Three Sites, One History: When Did the Sea Creatures Vanish?

Investigation question: Three outcrops a few kilometers apart each show only part of the region's rock record. Can you stitch them into one sequence, find the levels where whole groups of fossils disappear, and explain how that sequence organizes the region's history?

Scientific purpose

Students receive the field logs of three fictional outcrops, each exposing a different slice of the same stack of layers, and must work out which layers are shared, build one composite column, locate two mass-extinction levels where fossils vanish, and use two dated layers to attach years to parts of the sequence. They then write a scientific explanation of how the composite sequence organizes the region's history, separating what was observed from what was inferred. The lab is the chapter's direct practice of constructing an explanation from rock-strata evidence.

Why this helps students with this chapter

Textbook figures hand students a finished composite sequence, so the reasoning that produced it stays invisible. Here students do that reasoning themselves: they must decide which layers are the same, discover that rock type alone is not enough, find that a thin ash bed is the best anchor, and notice that the fossils change most at exactly the levels geologists use as boundaries. Writing the explanation afterward makes them separate what the rocks show from what they concluded.

Standards connections

MS-ESS1-4 (assesses, the explanation built from strata and fossil evidence; no period names are required): A California Grade 8 performance expectation, placed here because the Grade 7 textbook teaches it. The written explanation in step 9 asks students to use evidence from rock strata (order, shared key beds, fossil disappearances, dated layers) to explain how the sequence organizes the region's history, which is the PE as written. In line with its assessment boundary, students never need to name periods or epochs.

MS-ESS2-2 (supports, reading environmental change from rock type): The sequence records a sea becoming a desert, then a river plain; students read these surface changes from rock type as background for the Grade 7 standard on surface change over time.

Roles (rotate each session)

• Site reader: Reads each site log aloud, layer by layer from the top, and makes sure every layer's rock type, thickness, fossils and notes are heard before any matching begins.
• Matcher: Proposes which layers at different sites are the same, states the evidence for each match, and records the match in the matching table.
• Column drafter: Draws the composite column to scale on graph paper (1 cm = 2 m of rock, with the two thin beds drawn as lines), labels every layer and marks the fossil disappearances.
• Skeptic and reporter: Challenges each match ('could that be a different gray limestone?'), checks the final order against all three logs, and presents the column to the class.

Materials

Estimated cost: 3 to 5 per class (an estimate; Printing and cutting 14 strips per group on card stock (about $2 for a class), envelopes and graph paper from stock; colored pencils and rulers are reused.)

Safety

• Paper, glue and colored pencils only; no chemicals or heat. Keep glue sticks capped between uses.
• If scissors are used to cut strips in class, cut away from fingers and pass them handle first.
• No physical hazards beyond ordinary classroom materials; the fictional sites are described, not visited.

Teacher preparation and setup

1. Print the site-log table from the recording sheet once per group on card stock, cut it into one strip per layer (14 strips), and sort the strips into the three site envelopes. Shuffle within each envelope.
2. Decide whether to tell students the sites are fictional at the start (recommended) or at the debrief; either way, say that the method is exactly what geologists do in the western United States.
3. Prepare a hidden 'answer column' on chart paper to reveal at the debrief: nine layers, bottom to top, with the two extinction levels and the two dated beds marked.
4. Review the key-bed idea from slide 6 so students know that an ash bed with a matching chemical fingerprint can be treated as the same layer.

Investigation design

Student procedure

1. Open envelope A. Use the 'position at this site' column to put Site A's strips in order, top (youngest) to bottom (oldest). Repeat for Sites B and C. Each site is a separate stack of its own; do not mix sites yet.
2. Read each stack aloud. For every layer, note the rock type, thickness, fossils and any note about dating or a chemical fingerprint. Write in the first recording-sheet box which site shows the youngest rock and which shows the oldest, with your reason.
3. Find layers that appear at two sites. For each proposed match, write the evidence in the matching table: rock type, fossils, thickness and any fingerprint or date. A match needs at least two lines of evidence; rock type alone is not enough.
4. Slide the three stacks sideways so that matched layers sit level with one another. Check: does every layer above a match stay above it at both sites, and every layer below stay below? If not, the match is wrong.
5. Once the stacks line up, read off the complete order from the oldest layer at the bottom of Site C to the youngest at the top of Site A. Write the nine layers in order on the recording sheet.
6. Draw the composite column on graph paper at 1 cm = 2 m of rock, oldest at the bottom. Draw the two beds thinner than 10 cm as single lines and label them. Color each layer by rock type and sketch a symbol for each fossil type in the layers where it occurs.
7. Mark with an arrow every level above which a fossil type never appears again. Decide which of these levels are large enough to call a mass extinction (several types vanish together) and label them.
8. Write the two laboratory dates beside their layers. Then write an age range (older than, younger than, or between) beside every other layer, using only those two dates and the date of the basement rock.
9. Session 2: write your explanation (q9 on the analysis sheet) of how the composite sequence organizes the region's history. Mark each sentence E (something the logs record) or X (something you concluded). Present the column to another group and have their skeptic challenge one match.

Expected observations

• Composite order, oldest to youngest: speckled granite (1,100 Ma), black shale, gray limestone, brown sandy limestone, white ash Z-7 (252 Ma), red sandstone, gray mudstone, gray clay with glassy beads (66 Ma), yellow mudstone. Total about 36 m plus the granite.
• Matches: B3 = C1 (ash, fingerprint Z-7, same thickness, same position above the sandy limestone); B4 = C2 (sandy limestone, same fossils, same thickness); B5 = C3 (gray limestone, same fossils, same thickness); A3 = B1 (gray mudstone with dinosaur bones and conifer wood); A4 = B2 (red sandstone with footprints).
• Trilobites, crinoids and brachiopods last appear in the sandy limestone directly below the 252 Ma ash; dinosaur bones last appear in the gray mudstone directly below the 66 Ma clay. Most groups find both levels once the column is drawn.

Likely difficulties

• Groups try to combine all 14 strips into one stack immediately and double-count the shared layers; insist on three separate stacks first, then sliding them level.
• Some groups match Site B's gray limestone with Site A's gray mudstone because both are gray; the skeptic role and q3 exist for this.
• Students write a single age for the red sandstone instead of a range; remind them that only the two thin beds and the granite carry measured dates.
• The worn granite surface is overlooked; prompt with 'what happens to a rock surface that sits exposed for a long time before anything is deposited on it?'
• The 1 cm = 2 m scale makes the two thin beds invisible; students should draw them as lines and label them, not try to scale them.

Structured debrief

1. Reveal the answer column and have groups compare. Ask which match caused the most argument; typically the two limestones at Site B and Site C, because they are the same at both sites but students worry they are different gray limestones until they see both sit under the same sandy limestone and ash.
2. Ask each group which single layer did the most work. The ash with fingerprint Z-7 anchors B to C and carries a date; the glassy-bead clay carries the other date. Point out that two thin beds that took days to form pin down a 36-meter stack.
3. Walk the extinction levels together: sea-floor animals vanish just below the 252 Ma ash and never return above it; dinosaur bones stop below the 66 Ma clay. Explain that these are the same two levels geologists use for the end of the Paleozoic and Mesozoic eras, without requiring students to remember those names.
4. Collect three or four q9 paragraphs and have the class sort the sentences into E and X. The goal is to see that 'trilobites stop below the ash' is an observation while 'a mass extinction happened about 252 million years ago' is a conclusion from several observations together.

Recording sheet: what students fill in

Lab 2 recording sheet: Three sites, one history

The three sites below are fictional and simplified for this investigation, but the method is the one geologists use. Thickness is given in meters (m) or centimeters (cm). 'Lab date' means an age measured in years from the layer's own minerals; Ma means million years ago.

Analysis questions: answer key

Individual evidence of learning

Each student submits their own recording sheet (q1, q2, the matching table, the composite sequence table, the drawn column) and written answers to q3 through q10. The q9 explanation paragraph with its E/X labels is the primary evidence toward the standard; the composite table and q4 show whether the student located the extinction levels and used the dated beds correctly.

Assessment criteria

Accessibility adaptations

• Provide the site strips in large print with a colored stripe per rock type and a tactile sticker per fossil type for students with low vision; let a partner slide the stacks while the student calls the matches.
• Give a partly completed composite table (rows 1, 5 and 9 filled) and a column frame with the scale already marked for students who need more support.
• Allow the q9 explanation to be dictated or recorded, and provide the sentence frame 'The ___ stop below the ___, which shows ___ because ___.'
• Offer a bilingual word strip for limestone, mudstone, sandstone, shale, ash, fossil, older and younger, and pair the site-reader role with a student comfortable reading aloud.

Low-cost alternative

Skip the card strips and have groups work directly from the printed site-log table, sketching the three stacks on scrap paper and drawing the composite column on plain paper with a ruler. The reasoning is identical.

Model limitations

The sites are fictional and simplified: real outcrops have gaps, faults, layers that change thickness sideways and fossils that are rare or hard to identify. Real ash beds are matched by careful chemical analysis, not a printed code, and real dates carry uncertainty. The two extinction levels are placed to mirror the end-Permian and end-Cretaceous events, with one small invented difference: the sea-floor animals here vanish directly below the ash, while in real sequences the last appearances are often spread over a thicker interval. The activity has not been physically tested by the authors.

Teacher notes

Check the composite table against the expectedObservations order first; a wrong order usually traces to the limestone-mudstone decoy or to double-counting the shared red sandstone. For q9, grade the E/X labeling generously on borderline sentences (a statement combining an observation and a conclusion can be either) but not on sentences that present a conclusion as an observation. Keep the hidden answer column up for the next class's slide 20 task.