Worksheet

The Bluff Below the Weather Station: Fossils, Ash Beds and Deep Time

Grade 8 geologic time practice: fossil counts and dated ash beds in a storm-exposed bluff show how fossil changes mark boundaries in Earth's history.

Grade 8ScienceInteractive · Printable

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1 NGSS standard
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11 questions
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Format: Interactive · Printable
Grade: Grade 8
Subject: Science
Question types: Fill in the blank, Multiple choice, Circle the answer, Sorting, Short answer

About this worksheet

What students practice

Students use fossil counts in rock layers to find where a boundary in the geologic time scale belongs.
Students use layer order and dated ash beds to place rock layers and events in Earth's history.
Students scale Earth's 4.6-billion-year history to a model timeline.

How to use

Read the passage and study the bar graph together, then let students work through the tables on their own. Discuss question 4 as a class: why does one rock face need evidence from other continents?

Curriculum

NGSS standard covered

MS-ESS1-4

Explain how rock strata and fossils are used to organize Earth's 4.6-billion-year history.

Preview

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The Bluff Below the Weather Station: Fossils, Ash Beds and Deep Time

Use a fresh rock face, fossil counts and ash bed ages to place a lakeshore bluff in Earth's history.

Part 1: The fresh bluff face

Read the passage and study Table 1 and the bar graph. Then answer questions 1–5.

The Night the Bluff Slid

1Harrow Point Middle School stands on a high bluff above a large lake. Its weather club runs a station near the edge of the bluff and posts the readings to a forecast app that many families check before school. On one night in May, the station logged 96 mm of rain in six hours. By morning, a slice of the bluff about 40 m wide had slumped onto the beach below, leaving a fresh, steep face of rock. The station and its fence were safe, but the town closed the beach.

2Eighth grader Anouk Szabo posted photos of the new rock face on the club's app, and a geologist from the state survey, Dr. Ngozi Farrow-Lund, asked to visit. The club watched from behind the fence at the top while her team worked on the beach in hard hats, staying away from the base of the unstable slope and using a drone camera for close-ups. The face shows five flat, unbroken layers of rock that formed on an ancient sea floor. Two of the layers hold a thin bed of volcanic ash. Ash from a large eruption can settle over a huge area within days, and crystals in the ash can be dated in a lab.

3The team split fallen blocks from each layer and counted how many different kinds (species) of fossils each layer held. Their counts are in the bar graph. Of the 20 kinds of fossils in layer 2, only 3 also turn up in layer 3: one kind of clam, one kind of snail and a small brachiopod the team calls brachiopod L. Higher in the bluff, new kinds appear, including ammonites, coiled sea shells that the team did not find in layers 1 or 2.

4Dr. Farrow-Lund explained that geologists divide Earth's 4,600-million-year history into large spans such as eras. The spans are not equal blocks of time. Many of their boundaries are placed at levels where the fossils change sharply in rocks around the world, and dated ash beds near those levels tell geologists when the changes happened.

5Anouk now wants to add a "deep time" loop to the club's app: a 46-second animation of Earth's whole history, in which each second of the loop stands for 100 million years. The loop starts with Earth's formation at 0 seconds and ends with today at 46 seconds.

Table 1: Fossil Groups in Each Layer (listed from top to bottom)
Layer	Rock	Trilobites & horn corals	Brachiopods	Clams & snails	Ammonites
5 (top)	Tan sandstone, 4 m	—	—	Yes	Yes
4	Gray shale, 3 m	—	—	Yes	Yes
3	Dark shale, 2 m	—	Brachiopod L only	Yes	—
2	Limestone, 5 m	Yes	Yes, including L	Yes	—
1 (bottom)	Limestone, 6 m	Yes	Yes	Yes	—

Ash beds: ash bed Y lies near the top of layer 1 and is 253.8 million years old. Ash bed X lies 1 m above the bottom of layer 3 and is 251.6 million years old.

Use the bar graph.

Layer 3 holds fewer kinds of fossils than layer 2.

Use paragraph 3. Count the kinds of fossils in layer 2 that do not turn up in layer 3.

Of the 20 kinds of fossils in layer 2, % are missing from layer 3.

Use paragraphs 3 and 4 and the bar graph. Between which two layers would Dr. Farrow-Lund most likely look for a boundary of the geologic time scale?

A. Between layers 2 and 3, where most kinds from layer 2 disappear
B. Between layers 4 and 5, where gray shale changes to tan sandstone
C. Between layers 3 and 4, where the count of kinds rises by three
D. Between layers 1 and 2, where the number of kinds rises from 18 to 20

Dark shale like layer 3 often forms in deeper, muddier water than limestone. A club member says, "Maybe the sea here just got muddier and the animals moved away. That would not be a worldwide change." Which new evidence would best show that the drop in fossils was a worldwide change?

A. Blocks from layer 3 are darker and softer than blocks from layer 2.
B. Rock 500 m farther along the same bluff shows the same drop above layer 2.
C. Clams and snails are found both below and above the drop in this bluff.
D. Rocks in China and Italy show a similar drop just above ash about 252 million years old.

Circle each observation that, by itself, shows that layer 3 is younger than layer 1.

Layer 3 lies above layer 1, and the layers are flat and unbroken.Layer 3 holds fewer kinds of fossils than layer 1.Ash bed X in layer 3 was dated as younger than ash bed Y in layer 1.Layer 3 is a thinner layer than layer 1.Clams were found in both layer 1 and layer 3.

Part 2: Placing the bluff in Earth's history

Use Tables 2 and 3 together with Part 1 to answer questions 6–11.

Table 2: Some Fossil Groups Around the World (Ma = million years ago)
Fossil group	Oldest known	Youngest known
Trilobites	about 521 Ma	about 252 Ma
Horn corals	about 460 Ma	about 252 Ma
Ammonites	about 409 Ma	about 66 Ma
Clams	about 500 Ma	Still living today

Table 3: Main Divisions of Earth's History
Division	Began	Ended
Precambrian time	4,600 Ma	539 Ma
Paleozoic Era	539 Ma	252 Ma
Mesozoic Era	252 Ma	66 Ma
Cenozoic Era	66 Ma	Today

Use Tables 1 and 3. Which boundary on the time scale does the drop between layers 2 and 3 most likely match?

A. The end of Precambrian time (539 Ma), because trilobites are found just below the drop
B. The end of the Paleozoic Era (252 Ma), because that age falls between the two ash bed ages
C. The end of the Mesozoic Era (66 Ma), because ammonites are found in the layers above the drop

Use paragraph 5 and Table 3. In Anouk's loop, each second stands for 100 million years. Round to the nearest tenth.

In the 46-second loop, the whole Paleozoic Era would fill about s.

The loop starts with Earth's formation at 0 s and ends with today at 46 s. The drop in fossils happened about 252 million years ago. Do not round.

The drop in fossils would appear at s into the loop.

At a quarry 300 km away, a geologist finds a limestone layer with trilobites, horn corals and clams, but no ash bed. Use Tables 2 and 3. What can she conclude about when the layer formed?

A. It formed before 521 Ma, in Precambrian time, since trilobites are very old fossils.
B. It formed in the Paleozoic Era, between about 460 Ma and 252 Ma.
C. It formed at about the same time as layer 3, since clams are found in both layers.
D. It formed in the Mesozoic Era, after the drop, since clams lived on after it.

10.

Sort each event by when it happened.

Ash bed Y settles onto the sea floor.Ammonites swim above the mud that became layer 4.A slice of the bluff slumps onto the beach after 96 mm of rain.Horn corals grow on the sea floor that became layer 2.Ash bed X settles into the mud that became layer 3.Waves start to wear away the fallen blocks on the beach.

Before the drop in fossils	After the drop, millions of years ago	This spring

11.

On the club's app, a parent comments: "Those layers were probably all laid down in one big storm long ago, like the storm that just broke the bluff." Make a claim about this idea. Use evidence from the tables, the bar graph and the passage, and explain your reasoning about how layers, ash beds and fossils show how much time the bluff records.