Reading the Rocks!

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Reading the Rocks!
Geologists read layers of rocks like pages in a
book!
Floodplain Forest
Each rock tells a story of the environment in
which it formed!
River Channel
Beach Near Shore
Tropical Ocean
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A good example?
Volo Bog a book waiting to be read!
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As Volo Bog filled in with silt and clay over the
years
Tiny grains of pollen from nearby plants drifted
into the bog.
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And Ragweed joins the Oak and Grass pollen in the
last 200 years.
Oak and Grass Pollen are the most common here
Oak Pollen forms much of the next sample
Elm and Ash dominate the next sample
Spruce Pollen dominates the lowest sample
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When you read a book of geologic history
How do geologists KNOW that???
you may read something like 300 million years
ago, a tropical ocean washed the shores of a
forested river floodplain
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Reading the Rocks!
Imagine an area where the landscape is fairly
level, and all the rocks are flat-lying
Exposures of bedrock occur here and there
We can collect samples of the bedrock from those
exposures!
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Heres our field area
N
From a location in the SW, we collect..
a carbonate rock with abundant seashells.
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N
We find similar rocks here...
and here.
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N
These rocks clearly represent
a shallow marine environment!
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Here we find a dark, fine-grained rock.
N
and another.
Leaves within a fined-grained rock?
A forested muddy floodplain?
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Here we find a coarse-grained rock composed of
various minerals.
N
And here and here
Sand bars in river channels?
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The sediments and other evidence lead us to
hypothesize
N
a delta system entering a shallow tropical ocean!
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We have reconstructed our ancient landscape
N
Millions of years ago, a river flowed through
muddy, forested floodplain and entered a shallow
tropical ocean!
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The Grand Canyon rocks!
It really does! ?
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Cretaceous
MESOZOIC
Jurassic
Triassic
PALEOZOIC
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CENOZOIC
MESOZOIC
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Geologic Map
Misconception!!!
Geologic Cross-Section
The geologic map shows the units side-by-side!
How can we say one is older and the other is
younger?
Youngest
Oldest