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Mayor Dean's Geology Walk

 

Photographs above: Gary Layda, City Photographer

Nashville Dome

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Art Work by: Abraham Padilla, Vanderbilt University

 

Big Points about Nashville’s Geologic setting -­-­-­ controlled by events at 3 different times

I. ~470 = 330 million years ago the rocks were formed by accumulation
of sediment in ocean on continental shelf

  • Unit #3 – shells of marine animals deposited in a warm, shallow sea (think Bahamas, not Miss Delta) to form LIMESTONE
  • Unit #2 – a stagnant sea on the continental shelf, with abundant floating algae that sank to the bottom to form SHALE
  • Unit #1 – more well-mixed ocean water on the continental shelf, with silt and clay delivered by rivers to form SILTSTONE

II. 330 million years ago to later – much of the time above sea level

  • Unit #1's silt had pores between grains and it was flushed with silica-­‐rich water that deposited CHERT
  • Unit #2 organic matter transformed into natural gas and oil, and uranium from ground water attached to organic matter
  • Units # 1, 2, and 3 were gently folded into a broad upwarp (dome)

III. Current processes that have been acting for millions of years

  • EROSION – Units #1, 2, & 3 erode in very different styles
  • Unit #3 – dissolves; calcium carbonate dissolves in weak acid formed by rain + carbon dioxide; leaves very little soil Nashville has lots of rain, so limestone dissolves rapidly – has formed a big basin
  • Unit #2 – breaks down quickly – rarely see unit #2 rocks (Old Hickory Blvd in Bellevue, I--‐24 Joelton)
  • Unit #1 –TOUGH to erode because CHERT resists breakdown. Remains standing, forms steep slopes.

How this history affects Nashville today

  • Limestone dissolves leaving little soil, so no soil for landfill sites, little soil for septic drain fields – large yards originally
  • Limestone dissolves, little soil so bedrock near surface: causes runoff, exacerbates flooding; prohibits burial of electric lines
  • Unit #1 is tough to erode so that steep slopes form, are landslide prone, especially when water saturated
  • Unit #2 is eroded, moved downhill to form soil; its uranium decays to form radon gas; its organic matter = oil & natural gas

Produced by Molly Miller, Vanderbilt University; for more information about TN geology go to http://www.tn.gov/environment/tdg; http://tn.water.usgs.gov

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