Three Road Cuts, One Ancient Sea

Use layers and a fossil range chart to date and match rocks from a field trip.

Answer Key

The stops and the range chart

Read the passage and both tables. Use them to answer questions 1–5.

Fossil Hunting Along Ridge Road

1On a cool October morning, Ms. Rosalind Achebe's eighth-grade class rode a bus along Ridge Road with Dr. Tomas Varga, a geologist from the state survey. Long ago, workers blasted through the hills to build the road, leaving tall walls of bare rock called road cuts. The class had a permit to collect fossils at three stops. Everyone wore a hard hat, stayed at least 3 m from the rock walls and collected only from the piles of loose rock at the bottom.

2At each stop, Dr. Varga pointed out the layers and helped students match the loose pieces to the layer each had fallen from. The layers at all three stops are flat, with no signs of folding, tilting or faults. Most of the rocks are limestone, shale and sandstone that formed on the floor of a shallow sea. Students filled bags with shells of brachiopods, the stems of sea animals called crinoids, corals and pieces of trilobites.

3Back on the bus, Keziah Lindqvist asked how anyone could tell the age of a rock. Dr. Varga handed out a range chart from a field guide. Geologists build charts like this by finding the same fossil in many places, including in layers that also contain volcanic ash that can be dated. Each fossil species lived for only part of Earth's history. The chart shows the oldest and youngest rocks in which each species has been found. The abbreviation mya means million years ago.

4"When two species appear in the same layer," Dr. Varga explained, "that layer must have formed while both of them were alive. Their ranges overlap only for part of the time, so you can narrow the age." He also told the class that Earth is about 4.6 billion years old, which is 4,600 million years.

5Keziah noticed that one fossil, brachiopod M, was in her bag but not in the guide's list for other states. Dr. Varga said it has been found only in this one valley.

Field Guide Range Chart
Fossil	Oldest (mya)	Youngest (mya)	Where it is found
Brachiopod B	395	385	Across North America
Trilobite T	392	380	Across North America
Coral K	388	372	Across North America
Crinoid C	382	365	Across North America
Snail G	450	300	Across North America
Brachiopod M	386	383	This valley only

Layers at Each Stop (listed from top to bottom)
Stop	Layer	Rock	Fossils found
Stop 1	1A (top)	Sandstone	Crinoid C
Stop 1	1B	Limestone	Coral K, Trilobite T
Stop 1	1C (bottom)	Gray shale	Brachiopod B, Trilobite T
Stop 2	2A (top)	Gray shale	Crinoid C, Coral K
Stop 2	2B	Limestone	Trilobite T, Coral K
Stop 2	2C (bottom)	Sandstone	Snail G
Stop 3	3A (top)	Limestone	Crinoid C
Stop 3	3B (bottom)	Shale	Brachiopod B, Snail G

Which layer at Stop 1 is the oldest, and why?

A. Layer 1B, because limestone takes more time to form than shale or sandstone.
B. Layer 1A, because the rock at the top has been out in the weather the longest.
C. Layer 1C, because the layers are flat and the bottom layer was laid down first.
D. Layer 1A, because crinoids lived earlier than trilobites and brachiopods.

Use the range chart. Layer 1C holds both Brachiopod B and Trilobite T.

Layer 1C formed no earlier than 392 million years ago and no later than 385 million years ago.

Find the time window for layer 2A in the same way.

How many million years long is the time window in which layer 2A could have formed? 10 million years

A geologist in another state wants to know whether her rock layer formed at the same time as one on Ridge Road. Which fossil from the chart would be the most useful for matching the layers?

A. Brachiopod M, because its species lived for the shortest time
B. Brachiopod B, because it lived for a short time and is found widely
C. Crinoid C, because it shows up in the top layer at all three stops
D. Snail G, because it shows up in layers at two of the stops

Use the range chart. Which layer at Stop 1 has exactly the same time window as layer 2B?

A. Layer 1C, because both layers have a window that starts at 392 mya
B. Layer 1C, because it sits at the bottom like layer 2C
C. Layer 1A, because it is the top layer, just as 2A is at Stop 2
D. Layer 1B, because it holds the same two fossil species

Putting the time puzzle together

Use the passage, the range chart and the layer table.

Layer 2C holds only Snail G, which lived from 450 to 300 mya. What is the best way to narrow the age of layer 2C?

A. Use the middle of Snail G's range, so the layer is about 375 million years old.
B. Use the layer above it: 2C lies below layer 2B, so it formed before 2B, more than about 380 mya.
C. Use the rock type: sandstone formed at the same time as sandstone at Stop 1, so it formed after 1B.
D. Use its position at the bottom: the lowest layer at a stop formed when the snail first appeared.

Suppose layer 1C is about 385 million years old. Use paragraph 4. Round to the nearest whole percent.

About what percent of Earth's 4,600-million-year history has passed since layer 1C formed? 8%

Sort each statement by whether the field data support it.

Layer 1C is older than layer 1A.Layer 1B formed before layer 1C.Layers 3B and 1C could have formed at the same time.Snail G narrows the age of layer 3B more than Brachiopod B does.Layer 3A is younger than layer 3B.Layer 2C formed less than 380 million years ago.

The data support it	The data show it is false
Layer 1C is older than layer 1A. Layers 3B and 1C could have formed at the same time. Layer 3A is younger than layer 3B.	Layer 1B formed before layer 1C. Snail G narrows the age of layer 3B more than Brachiopod B does. Layer 2C formed less than 380 million years ago.

A student finds a piece of Trilobite T in the gravel by the bus, far from the road cuts. Why can't the class use it to date a layer?

A. No one knows which layer it came from, so it is not linked to any layer.
B. It probably washed in from the ocean a few years ago.
C. Trilobites are too old to have dates in the range chart.
D. Loose fossils on the ground are younger than the fossils inside layers.

10.

Keziah says, "Layers 1C and 2A are both gray shale, so they must have formed at the same time." Make a claim about her idea. Use evidence from the range chart and the layer table, and explain your reasoning.

Keziah is not right. Layer 1C has Brachiopod B and Trilobite T, so it formed between 392 and 385 mya. Layer 2A has Crinoid C and Coral K, so it formed between 382 and 372 mya. The two time windows do not overlap, so 2A is younger than 1C. The same kind of rock can form at different times whenever the same conditions return, so fossils are better evidence of age than rock type.