Rock Record

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How Are Organisms Preserved?• Fossils are the

remains or traces of organisms long ago.

• Most fossils found in sedimentary rock that was once underwater.

• When the fossil decays, it leaves a hollow opening in the rock.– Hollow opening – mold– Cast forms when sand

or mud fill up the mold and harden.

Alternative Ways to Form Fossils

• Ice– Organism dies and

covered by snow or ice.

• Tar– Animals fell into these

pits of tar that is pools of oil and sediment bubbling to the surface.

• Amber– Insects were commonly

trapped in the sticky tree sap flowing down the tree.

– The tree sap would dry and harden and eventually turn to stone, perfectly preserving the insect.

Fossils Help Date Rocks• Law of superposition: each

undisturbed rock layer is older than the one above it.

590 – 250 million years 500 –335 million years

~ 250 million years old

~ 335 million years old

Rock’s Absolute Age• Absolute age –

true age of a rock layer.

1. Half Life• Amount of time it

takes for half of the element to decay.

2. Carbon-14 Dating• All organisms

have carbon.• Carbon’s half life

is 5,800 years.

Geologic Time Scale

• Geologic time scale outlines the major divisions of Earth’s history.

Trip Thru Geologic TimeQuarternaryTertiary

CretaceousJurrasicTriasic

PermianCarboniferousDevonianSilurianOrodivicianCambrian

PRECAMBRIAN

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