REFSTUDY GUIDE: Big IdeasUse to answer form questions
1.The law of superposition states that in undisturbed rock sequences, older layers lie beneath younger ones. Scientists pair this principle with index fossils — widespread species that lived for short time intervals — to assign relative ages to sedimentary strata. Comparing these fossil assemblages across sites allows geologists to determine which layers formed during the same time period even on separate continents.
2.The fossil record documents the succession of life forms through geologic time, with older organisms preserved in deeper strata. Mass extinction events appear as sudden disappearances of many species at specific rock boundaries, such as the end-Cretaceous layer that marks the loss of non-avian dinosaurs. Scientists use these sharp extinction horizons as global time markers to build and refine the geologic time scale.
3.Index fossils are remains of organisms that lived over a wide geographic area for a geologically brief span of time. When the same index fossil appears in rock layers on different continents, those layers must have formed during the same interval of time. This biostratigraphy technique allows geologists to correlate distant rock formations globally without requiring radiometric dating.
DRAWKEY TERMSGoogle each → Images tab → sketch what you see
OTHER VOCAB: geologic time scale · relative dating · extinction · sediment
Geologic Time Scale
System for dividing Earth's 4.6-billion-year history into eras, periods, and epochs
Stratigraphy
Study of rock layers and their sequence
Superposition
Law that younger layers are on top of older layers
Index Fossil
Fossil used to identify and date rock layers
WRITEFOCUS QUESTIONYour initial thinking
A geologist exposes a Missouri cliff face with five visible rock layers — a single trilobite fossil sits at the bottom, a dinosaur footprint is preserved halfway up, and the topmost layer holds modern plant pollen. The rocks tell a continuous story across hundreds of millions of years.
How do we know what Earth looked like millions of years ago?
I think...scientists read Earth's history by reading rock layers. Older rocks sit on the bottom (the law of superposition), so the FOSSILS inside each layer tell us what lived during that time. The deeper the layer, the further back in time you're looking.
WRITEAFTER HOOK FORMReflect before continuing
I learned that...Earth is 4.6 BILLION years old, and rock layers act like the pages of a history book. The deeper you dig, the older the page. Humans only appear in the very last 'second' of Earth's history when you compress all of geologic time into a 24-hour clock.
Station 1 — Stratigraphy Investigation
I observed that...in the Missouri cliff face, the trilobite was in the BOTTOM layer, the dinosaur footprint was halfway up, and modern plant pollen was on TOP — exactly the order they appeared on Earth (oldest to youngest).
This shows that...the Law of Superposition works: in undisturbed sedimentary rock, younger layers form ON TOP of older layers. The fossils inside each layer record what was alive when that sediment was being deposited.
The key concept is...Stratigraphy uses superposition to read rock layers like a stack of newspapers — the bottom one is the oldest, the top one is the newest. This gives RELATIVE ages (older/younger), not exact dates in years.
CHECKDAY 1 CHECK-INBefore you leave
Q1:Based on today's investigation of geologic time & fossil evidence,
explain one key finding using evidence from your data. Connect your
answer to this week's focus question: How do we know what Earth
looked like millions of years ago?
We can read Earth's past because sedimentary rock layers stack up over time. The Law of Superposition says deeper = older, so the trilobite at the bottom of the Missouri cliff lived FIRST, then the dinosaur (footprint in the middle layer), and modern plants are at the top. The fossils in each layer tell us what was alive when that sediment was being laid down — turning the rocks into a timeline we can read in order, even without knowing the exact year.
Confidence:
Geologic Time & Fossil Evidence (Day 2)ANSWER KEY
Name: Period:
Station 2 — Fossil Record Analysis
The data shows...trilobites lived only during the Paleozoic Era (~540–252 mya) and then went extinct. When we find a trilobite anywhere in the world, that layer MUST be Paleozoic. That short, wide range is what makes trilobites a great index fossil.
This connects to...the fossil record showing mass extinctions — the end-Permian (252 mya) wiped out ~96% of marine species, and the end-Cretaceous (66 mya) killed the non-avian dinosaurs. These sharp 'extinction horizons' are global time markers geologists use to line up rocks on different continents.
The main idea is...the same index fossils show up in rocks on continents that are now thousands of miles apart (like matching trilobites in Missouri and Morocco). Those layers must have formed at the SAME TIME, even though the continents have drifted since.
I can apply this by...looking at any cliff or roadcut and predicting which fossils are older without measuring a single year — just by checking which layers are deeper. I can also match rocks across distant locations using index fossils.
DRAWCONCEPT MODELShow your understanding visually
Draw a model that shows the main concept. Include labels and arrows
to explain relationships.
Cross-section of 5 rock layers (A–E from bottom to top). Layer A (deepest) labeled with trilobite fossil and 'Paleozoic, ~500 mya'. Layer C (middle) shows dinosaur footprint labeled 'Mesozoic, ~150 mya'. Layer E (top) shows modern grass pollen labeled 'Cenozoic, today'. Arrow on the right pointing UP labeled 'YOUNGER' and arrow pointing DOWN labeled 'OLDER' (Superposition). Side note: 'Index fossils = short time range + wide geographic range.'
Labels included
Relationships shown
Key terms used
CHECKTEST YOUR THINKINGRate each: FALSE ← 1-2-3-4-5 → TRUE
1. Fossils are only bones or shells — soft-bodied organisms and behavior never leave a trace.
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2. Older rocks are always deeper in undisturbed layers (law of superposition).
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3. Rocks don't change once formed — they stay the same type permanently.
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4. Index fossils help scientists match rock layers across different continents.
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5. The fossil record shows that most species that ever lived on Earth are now extinct.
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Pick any TWO statements -- especially any you rated 2, 3, or 4. Justify your rating with evidence:
Statement #___ -- I rated it ___ because:
Statement #___ -- I rated it ___ because:
THINKSCIENTIST CIRCLE DEBRIEFReflect after your Scientist Circle discussion
AHA! Moment
The one I changed my thinking most about is...fossils don't have to be giant dinosaur bones — even pollen and footprints count, and that gives us evidence about almost every layer of rock, not just the famous ones.
Remaining Question
I still wonder...If the continents have moved, how do scientists figure out where the rock layers ORIGINALLY were when those animals were alive?