Geology of Devils Tower National Monument - Sedimentation

swedishstreakMechanics

Feb 22, 2014 (3 years and 5 months ago)

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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

Devils Tower

This was the

United States

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
rocks


And become

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.

Older rocks

Younger 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.

Older rocks

Younger 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
M
illion
Y
ears
A
go. (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

magma

Future Little
Missouri

Buttes

Future Devils
Tower

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.

Erosion


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

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.

It is important to note the upper
portion of the tower appears

more
weathered

with numerous

vertical and horizontal cracks.


Why do you think this is?

Erosion (cont.)


Some
Geologists

think that

when the vertical cracks formed, the
horizontal cracks also formed.

Maybe the top was exposed to
weathering

longer than the bottom
was.

Devils Tower Today