GEOLOGY OF DEVILS TOWER NATIONAL MONUMENT

Sedimentation

Pat FrolanderJohn AloisioMarlon PooleLee 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 theUnited 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 siltthat settle to the bottom of an ocean

Which become rock again

Organic : LIMESTONESedimentary 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.

OrganicSedimentary 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 rockIncludes 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 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

magma

Future Little MissouriButtes

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

Maybe the top was exposed to weathering longer than the bottom was.

Devils Tower Today