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Week 10 · Reading the Layers

Friday

Fossils and everyday layers
// Oldest at the bottom, youngest on top
⏱ about 20 min

Friday: Fossils and Everyday Layers

Rocket digs through the recycling bin on the Sky Deck. "Newspaper from Monday under Tuesday under Wednesday. Superposition in a bin!"

Raven laughs. "And each one has a different story inside. What do you notice about the pictures?"

"They change," Rocket says. "Each layer has its own news."

Nova hovers over the bin. "Rock layers have their own news too," she says. "Fossils. Different layers hold different kinds."

"So a shell in one layer and a leaf in another are like dates on the papers," Raven says.

Rocket nods slowly. "Not dates. Order." Nova hums. "Exactly. Older and younger, from what is inside."

Fossils in the layers

The survey says a fossil is the recognizable remains or other evidence of past life: bones, shells, leaves, tracks, burrows or impressions.

Two hundred years ago, the English surveyor William Smith found that certain layers held fossils unlike those in other layers.

Rocks of the same age may contain the same fossils even when they are separated by long distances. Smith used key fossils as markers to match layers.

The Law of Fossil Succession says the kinds of plants and animals found as fossils change through time. Find the same kinds in two places, and the rocks are the same age.

Going down through time

The survey describes a walk back through the layers. Start at the present and look at older and older rock.

First you reach a level with no human fossils. Keep going and you lose flowering plants, then birds, then mammals, then reptiles.

Deeper still there are no four-footed animals, no land plants, no fishes, no shells and finally no animals at all.

Fossils with a short, well-known time of existence are called index fossils. They are the best markers for matching layers.

WALKING DOWN THROUGH THE LAYERS
  • ?Deeper, no birds but still mammals
  • ?Deeper, no mammals but still reptiles
  • ?The top layers hold human fossils
  • ?Deeper, no flowering plants but still birds
  • ?Near the bottom, no land plants and no fishes, only shells
  • ?Below that, no humans but still flowering plants
WHY THIS EXERCISEThe fossil record is a sequence. Reading it downward is reading time backward.
FOSSILS AS EVIDENCE
  • Read the question.
  • Tap your answer.
Two rock layers in different places hold the same kinds of fossils. What do geologists conclude?
Which fossil makes the best marker for matching layers?
A footprint pressed into mud that turned to rock is which?
Week reviewTrue or false?
The largest and heaviest particles settle first.?
In an undisturbed stack, the bottom bed is the youngest.?
A feature that cuts across layers is younger than those layers.?
Rock strata and fossils give relative dates, not years.?
An unconformity is a gap in the rock record.?
WHY THIS EXERCISEThese five ideas let you read any stack of layers in order.
Which kind of date do rock layers and fossils give: relative or absolute? Type one word.
WHY THIS EXERCISEThis is the heart of the standard. Numbers of years need other tools.
Try it
Find a layered thing at home: a stack of mail, a laundry pile, a layered sandwich made by a grown-up. Say which layer came first and how you know.
Check your sediment bottle one last time. Is there a thin film of fine mud on top? Record yes or no.
Draw six rock layers. Put a shell in the bottom one, a fish above it, then a fern, a reptile, a bird and a flower.

Strong week, stratigrapher. Tomorrow is Sky Night: a Moon check and a family game of putting events in order.

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