GEOLOGY OF DEVILS TOWER
Devils Tower used to be under water in
a shallow sea about 145 to 65 million
This was North
America during the
late Cretaceous time.
This was the
The bottoms of the ocean were covered in deep sand or
The Mississippi pouring sediment into the
Atlantic Ocean. (ESEA esa.int)
There are 3 categories of
:Big rocks break up into small
sand, mud or silt
that settle to the bottom of an ocean
Which become rock again
Sedimentary rock made from the shells of billions and
(foraminifera) over millions of years.
Limestone can also be
when the calcium from the shells
This is what it looks like today
Coal is an
rock made from ancient forests
Sediments can be
As an ocean dried up,
called salts were left
Chemical rocks include
The Gypsum Springs Formation today.
Gypsum is found in many
places including here at
The White Sands in New
Mexico is an example of a
large deposit of gypsum.
of our school walls
of gypsum, it is
used to make sheet rock.
Let’s review: What are the three categories of sedimentary rock?
, made from
pieces of broken or
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
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.
These are the sedimentary
rocks you see today at
Which rocks are the oldest?
The sea disappeared as the Rocky Mountains, with the Black Hills,
began to form, about 66
go. (MYA) The land
covering Devils Tower was no longer under water.
This mountain building was called the
Molten rock that cools and becomes
hardened is called
Igneous rock that cools quickly above ground is
Igneous rock that
This magma caused
to form, making
Bear Butte near Sturgis, S.D.
Today we see
layers 18, 19 and
Where are layers
17 on top of
Layers 18, 19 and 20
Are now being eroded.
At the end of the last Ice Age (10,000 years ago) 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
eroded from above
and around the
Have WE changed the
amount of erosion at
Yes. Humans have dammed the Belle Fourche River, creating Keyhole
Reservoir, and preventing the floods that cause the greatest amount of
It is important to note the upper
portion of the tower appears
vertical and horizontal cracks.
Why do you think this is?
when the vertical cracks formed, the
horizontal cracks also formed.
Maybe the top was exposed to
longer than the bottom
Devils Tower Today