GeologyOfDevilsTowerNationalMonument sedimentation

GEOLOGY OF DEVILS TOWER
NATIONAL MONUMENT
Sedimentation

Pat Frolander
John Aloisio
Marlon Poole
Lee McDowell


Devils Tower used to be under water in
a shallow sea about 145 to 65 million
years ago
This was North
America during the
late Cretaceous time.

thelivingmoon.com


This was the
United States

Devils Tower

USGS pubs.usgs.gov


The bottoms of the ocean were covered in deep sand or
mud called sediments.

The Mississippi pouring sediment into the
Atlantic Ocean. (ESEA esa.int)


There are 3 categories of
sedimentary rocks:
• Clastic
• Organic
• Chemical


Clastic:Big rocks break up into small
And become
rocks sand,
mud or silt
that settle to the bottom of an ocean

Which become rock again


Organic : LIMESTONE
Sedimentary rock made from the shells of billions and
billions of microscopic ocean organisms called plankton
(foraminifera) over millions of years.
Limestone can also be chemical,
when the calcium from the shells
becomes rock.


This is what it looks like today

Coal is an Organic rock made from ancient forests
From this
To this.

Organic
Sedimentary
rock


Sediments can be Chemical
As an ocean dried up,
different sediments
called salts were left
behind.
Chemical rocks include
• Table salt
• Gypsum
• Some Limestone


The Gypsum Springs Formation today.
Gypsum is found in many
places including here at
Devils Tower.
The White Sands in New
Mexico is an example of a
large deposit of gypsum.
Some of our school walls

are made of gypsum, it is
used to make sheet rock.


Let’s review: What are the three categories of sedimentary rock?

1. Clastic, made from the smallest pieces of broken or weathered rock
Includes sandstone, siltstone and shale.

2. Oganic, from ancient living things, includes coal and limestone.
3. Chemical, from when oceans dried up, includes gypsum and table salt
and limestone.


Younger rocks

Older rocks


When layers form, the older rocks are on the bottom, and the younger
rocks are found on top. This is called the
Law of Superposition.
Younger rocks

Older rocks


These are the sedimentary
rocks you see today at
Devils Tower

Which rocks are the oldest?

The Spearfish
formation


The sea disappeared as the Rocky Mountains, with the Black Hills,
began to form, about 66 – 45 Million Years Ago. (MYA) The land
covering Devils Tower was no longer under water.
This mountain building was called the Laramide Orogeny

Devils Tower

Rocky
Mountains


magma


Magma


Future Devils
Tower

Future Little
Missouri
Buttes

magma


Molten rock that cools and becomes
hardened is called
Igneous Rock.


Igneous rock that cools quickly above ground is
called Extrusive.

Igneous rock that
cools slowly
underground is
called Intrusive.


This magma caused bulges to form, making dome mountains such as
Bear Butte near Sturgis, S.D.


Today we see
layers 18, 19 and
20.
Where are layers
1 – 17??
The Sedimentary
layers 1-17 on top of
Devils Tower
eroded away, leaving

the harder, igneous
rock standing.
Layers 18, 19 and 20
Are now being eroded.


At the end of the last Ice Age (10,000 years ago) the outwash from the
melting glaciers increased the flow of rivers throughout the North
American continent. This is when geologists assume the majority of the
sedimentary rocks in this area were eroded away.


Today, the Belle
Fourche River does
not seem large
enough to wash
away the vast
amount of
sedimentary rock
eroded from above
and around the
Tower

Erosion

Have WE changed the
amount of erosion at
Devils Tower?
Yes. Humans have dammed the Belle Fourche River, creating Keyhole
Reservoir, and preventing the floods that cause the greatest amount of

erosion.