Week 2: Geologic Time & Fossil Evidence

Grade 7 Science | Rosche

MS-ESS1-4 Earth's Place in the Universe

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The Phenomenon: The Deep Time Mystery

Anchoring Context & Focus Question

How do we know what Earth looked like millions of years ago?

Animated diagram showing three rock layers depositing from oldest (bottom shale) to youngest (top sandstone), illustrating the principle of superposition — younger rock forms on top of older rock.
Animated: layers deposit oldest first. Watch once, then predict which layer in your site is oldest.
Animated explanation of the week's focus question. Watch then reason about why this happens.
Key Vocabulary (7 terms) — Practice Tool

Cognate Strategy: Many science words look similar in English and Spanish — use your Spanish to learn science!

Term Spanish Definition
stratigraphy estratigrafía Study of rock layers and their sequence
superposition superposición Law that younger layers are on top of older layers
index fossil fósil índice Fossil used to identify and date rock layers
geologic time scale escala de tiempo geológico Timeline dividing Earth's 4.6 billion year history
relative dating datación relativa Determining if something is older or younger without exact age
extinction extinción Complete disappearance of a species
sediment sedimento Small particles of rock, minerals, or organic material
Concept map showing how this week's vocabulary terms connect: stratigraphy underlies the law of superposition; index fossils enable relative dating; the geologic time scale frames extinction events; sediment layers carry all of this evidence.
How this week's terms connect — strata + fossils → time.

Hook – The Deep Time Mystery

Compare the geologic time scale to human history and generate hypotheses about how scientists reconstruct Earth's past.

The Deep Time Mystery

Earth is 4.6 billion years old — a number so large it's hard to imagine. If Earth's entire history were compressed into one calendar year, humans would appear in the last 11 seconds of December 31st. Geologists read this enormous story by studying rock layers, fossils, and the law of superposition: the deeper the layer, the older it is. Missouri's own limestone bluffs along the Mississippi River record 300 million years of history right here in St. Louis.

Earth's 4.6-billion-year history compressed into a 24-hour clock face. Single cells appear at 4 AM, complex cells at 8 PM, dinosaurs go extinct at 11:39 PM, and modern humans appear in the last 1.17 seconds before midnight.
Deep time made visceral: humans = the last 1.17 seconds of Earth's day.

Cycle 7 Week 1 Connection

Last week you classified rocks by formation process — igneous, sedimentary, metamorphic. This week we use that knowledge: sedimentary rocks are Earth's history book. Each layer records an ancient environment, and the fossils trapped inside tell us what creatures were alive at that time. Layers also record catastrophic events like volcanic eruptions and mass extinctions.

What You'll Do

  1. Examine the geologic time scale and place major events on a proportional timeline
  2. Apply the law of superposition to read rock layer sequences
  3. Use index fossils to match and date rock layers from different locations
  4. Analyze the fossil record to identify mass extinction events
  5. Submit your reasoning about how we reconstruct Earth's past
COMPLETE THE HOOK FORM

Explore geologic time scales and make predictions about reading Earth's history.

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Worked Example – Reading Rock Layers Using Superposition

Worked Example and Simulation: Reading Rock Layers Using Superposition

Problem: A geologist exposes a cliff face in rural Missouri with five visible rock layers. Layer A is at the bottom, Layer E is at the top. Layer A contains trilobite fossils. Layer C contains dinosaur tracks. Layer E contains modern plant pollen. She asks: "What is the age order of these layers, and what can the fossils tell us about when each layer formed?"

Worked example cliff face with five undisturbed layers labeled A through E. Layer A at the bottom is oldest and contains trilobites; Layer C contains dinosaur tracks; Layer E at the top is youngest and contains modern pollen.
Use the visual first: bottom-to-top position gives the relative age order before fossil dates refine the story.

Step 1: Apply the Law of Superposition

In undisturbed rock layers, older layers are on the bottom, younger layers are on top. This is the Law of Superposition. So the age order from oldest to youngest is: A → B → C → D → E. Layer A (trilobites) is the oldest; Layer E (modern pollen) is the youngest.

Step 2: Use Index Fossils to Add Dates

Index fossils are organisms that lived during a known, short time window. Trilobites (Layer A) went extinct at the end of the Paleozoic Era (~252 million years ago), so Layer A formed before 252 mya. Dinosaur tracks (Layer C) place that layer in the Mesozoic Era (252–66 mya). Modern pollen (Layer E) is Cenozoic (66 mya–present). The rock layers tell a continuous story through time.

Step 3: Watch for Gaps (Unconformities)

If layers are missing — for example, no Mesozoic layer appears between Paleozoic and Cenozoic — that gap is called an unconformity (discordancia). Erosion removed those rocks. An unconformity means time passed but left no record in that location. Real geologists always look for missing time before drawing conclusions.

Simulation: Geologic Time

PREDICT (before)

Look at the simulation controls. Before changing any variables, predict what will happen when you adjust them. Write your prediction down.

OBSERVE (during)

Change one variable at a time. Record what happens after each change. Use the data journal to capture at least 3 trials.

EXPLAIN (after)

Compare your observations with your prediction. Use scientific vocabulary to explain the patterns you found. What surprised you? What confirmed your thinking?

Your Simulation Summary
Your prediction: —
Trial results: —
Average result: —
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Station 1 – Stratigraphy Investigation

Use superposition and index fossils to determine the relative ages of rock layers.

Station 1 — Stratigraphy Investigation

You are a geologist studying two sediment cores drilled 50 miles apart in Missouri — one near St. Louis, one near the Ozarks. Both cores show multiple rock layers. Your task: use the Law of Superposition and index fossils to match layers between the two cores and determine their relative ages. This is the same technique scientists used to map Earth's rock record long before radiometric dating existed.

Two stratigraphic cores side-by-side — St. Louis and Ozarks. Coloured layers run across both cores; matching trilobite and ammonite fossils mark layers of equal age between the two locations.
Same fossil ⇒ same age — biostratigraphic correlation across 50 miles.

Evidence Checklist — For Each Rock Layer

  1. Position: Is this layer near the top (younger) or bottom (older) of the core?
  2. Index fossil present? If yes, record the fossil name and its known time range
  3. Relative age claim: Write a statement comparing this layer to adjacent layers (e.g., "Layer B is older than Layer C because it is below it")
  4. Correlation: Does a matching index fossil appear in the other core? If so, those layers are the same age
COMPLETE THE STATION 1 FORM

Analyze layered sediment cores and use index fossils to date rock layers.

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Station 2 – Fossil Record Analysis

Interpret fossil distribution patterns and identify major extinction events.

Station 2 — Reading the Fossil Record

The fossil record is not evenly distributed — some layers are packed with diverse species; others show a sudden collapse. These collapses are mass extinction events (extinciones masivas). Scientists count the number of species in each rock layer period and plot it over time to find these patterns. You will analyze a simplified fossil diversity chart and answer analytical questions about what the data reveals.

Line graph of marine genus diversity across the last 540 million years, showing five sharp drops at the Big Five mass extinctions, with the largest at the end-Permian event.
The Big Five: each crash was a near-reset of complex life.

Patterns to Identify

  1. Locate two major extinction events in the diversity chart — identify the era boundary where each occurs
  2. Recovery pattern: After each extinction, how long did it take for diversity to recover? Compare the two events
  3. Geologic Time Scale placement: Label each extinction event on the provided time scale with the correct era transition (e.g., Permian–Triassic, Cretaceous–Paleogene)
  4. Modern connection: How does current species loss compare to historical extinction rates? Use the chart data to support your argument
COMPLETE THE STATION 2 FORM

Analyze fossil distribution maps and extinction event data.

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Station 3 – Design a Geologic Investigation

Apply dating principles to solve an age determination problem with unknown strata.

Station 3 — Design a Geologic Investigation

You have discovered a new rock formation with three distinct layers. Your mission: design a complete investigation to determine the relative ages of the layers, reconstruct the ancient environment of each layer, and explain what geologic events shaped this site. This is the same work real field geologists do when they encounter an unexplored outcrop.

Cliff outcrop with three exposed rock layers: A (red sandstone, footprints, top), B (gray limestone, seashells, middle), C (black shale, plants, bottom).
Your investigation site: lithology + fossils → environment + relative age.

The Rock Formation: Your Field Site

  • Layer A (top): Red sandstone with footprints
  • Layer B (middle): Gray limestone with seashells
  • Layer C (bottom): Black shale with plant fossils

Your Investigation Must Answer

  1. Relative ages: Which layer is oldest? Which is youngest? How do you know?
  2. Environments: What kind of environment did each layer form in? Use rock type and fossil clues.
  3. Timeline: Put layers in order from oldest to youngest and describe how this location changed over time.
  4. Limitations: What additional evidence would you need to get absolute (exact) ages?
  5. Cross-cutting: If a magma intrusion cuts through all three layers, what does that tell you?
COMPLETE THE STATION 3 FORM

Design an investigation to determine the age of unknown rock layers.

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Exit Ticket – Earth History Integration

Synthesize learning about geologic time and fossil evidence.

Exit Ticket — Earth History Integration

Demonstrate your mastery of geologic time and fossil evidence through 2 new-content questions (stratigraphy & index fossils), 2 spiral questions from prior learning (MS-ESS2-6 climate factors and Cycle 6 plate tectonics), and 1 integration question that connects rock types, geologic time, and plate tectonics using the SEP practice of constructing explanations (SEP-6).

Synthesis diagram: rock cycle → sediment layers → fossils → geologic time scale → mass extinctions, with arrows showing how each concept feeds into the next.
Exit-ticket synthesis: from rocks to time to extinctions.

Question Structure (23 points)

  • [NEW] Q1–Q2 (8 pts): Stratigraphy & index fossil identification — this week's core content
  • [SPIRAL] Q3 (8 pts): MS-ESS2-6 — distance from water and climate moderation (IXL data gap reteach)
  • [SPIRAL] Q4 (8 pts): Cycle 6 — continental drift and fossil distribution as evidence
  • [INTEGRATION + SEP-6] Q5 (7 pts): Construct an explanation connecting rock types, geologic time, and plate tectonics — Himalayan marine fossils scenario
COMPLETE THE EXIT TICKET

Demonstrate mastery of geologic time and fossil evidence concepts.

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Enrichment & Extension
Optional content if you finish early or want to go deeper.

Scientist Spotlight: William Smith

William Smith (1769–1839) was an English canal surveyor and self-taught geologist who created the world's first large-scale geologic map of England, Wales, and part of Scotland in 1815 — entirely on his own, without a university degree, working as a laborer. While supervising canal digs across England, Smith noticed that the same types of fossils always appeared in the same sequence of rock layers, no matter where he was. He realized that fossils could be used as clocks to identify and match rock layers hundreds of miles apart — a principle called biostratigraphy (bioestratigrafía). His colleagues in the Geological Society initially stole his ideas without credit because of his working-class background. Eventually, he was recognized and called "The Father of English Geology." His story reminds us that observational skill and persistence can produce discoveries that transform science — regardless of formal credentials.

Cross-section of England's strata as William Smith mapped it: numbered colored bands stretching across counties, with index fossils paired to each layer — the world's first large-scale geologic map (1815).
Smith's 1815 map: same fossils → same age, hundreds of miles apart.

Environmental Justice: West Lake Landfill — Reading Contamination in Layers

Just 15 miles north of our school, the West Lake Landfill in Bridgeton, Missouri contains approximately 8,700 tons of radioactive waste illegally buried in the 1970s — leftover from the same Manhattan Project uranium processing that contaminated Coldwater Creek. Geologists and environmental scientists have used the same layering principles you studied today to map exactly how deep the contamination is and how it has migrated through different soil layers over decades. Like reading rock strata, scientists drill cores, collect samples at each depth, and use the layer sequence to reconstruct when contamination reached different zones. The Bridgeton community — predominantly working-class families of color — has fought for decades to get this site cleaned up. Understanding how geologists read layers is not just an academic skill: it is a tool for accountability, environmental monitoring, and protecting communities from harm.

Schematic cross-section of the West Lake Landfill: surface soil, then a band of buried 1970s radioactive waste, with arrows showing migration of contaminants downward through soil layers over decades.
West Lake Landfill: reading contamination layers, just like rock strata.
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Week 2 Complete!

Next Week: Radiometric Dating & Absolute Age — how can scientists determine the exact age of a dinosaur bone?