.
[03:46.82]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. [03:56.80]But there were thousands of years of wind between the two rainy periods … reshaping the dunes. [04:03.70]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.
[04:17.24]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. [04:30.03]But … where did these animals come from? [04:33.14]Well, the theory that has been suggested is that they migrated in from nearby habitats where they were already living. [04:42.02]Then, as the lakes dried up, they died out.
[04:46.18]The study makes a couple of interesting points about the fossils... which I hope will be looked at in future studies. [04:53.63]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.
[05:07.65]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, [05:22.30]so they survive on much less.[05:24.55]Interestingly, there are clam and snail shells, but no fossils of fish.  [05:31.39]We're not sure why. [05:32.80]Uh, maybe there was a problem with the water … [05:36.30]maybe it was too salty.[05:38.13]That's certainly true of other desert lakes.