Our other field trips have made it very clear, that when the rocks we see were deposited, the Nashville Basin was covered by an ocean.

Until now, we have not addressed the question of what difference this makes for modern Tennesseans.  However, it has made a significant difference.

Middle Tennessee's two most important mineral resources, limestone and phosphate both form only under the ocean. Because the value of the field trips is dependent on understanding the importance of these resources, I have included more background for these field trips than for others.

How are limestone and phosphate formed?
Limestone is a rock formed primarily from the shells of, mostly microscopic organisms which accumulate at the bottom of the ocean.  Because it is soluble in acid it forms only when the water pH is greater than 7.5 (slightly basic)

• It forms only in sea water (which is slightly basic), not in river water (which even before industrialization was slightly acidic).
• It will not form in very cold or very deep water

Phosphates are sedimentary rocks which contain more than 15-20% phosphate (P2O5).  Like limestone, phosphates are formed both by chemical precipitation and by biochemical processes.  Although phosphorous accumulates in bones and teeth, the major source of phosphorous appears to be that used in soft tissue for energy storage (conversion of ADP into ATP for energy storage).  In shallow parts of the ocean, these soft body parts are quickly eaten by other organisms. For this reason, phosphate accumulates only under reducing conditions in deep anoxic water.  The final stage of phosphate rock formation involves reworking of the sediment under high energy conditions, which concentrates the phosphorous in the rock.  High energy conditions occur primarily in shallow water.  What this means is that high quality phosphate rock can only form in the places where deep ocean water is upwelling into shallower, high energy water.

Given such complex conditions, phosphate is obviously a great deal rarer than limestone.

The P's on the map show where phosphate rock is now forming.  It is forming on the west coast of South America and Africa, where cold Antarctic currents rise to the surface.
 
 
 
 
 
 
 
 
 
 

What use is middle Tennessee's limestone and phosphate?

Limestone is used in many ways.  Since the founding of Nashville, limestone has been used as a building stone.  The state capitol building was built in the middle of the 19th century from limestone from the Bigby member of the Bigby-Cannon Formation (Nashville Group)  In fact, during the 19th century the Bigby member was called "Capitol Limestone."
 
 
 
 

Hume-Fogg High School (Broadway between 7th and 8th) was built in 1912 of the lower Dove-colored member of the Bigby-Cannon Formation. (the lower of the two light colored strata in our normal fault.
 
 
 
 
 
 
 
 
 
 
 

In recent years, most of the limestone quarried in Nashville is sold as crushed stone.  This is used for many purposes.
 
 
 
 

• It is used in road building, for creating the foundation for the road and mixed with the asphalt to pave the road.

• It is used in agriculture, spread on fields to reduce the soil's acidity.

• It is used in making cement

The major use of phosphate is in agricultural fertilizer.  However, phosphate has many other uses.  (photo is an exhibit at the Phosphate Museum.)

This site gives a good summary of the many uses of phosphate
 

In 1888, phosphate was discovered in Maury County, Tennessee along the Bigby River in the stratum then known as "Capitol Limestone."  I am not sure that they changed the stratum's name to Bigby Member in honor of the discovery of phosphate, but I suspect they may have.

Let's start our field trip, going to Maury County to take a look.
 
 
 

Visiting the Phosphate Museum in Mount Pleasant.

Mount Pleasant is in southern Maury County.  It takes an hour to an hour and a half to get there.

• Drive south on I-65 to Exit 46 and follow the signs to Columbia.
• From Columbia, take rt. 43 to Mount Pleasant

This is a typical local Museum, which celebrates the history of its town.  Phosphate is a very large part of the museum, because, in the early part of the 20th century, Mount Pleasant was "The Phosphate Capital of the World."

The first floor of the museum exhibits photographs and artifacts of the history of phosphate mining in the area, such as the Monsanto jug shown here, and the products produced with phosphate, shown above.  Many of the materials in the museum, are included in  Images from our Past:  A Photographic Review of Historic Mount Pleasant, Tennessee published by the Museum in 1999 ($12.50)
 
 
 
 

Phosphate was discovered by a prospector in Maury County in 1888.  Chemical fertilizers had been introduced into American agriculture in the 1840's, and by 1890, production of fertilizer had become big business.  Many local mining companies were formed to exploit the new opportunities. Between 1890 and 1900, the population of Mount Pleasant increased from 400 to 2,000.  In early years, mining was done by hand and phosphate was taken from the pit by mules.
 
 
 

By the 1930's local mines had been bought up by a few national companies.  Production had been mechanized.  This is the International Agriculture Corporation Plant.
 
 

Beginning in the 1970's phosphate mines started closing in Maury County.  The museum staff reports that the opening of new quarries in Florida where cost of extraction was lower made mining in Tennessee unprofitable.  The industry closed down entirely in 1990.

Sam Boggs in his Principles of Sedimentology and Stratigraphy still lists middle Tennessee as one of six major deposits of phosphate in the United States.
 
 

Visiting Limestone Quarries in Nashville

Limestone has been quarried in middle Tennessee since the arrival of whites in the early 19th century.  As with phosphate mining, in the beginning there were many small quarries.  The remnants of dozens of them can be found within Nashville itself.

In recent years, most working quarries have been acquired by Vulcan Materials and The Rogers Group.

Both of these companies offer tours of their quarries  for students.  Contact the quarry you want to visit, and they will get you in touch with the tour organizers.
 
 

Phosphate as a Guide in Understanding Geologic History Phosphate has an entirely non economic value.  Because it forms only under conditions in which a high energy, shallow environment is near to deep water, its  presence indicates that those conditions existed. While limestone is found in all Ordovician strata in middle Tennessee, phosphate is only found in the Nashville Group and above, i.e. it is only found above the Taconic Orogeny.  This is taken as evidence that the Taconic Orogeny squeezed the Nashville Dome, increasing its slope, creating a ramp on the western side of the dome, which allowed water from the deep ocean to well up into what is now middle Tennessee  (Steven M Holland and mark E Patkowsky, "Distal Orogenic Effects on Peripheral Bulge Sedimention:  Middle and Upper Ordovician of the Nashville Dome" Journal of Sedimentary Research, Vol 67, # 2, 1997)