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Trapped in tar: a big dig is revealing Los Angeles's wild and deadly past.

This past January, workers began construction on a new subway station in the heart of Los Angeles, California. They're being more careful than usual as they dig beneath the city. Their machines could hit a mammoth!

Why are these ancient animals lurking beneath L.A.? About 40,000 years ago, during Earth's last ice age, California was teeming with mammoths and other enormous mammals (see Ice Age Who's Who, page 12). And some of these giants stuck around--literally.

Los Angeles is dotted with sticky tar deposits that formed naturally over millions of years. The gooey pools trapped unlucky animals passing by. Everything from massive mammoths to tiny mice died and were preserved in the ancient sludge.

Today, anyone who digs in the area has to be on the lookout. "It's likely subway workers will not only find fossils, but potentially whole new tar pits containing tens of thousands of fossils," says Emily Lindsey. She's the lead paleontologist at the La Brea Tar Pits and Museum in downtown L.A. Scientists there are using the tar pits to piece together the city's wild past.

In the Pits

Tar pits form very slowly. They start as natural oil fields deep underground. Over time, pressure from the rocks above forces some of the oil to the surface. The lighter parts of the oil evaporate, leaving behind pools of thick, sticky tar.

Visitors to the La Brea Tar Pits can see--and smell--the large, muddy-looking tar pits around the property. They can also watch paleontologists dig up fossils. Scientists find huge extinct mammals, like giant sloths and mammoths. They also find animals that still roam California today, like badgers, snakes, and birds.

Many of the preserved animals are predators. Dire wolves, an extinct species of wolf, are the most common. They were likely drawn to the area because they wanted to eat animals that were stuck in the tar. But when they went after their meal, they often got trapped too.

Digging Up Clues

Since 1875, paleontologists have unearthed more than 3.5 million fossils from the tar pits. Each find provides clues about California's past. For example, scientists have uncovered fossils of plants that live only in cold or foggy areas. That means that L.A.'s climate was much cooler and wetter thousands of years ago.

At the new subway site, across the street from the museum, workers don't want to damage any buried fossils. So the city hired monitors--people trained to look for fossils during construction. They watch the dirt as machines dig.

"Most people probably wouldn't notice a fossil. It's the same color as the dirt," says Ashley Leger. She's the paleontologist overseeing the process. "Monitors know the texture and shape of bones," she says.

Construction workers take extra care too. On most projects, they use an excavator's big claw to dig meter (3 feet) deep with each swipe. At this site, they dig just 15 centimeters (6 inches) at a time. "If it weren't for this process, the fossils would be in a million pieces," says Leger. "We want them to be saved for people from all over the world to study."

Look Out Below

The more fossils workers find, the more scientists will learn about a time when ice age giants roamed the land. "It's fascinating to sit in the tall buildings surrounding the tar pits and think about how this area once had mammoths wandering around," says Leger.

Those mammoths may be long gone, but the tar pits are still active. New sticky pools bubble up all the time. Workers place bright-yellow cones around them to warn visitors to watch their step.

"Things still get trapped in them--bugs, squirrels, and even trash," says Lindsey. "It's a modern-day record of life."

Ice Age Who's Who

Like mammoths. These animals Lived around the tar pit during Earth's last ice age, They became extinct 13,000 to 10,000 years ago.

SABER-TOOTHED CAT: These distant relatives of modern-day cats used their long teeth to slice open prey.

DIRE WOLF: Hunting in packs helped these dog-like animals take down big prey such as horses and bison.

GROUND SLOTH: These grass-eating giants stood 6 feet tall and weighed as much as a mid-sized car.

words to know

ice age--one of several periods in Earth's history when glaciers covered a large part of Earth

paleontologist--a scientist who studies the remains of plants or animals that lived in the past

evaporate--to turn from a liquid into a gas

fossil--the preserved remains of a living thing

climate--the average weather conditions of an area

hands-on science

Sticky Spots

How does temperature affect a tar pit?

Observe: Mammoths and other ice age animals got trapped in sticky tar pits.

Ask a Research Question: How does temperature affect the consistency of a tar pit?

Form Hypotheses Based on These Questions: Will a mammoth's foot sink faster into a warm tar pit or a cool one? Which will be easier to escape?

Materials: smock * newspaper * two same-size drinking glasses * measuring cup * molasses * spoon * two same-size bowls * warm water * ice water * two 3-ounce paper cups * 60 pennies * two same-size rubber bands * pencil and paper


1. Put on a smock and cover the desk with newspaper.

2. Make two model tar pits. Measure 1/4 cup of molasses into each glass, using the spoon to scrape it all in. The sticky molasses represents tar.

3. Fill one bowl about an inch deep with arm water. Fill the other with the same amount of ice water. Place a tar pit in each bowl. Let them sit for 10 minutes.

4. Meanwhile, make "mammoth feet." Put 30 pennies in each paper cup. Wrap a rubber band around each cup to form a handle over the opening. Make sure the rubber band is in the center of the cup.

5. After 10 minutes, remove the glasses from the water and place them on the newspaper. Pick up both "feet" by their handles. Lower one into each glass until the bottoms of the cups just touch the tar. Then drop them at the same time. Which sinks faster? Record your observations.

6. Now test how much force it takes to escape each tar pit. Push the mammoth feet to the bottoms of the glasses. Hook a finger under the handle of each foot and slowly pull up. Which is easier to remove? Record your observations.

Results: In which tar pit did the mammoth foot sink faster? Which was harder to escape?


1. How did the temperature of a tar pit change its effect on a mammoth foot?

2. Based on your results, when do you think animals would be most likely to get trapped in tar pits? Explain your reasoning.


* Hands-On: Sticky Spots (Student Edition, p. 13) Conclusions: 1. Warming the molasses made it thinner and less sticky. That made the mammoth's foot sink faster, but it also made it easier to escape the molasses. 2. Answers should be supported by evidence. Students could argue that animals are more likely to get trapped during warm weather because they would sink faster in tar pits. They could also argue that animals would be more likely to get stuck in cool weather because tar pits would be harder to escape.

(1) Which word does NOT describe the La Brea Tar Pits?

(A) thick

(B) ancient

(C) solid

(D) oily

(2) The fossils most often found in the tar pits are from--

(A) insects

(B) squirrels

(C) mammoths

(D) dire wolves

(3) Why do scientists want to remove fossils found at a construction site near the tar pits?

(A) to preserve the tar pits

(B) to attract visitors

(C) to make trains go faster

(D) to learn about the past

Trapped in Tar: 1. c 2. d 3. d

READING LEVELS: Lexile Level 930 / Guided Reading Level S

NEED A LOWER READING LEVEL? To access this article at a lower reading level, go to


Use a model to investigate how sticky tar pits trap animals and form fossils.



Core Idea: ESS2.A: The history of planet Earth

Practice: Developing and using models

Concept: Energy and matter


Reading Informational Text: 4. Determine the meaning of general academic and domain-specific words or phrases from a text.


Science: 3.2A, 3.5C, 4.2A, 4.5B, 5.2A, 5.7D, 6.2A

ELA: 3.4, 4.2, 5.2, 6.2

Lesson Plan

(1) Do a demonstration to define viscosity.

Place a plastic shoebox on your desk in front of the class. Fill a small glass with about V* cup of water and another with about Vi cup of molasses. Hold up the glasses and tell the class what they contain.

Ask: If I pour the contents of these glasses into the box at the same time, which liquid will reach the box first? (water)

Pour the glasses' contents into the box. Point out that the molasses flows more slowly. Ask: What properties of molasses make it flow more slowly? (thickness, stickiness)

Write the word viscosity on the board. Explain that this word describes how much a material resists flowing. Molasses is very viscous. Ask: Would it be easier to climb out of a swimming pool full of water or molasses? (water)

(2) Read and review the article.

Explain that tar is another very viscous material. Read the article as a class, switching readers after each paragraph. Then ask:

* What words or phrases from the article tell you that tar pits are viscous? (Answers could include the words gooey, sludge, thick, and sticky, and descriptions of animals getting stuck in tar.)

* How do tar pits form? (Tar pits begin as natural oil deposits underground. Pressure from rocks above pushes some of the oil to the surface. Exposure to air causes parts of the oil to evaporate, leaving sticky tar.)

* How do the tar pits help scientists learn about the past? (The tar pits contain fossils of animals that lived during the last ice age.)

(3) Use a skills sheet to practice analyzing fossils.

To better understand how scientists interpret data from tar pits, have students complete the "Explaining Evidence" skills sheet (T7). Go over the answers as a class.

(4) Do the hands-on investigation.

Explain that temperature can affect the viscosity of a material. Divide students into small groups and have them complete the hands-on experiment "Sticky Spots" on page 13 of the Student Edition.

TEACHING TOOLS available at

Skills sheets:

Explaining Evidence (T7): Answer questions about a diagram showing fossils in layers of rock.

Think It Through (T11): Answer critical-thinking questions about the article.

Word Workout (online only): Define and practice using content-area vocabulary.


Excavation 101: The La Brea Tar Pits: Watch a paleontologist who works on the La Brea Tar Pits explain her research.

Caption: Columbian mammoths, cousins of woolly mammoths, were one of many species that got stuck in tar pits thousands of years ago.

Caption: Today, scientists are learning about ancient animals by studying fossils preserved in the tar.

Caption: Scientists and volunteers dig for fossils in a tar pit.

Caption: Saber-toothed cat skull


Explaining Evidence

In "Trapped in Tar" (pp. 10-13), you read about how scientists are studying fossils to learn about life during Earth's last ice age. Scientists use fossils as evidence to draw inferences about the past. The diagram below shows fossils that appear within rock layers of varying ages. The deeper a rock layer is underground, the older it is. Use the diagram to draw your own inferences and answer the questions.

1. Which layer of rock is older: Layer C or Layer E?

2. Observe the fossils in Layer C. What type of life existed during that time?

3. Observe the fossils in Layer A. What type of life existed during that time?

4. Based on your observations, make a claim about how the environment in this area changed over time.

5. What evidence supports your claim?

* Explaining Evidence (Reproducible, T7)

1. layer E 2. fish and other aquatic animals 3. animals that walk on land 4. Answers will vary but may include: In this region, the landscape changed over time from being underwater to being dry land. 5. Layer C is from an earlier time period than layer A, so the fish lived in the area before the land animals.


Think It Through

Directions: Use the article "Trapped in Tar" (pp. 10-13) to answer each question.

1. The article asks, "Why are these ancient animals lurking beneath the city?" What does the word lurking mean in this sentence?

(A) lying in a hidden place

(B) traveling from one place to another

(C) declining in health

(D) escaping by digging

2. Which line from the article supports the answer to Question 1?

(A) "And some of these giants stuck around--literally."

(B) "Today, anyone who digs in the area has to be on the lookout."

(C) "Tar pits form very slowly."

(D) "Over time, pressure from the rocks above forces some of the oil to the surface."

3. Which sentence BEST represents the main idea of this article?

(A) Digging under L.A. exposes fossils that provide clues about Earth's past.

(B) The movie Ice Age is based on a true story.

(C) Paleontologists find more predator fossils than prey fossils while digging in L.A.

(D) Construction crews working in L.A. have to be careful not to damage fossils.

4. How do the photographs in the article support the article's main idea? Use details from the images to support your answer.

5. What are two ways that the presence of fossils under Los Angeles is affecting how the city is constructing its new subway? Use details from the article in your answer.

* Think It Through (Reproducible, T11)

1. a 2. b 3. a 4. The photos support the main idea by showing researchers excavating and examining fossils from tar pits. The illustrations on page 11 and in the sidebar on page 12 show what the animals that were trapped in the tar pits might have looked like. 5. The city hired monitors to look for fossils during construction. Workers have also been digging up small sections of ground at a time to avoid damaging fossils.
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Title Annotation:earth science
Author:Barth, Amy
Geographic Code:1USA
Date:May 1, 2017
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