This exercise can be used for classroom teaching with an accompanying box of rocks and fossils.

If you hunt for fossils in middle Tennessee, you will notice that you find very different fossils in counties. These different fossils are not randomly distributed but form three distinct areas shown in this chart. Why do we find different kinds of fossils
 

	Fossils of the Nashville and Maysville Groups
Fossils of the Fort Payne Formation	Fossils of the Stones River Group

We find such different fossils because middle Tennessee is hilly and different strata are exposed in different places.  If you look at a cross section of the Nashville Basin, you will see that the three types of fossils are in three different layers, or strata.

Why Study Strata?
Aside from being helpful in hunting fossils, strata provide a historical record of Nashville's early history. To be able to understand it, we need to learn to read the rocks.
 
 
 

How are Strata Formed?
Strata are formed when sediment is deposited. In general, sediment erodes from high places to low places, so it deposits in low places, in the ocean, in lakes, in river basins and valleys. As large amounts of this sediment accumulates, pressure and dissolved minerals cement it into rock. The rocks and fossils that we find in each stratum tell us about conditions during the time that the stratum was deposited.

Subsequent layers of sediment settle on top of earlier ones so that newer strata lie on top of older ones. This is called the Law of Superposition.

When sediment is deposited, the layers that it forms are horizontal. If no tectonic, mountain building forces impact the strata, they remain horizontal, as they have in Nashville. This basic understanding of the nature of strata was discovered by Nicholas Steno one of the founders of geology.  For a slideshow on Steno's contributions by Dr Ben Waggoner University of Central Arkansas follow this link.
 

The strata that we find in Nashville represent a record of periods during which middle Tennessee was under water or close to it. When Nashville was high above the water no sediment was deposited, and existing sediment eroded as it does today. Thus, stratigraphy gives us a history with whole chapters missing.

For more information on the formation of sedimentary rocks follow this link.
 
 

How are fossils formed?
Hard parts of animals or plants get buried in the accumulating sediment and preserved as fossils. Finding different fossils in different strata means that different animals were alive in each of the periods of deposition. For more information and an exercise on fossilization, follow this link.
 
 
 
 

Identifying Strata Using Fossils
Although it is not always easy, it is possible to learn to identify strata using fossils.

Start by studying my Fossils by Stratum page.

Next let's identify some pictures of fossil slabs found around town.  Click on the images to see larger pictures.  Can you tell what stratum they are from?

Specimen 1	Specimen 2	Specimen 3
Specimen 4	Specimen 5	Specimen 6
Specimen 7	Specimen 8	Specimen 9

Now you're ready to start looking at real fossil slabs. If you don't have any in your classroom, you can find them on many roadcuts in the area. Fossil sites for each of the strata exposed in middle Tennessee are listed in Fossils by Stratum.

After you have tried to figure them out for yourself, you can look at the solution.
 
 
 

How old are these fossils?
 

Ken McKinney has an excellent exercise on relative and absolute dating of fossils in Learning from the Fossil Record.
 
 

Relative Dating
Relative dating is an application of the law of superposition. When horizontal strata are undisturbed, as they are in Nashville, lower layers are older than higher ones. It does not tell us how old a stratum is, but it allows us to put strata in chronological order.

NOTE:  This chart is simplified.  Between the Maysville Group and the Chattanooga Shale, there are several thin strata which only appear in some places.  Vertical distances are vastly exaggerated.

• Nashville Group on top of Stones River Group - Murfreesboro Pike at Plus Park
• Chattanooga Shale on top of Richmond Group - Old Hickory Blvd in Bellevue
• Fort Payne Formation on top of Chattanooga Shale - Ridgetop

Put the sample slabs in order:
 

1.  Earliest _____________________________________________________________

2.  Second _____________________________________________________________

3.  Most Recent _________________________________________________________

Absolute Dating

Sedimentary rocks, as a rule cannot be dated.  However two middle Tennessee strata can be dated.

Bentonite:  Between the Stones River Group and the Nashville Group lie four thin layers of a clay called bentonite.  Bentonite is the residue of volcanic ash, and these four layers are from volcanoes which erupted during a major mountain building episode called the Taconic Orogeny (oro=mountain:  geny=birth).  Because bentonite is igneous, it can be dated.  The largest of these four eruptions has been dated at 454 million years ago.

The fact that the bentonite is between the Stones River and the Nashville Groups is not an accident.  The Taconic Orogeny, which was caused by the collision of what is now North America with an island arc put pressure on the continent, causing a rise in the Nashville Dome.  This brought colder water for deeper in the ocean into what is now middle Tennessee, changing a tropical, low energy environment into a temperate, high energy environment.  The animals which had thrived during the deposition of Stones River were replaced by others, more suited to the changed environment.   This is based on the research of Steven Holland and Mark Patzkowsky  Follow the link to find out more about their work.  (This is advanced material).
 
 

Chattanooga Shale:  The Chattanooga Shale is a distinctive black shale which in the Nashville area is located directly under the Fort Payne Formation.  Because it contains significant amount of Uranium, it can also be dated.  It is dated at about 360 million years ago.

Using the chart of geologic time, mark the dates of the bentonite and the Chattanooga Shale and make your best guess about the period in which each of the three stata are located
 

Cenozoic     Era	Quaternary Period 2 my - present
	Tertiary  Period 66 my - 2 my
Mesozoic Era	Cretaceous Period 144 my - 66 my
	Jurassic Period 208 my - 144 my
	Triassic Period 245 my - 208 my
Paleozoic Era	Permian Period 286 my - 245 my
	Pennsylvanian Period 320 my - 286 my
	Mississipian Period 360 my - 320 my
	Devonian Period 408 my - 360 my
	Silurian Period 438 my - 408 my
	Ordovician  Period 505 my - 438 my
	Cambrian Period 570 my - 505 my

Our original relative dating predicted that the Fort Payne Formation would be later than the Stones River and Nashville Groups.  Do the dated strata confirm this or contradict this?

Look up the fossils of the Stones River and Nashville Groups in a fossil identification handbook.  In what period(s) does the book say the fossils are found?  Does this confirm your placement?

Why don't we find any Cambrian strata here in Nashville?

Why don't we find Mesozoic or Cenozoic strata in Nashville?

Why don't we find Silurian strata in Nashville?