Reading the Rocks!

Geologists read layers of rocks like pages in a book!

Each rock tells a story of the environment in which it formed!

Tropical Ocean

Beach & Near Shore

River Channel

Floodplain Forest

Volo Bog… a book waiting to be read!

A good example?

As Volo Bog filled in with silt and clay over the years…

Tiny grains of pollen from nearby plants drifted into the bog.

Spruce Pollen dominates the lowest sample…

Elm and Ash dominate the next sample…

Oak Pollen forms much of the next sample…

Oak and Grass Pollen are the most common here…

And Ragweed joins the Oak and Grass pollen in the last 200 years.

When you read a book of geologic history…

…you may read something like :

300 million years ago, a tropical ocean washed the shores of a forested river floodplain…

How do geologists KNOW that???

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!

Here’s our field area…

From a location in the SW, we collect..

N

…a carbonate rock with abundant seashells.

We find similar rocks here...

N

…and here.

These rocks clearly represent…

N

…a shallow marine environment!

Here we find a dark, fine-grained rock.

N

Leaves within a fined-grained rock…?

A forested muddy floodplain?

…and another.

Here we find a coarse-grained rock composed of various minerals.

N

And here and here…

Sand bars in river channels?

The sediments and other evidence lead us to hypothesize…

N

…a delta system entering a shallow tropical ocean!

We have reconstructed our ancient landscape…

N

…Millions of years ago, a river flowed through muddy, forested floodplain and entered a shallow tropical ocean!

Vishnu Schist

Bright Angel

Redwall

Supai

KaibabToroweapCoconino

Mountain Building

Shore sand

Off shore

Land\Marine

Marine

Marine

Shallow Sea

The Grand Canyon rocks!It really does!

Desert

MESOZOIC

PALEOZOIC

Cretaceous

Jurassic

Triassic

CENOZOIC

MESOZOIC

Geologic Map

Oldest

Youngest

Misconception!!!

The geologic map shows the units side-by-side! How can we say one is older and the other is younger?

Geologic Cross-Section