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

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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)
LayerRockTrilobites & horn coralsBrachiopodsClams & snailsAmmonites
5 (top)Tan sandstone, 4 m——YesYes
4Gray shale, 3 m——YesYes
3Dark shale, 2 m—Brachiopod L onlyYes—
2Limestone, 5 mYesYes, including LYes—
1 (bottom)Limestone, 6 mYesYesYes—
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.
Kinds of Fossils in Each Layer02468101214161820Layer 1Layer 2Layer 3Layer 4Layer 5Layer (bottom to top)Kinds of fossils (species)
1.
Use the bar graph.

Layer 3 holds fewer kinds of fossils than layer 2.

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.

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?
Question 3 options
4.
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?
Question 4 options
5.
Circle each observation that, by itself, shows that layer 3 is younger than layer 1.

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 groupOldest knownYoungest known
Trilobitesabout 521 Maabout 252 Ma
Horn coralsabout 460 Maabout 252 Ma
Ammonitesabout 409 Maabout 66 Ma
Clamsabout 500 MaStill living today
Table 3: Main Divisions of Earth's History
DivisionBeganEnded
Precambrian time4,600 Ma539 Ma
Paleozoic Era539 Ma252 Ma
Mesozoic Era252 Ma66 Ma
Cenozoic Era66 MaToday
6.
Use Tables 1 and 3. Which boundary on the time scale does the drop between layers 2 and 3 most likely match?
Question 6 options
7.
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.

8.
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.

9.
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?
Question 9 options
10.
Sort each event by when it happened.
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.