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GEOLOGY OF DEVILS TOWER NATIONAL MONUMENTSedimentationPat FrolanderJohn AloisioMarlon PooleLee McDowellDevils 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 TowerThis was theUnited States USGS pubs.usgs.govThe 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:ClasticOrganic ChemicalClastic:Big rocks break up into small rocks

And becomesand, 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 includeTable salt Gypsum Some LimestoneThe 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.

Lets review: What are the three categories of sedimentary rock?

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 rocksYounger rocksWhen layers form, the older rocks are on the bottom, and the younger rocks are found on top. This is called theLaw of Superposition.

Older rocksYounger rocks

These are the sedimentary rocks you see today at Devils TowerWhich rocks are the oldest?

The Spearfish formationThe 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 TowerRocky Mountains

magma

Magma

magmaFuture Little MissouriButtesFuture Devils TowerMolten rock that cools and becomes hardened is calledIgneous 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 20Are 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 TowerHave 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