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Millions of years ago, long before the city of Los Angeles existed, the area was covered by the Pacific Ocean．Over time, it turned from sea to land．Oil came to the surface through cracks in the ground and pooled in low-lying areas, which are today known as the La Brea Tar Pits.
In warm periods, the oil that came from the ground became sticky and the surface became covered with leaves, dust, and even water．When animals came to drink, they became trapped．Predators that preyed on the trapped animals often became trapped themselves．This sticky area was perfect for fossilizing and preserving the remains of these animals．
Now the La Brea Tar Pits are one of the best places for excavating fossils．More than 3 million fossils have been found there since the early 1900s．There are some really big fossils, like those from animals like mammoths, saber-toothed tigers, and short-faced bears．However, fossils of plants, insects, and smaller animals are also valuable to paleontologists．
Microfossils help scientists form a picture of what life was like in the area around Los Angeles 40000 years ago．For example, by analyzing these fossils, scientists learned that the climate was moister and cooler than today, but not so much different．
To find out how old fossils are, scientists use a process called radiometric dating．Living things contain carbon．A small portion of this carbon is called carbon 14, which is unstable．It takes 5730 years for half of the carbon 14 to become stable, then the same amount of time for the remaining half to become stable, and so on．So measuring the amount of unstable carbon 14 remaining in a fossil allows scientists to accurately date it．By doing this they found that most fossils in the La Brea Tar Pits were between 8000 and 38000 years old．
Word List
excavating： digging things up
paleontologists： scientists who study life from the past
microfossils： very small fossils
You Practice
I．By the numbers—What do these numbers mean in the reading?
1）3 million ____________________
2）40000 ____________________
3）carbon 14 ____________________
4）5730 ____________________
5）8000 to 38000 ____________________
II．Answer the following questions.
1）Where are the La Brea Tar Pits?
2）How did so many animals become trapped at La Brea Tar Pits?
3）What kinds of fossils have been found at La Brea Tar Pits?
4）How is carbon 14 useful?
5）What was the weather like at La Brea Tar Pits 40000 years ago?
III．True / False / Not Given—Write T if the sentence is true, F if it is not true, or NG if there is no information about this sentence.
____ 1）The La Brea Tar Pits existed when dinosaurs roamed the Earth．
____ 2）Small fossils can be very important．
____ 3）There are not many fossils remaining at La Brea Tar Pits．
____ 4）Radiometric dating cannot be used for fossils that are millions of years old．
____ 5）Los Angeles has existed for about 38000 years．