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Siccar Point - Scotland

THE 6 RULES OF RELATIVE AGE DATING

1. The Law of Original Horizontality

– most rocks laid down flat, and tilted only later during geologic events

2. The Law of Superposition– oldest rocks are on the bottom

THE 6 RULES OF RELATIVE AGE DATING

3. Law of Lateral Continuity– most rock beds don’t end suddenly when

originally laid down

4. Law of Cross-cutting relationships

– A rock being cut by another rock or feature is older than that rock or feature

Lateral continuity says these layers have been eroded

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I-70 roadcutNear Denver

• NOTE: need to be assured of the structural geology to ensure which way is up - can do this with sedimentary structures which indicate “up”and fossils or other time indicators.

Upside down

Rightside up

Graded bedding

Fossils such as tree stumps, animal burrows, root traces.

StratigraphicSuperposition

Channel fill - cut out of beds and filling with younger beds. Scours or channels are at scales from millimeters deep to hundreds of meters deep.

Up

Stratigraphic SuperpositionCross bedding - sediment deposited on slight slope in water (submarine sand dunes) or air (sand dunes).

Which way did the water move here - how do we tell “up”?

I-70 Roadcut

Stratigraphic SuperpositionCross bedding - sediment deposited on slight slope in water

(submarine sand dunes) or air (sand dunes).

Waterdirection

I-70 Roadcut

Up

Stratigraphic SuperpositionCross bedding in ancient subaerial sand dunes

Zion Nat. Park

Stratigraphic up

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Other “up indicators”

• Mud cracks• Raindrop impressions• Infilled voids

– Sediment fills lower half, crystals can fill upper half.

• Flute and scrape casts and molds.• Vesicles at the top of a lava flow.

RELATIVE AGE DATING RULES CONTINUED

5. Law of Inclusions–If rock A includes pieces of rock B,

then rock B is older than rock A.

6. The Law of Fossil Succession–Fossil assemblages change in an

ordered succession through time.

Fossils• Fossil = a remnant, impression, or trace

of an animal or plant of past geological ages that has been preserved in the earth’s crust

• Trace fossil is when the organism itself is not preserved but only evidence of its existence (dinosaur tracks, worm burrows, coprolite)

William SmithSurveyorEarly 1800’s

PALEOZOIC

MESOZOIC

CENOZOIC

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Index Fossil• A fossil that can tell you what part

of geologic history the rocks were formed in

• A good index fossil exists for a limited period of geologic history, is easily preserved, and can be preserved in many different sedimentary environments

Ecological indicator fossil• Can tell you what kind of an

environment the rocks were formed in• A good ecological indicator exists

through a long period of geologic history, and has a limited tolerance for variation in their environment (must live in a specific type of environment)

Biocorrelation Missing time in the Grand Canyon’s stratigraphyin pink

UNCONFORMITIES• DEFINITION: A break in the

stratigraphic rock record = missing time• TYPES:

–Angular Unconformity –Disconformity–Nonconformity –Paraconformity

Angular Unconformity Disconformity

Paraconformity Nonconformity

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Grand Canyon cross-section

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