This stop takes us to a site where we can find evidence of some of the largest pyroclastic volcanic eruptions in the history of the world.

Before we focus in on the volcanic ash,take a look around the site.

How would you describe the difference between the rock in the bottom two thirds of the exposure with that at the top?

The bottom two thirds of the exposure consist of massively bedded limestone.  It is almostpure limestone, with very little shalemixed in.  It is called Lower Carters Limestone.

The top one third consists of limestone with partings of shale.  It is called Upper Carters Limestone.

Rock in one strata differs from that of another because the depositional environment is different.  Consult what you know about limestone and shale to describe the difference between the two depositional environments.

Now take a look at some of the fossils.  The most spectacular fossils at this site are the corals. There is a large one, about 18" across in the face of the rock about at eye level.  This is Columnaria, a common coral.
 
 
 
 
 
 

The most typical fossils of this site are what are called "trace fossils".  Those at this site are burrows of some invertebrate.

In some cases, the burrows are still hollow.

More commonly, the burrows were filled in by other sediment.  You find these fossils in almost every rock you pick up.

Now look at the material between lower and upper Carters Limestone strata.  How does it differ from the from the strata above and below it?  Why do you think so many plants are growing in it?

There is a second layer of this substance about five feet above the ground.  Being very careful not to fall over the building materials, get close to the outcrop and collect some from this lower band.

You will notice that it's not rock at all but clay.  This clay, called bentonite, is formed from volcanic ash.

• How do you think the volcanic ash got into the limestone?
• Do these layers come from one eruption or two?
• Can you tell what kind of volcanoes they came from?
• Do you think that the upper volcanic eruption might have caused the change in the depositional environment which led to the change in rock type?  Why do you think there was no change in the rocks after the lower eruption?

More recent research has very much expanded our understanding of these volcanoes.  A recent book Ordovician K-bentonites of Eastern North America by Dennis R Kolata, Warren D. Huff and Stig M. Bergstrom, summarizes the current stte of our knowledge.  It is available from the GSA or online bookstores.
 

• Bentonite can be chemically analyzed and individual volcanoes can be identified from this analysis.
• The bentonite we see in Nashville was part of a series of 60 eruptions which covered most of eastern North America and part of Northern Europe with ash.
• The bentonite from these eruptions can be dated radiometrically, and this dating shows that the eruptions occurred in a brief period (geologically speaking) about 450 million years ago

• These eruptions were caused by a major tectonic collision between what is now North America with an island arc.  This collision is called the Taconic Orogeny.