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Outer Nashville Basin

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About the Outer Nashville Basin

The Outer Nashville Basin is a roughly donut-shaped region centered around an area east of the city of Nashville. It is mostly located in Tennessee, but with small sections extending into southern Kentucky and northern Alabama.

This region has diverse topography, ranging from gently-rolling hills and plains to steeper hills and highly-dissected escarpments. This region is mostly underlain by phosphate-rich limestone and shaly limestone, with chert and cherty limestone on the higher knobs; there is also some shale. Soils here are mostly formed on solution residuum and silty clays derived from the bedrock. Soils here are diverse, but tend to be rich in mineral nutrients.

This region was originally mostly forested, with oak-hickory forest over most of the region, but the forests having some commonalities with mixed mesophytic forests in the east. Dominant species were white oak (Quercus alba) and northern red oak (Quercus rubra), also with tuliptree (Liriodendron tulipifera), hickories (Carya sp.), eastern redcedar (Juniperus virginiana), black locust (Robinia pseudoacacia), sugar maple (Acer saccharum), honeylocust (Gleditsia triacanthos). Floodplains and bottomland terraces supported American sycamore (Platanus occidentalis), red maple (Acer rubrum), river birch (Betula nigra), American sweetgum (Liquidambar styraciflua), and tuliptree, with understories of giant cane (Arundinaria gigantea).

Presently this region has a mosaic of different types of land use, including urban areas, pasture, forest, and cropland. The cropland, mostly located on bottomlands and lower slopes, produces corn, tobacco, hay, and garden crops. There is also some phosphate mining. This region contains nearly all of the Nashville, TN metro area, with just under 2 million people; this figure includes the smaller city of Franklin, TN and its suburbs. This metro area primarily has a service economy, and most of the environmental impacts are associated with sprawling residential development. However, the rest of the region is rural and sparsely populated, with only smaller towns.

Forest cover has been reduced greatly, especially in the bottomland areas; most remaining forests are on the steeper uplands. These forests presumably have a similar composition to the original forest cover here. Eastern redcedar, an early-successional species that thrives on limestone soils, has become more common throughout the region, and often forms large stands, especially on recently-abandoned lands. Abandoned fields initially support broomsedge bluestem (Andropogon virginicus) and sumac, which develops into early-successional forests of redcedar and black locust.

This region entirely surrounds the Inner Nashville Basin, which is lower, flatter, and has soils lower in phosphates. This region is surrounded to the north, east, and south by the Eastern Highland Rim, and to the west by the Western Highland Rim. Between these, there is also a small border to the north with the Western Pennyroyal Karst Plain, a flatter area that is heavily utilized for agriculture.

References

1. Woods, A.J., Omernik, J.M., Martin, W.H., Pond, G.J., Andrews, W.M., Call, S.M, Comstock, J.A., and Taylor, D.D. "Ecoregions of Kentucky (Poster)", U.S. Geological Survey, Reston, VA (2002) Web.

2. Griffith, G.E.; Omernik, J.M.; and Azevedo, S.H. "Ecoregions of Tennessee (Poster)", U.S. Geological Survey, Reston, VA (1998) Web.