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...
WRITEAFTER HOOK FORMReflect before continuing
I learned that...
Station 1 — Stratigraphy Investigation
I observed that...
This shows that...
The key concept is...
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?
Confidence:
Geologic Time & Fossil Evidence (Day 2)
Name: Period:
Station 2 — Fossil Record Analysis
The data shows...
This connects to...
The main idea is...
I can apply this by...
DRAWCONCEPT MODELShow your understanding visually
Draw a model that shows the main concept. Include labels and arrows
to explain relationships.
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