The Nashville Basin
An escarpment is defined as “a long cliff or steep
slope separating two comparatively level or more gently sloping
surfaces and resulting from erosion or faulting.” The
Highland Rim encircling the Nashville Basin of middle Tennessee
aptly fits this description as one passes either into or out of the
region. The maps below illustrate the geologic situation shared by
both Kentucky and Tennessee. [1]
Geologically speaking, the basin in both regions actually formed
from a dome. The rock layers in Middle Tennessee were warped into a
large, gently sloping anticlinal dome. After a very long
period of time (and continued pressure) the dome folded and
eventually cracked. These fractures and cracks broke down more
quickly as a result of weathering and eroded more easily than the
surrounding Highland Rim. A basin thus developed from the
eroding of these points of stress over millions of years. The
region eventually enlarged to its present size of about 4,850
square miles. [2]
Below is a satellite image of the Richat dome structure of
Mauritania which shows how the Nashville dome might have
looked several million years ago.[3] A caveat: the
structure was pushed up by a volcano while the Nashville dome was
due to warping.
The Highland Rim
Surrounding the Nashville (a.k.a. Central) Basin, the Highland
Rim is an elevated plain with steep slopes marking the descent into
the basin. Underground streams and caves are commonplace beneath
the surface of this region. The Highland Rim is divided into
eastern and western portions by the Nashville Basin. The eastern
section, about 15-20 miles wide, has a flat to slightly rolling
terrain because the limestone strata underlying the area are almost
flat or dip toward the east. The karst features of the Eastern
Highland Rim are a prominent topographic aspect of the region. The
Western Highland Rim encloses the Nashville Basin on the west. Its
terrain, dissected by many streams and rivers (e.g. the Buffalo
River), is more rolling than that of the Eastern Rim.
[2]
The Highland Rim is called a cuesta, a term which
describes the ridges formed by the tilted rock layers. Cuestas
usually have steep slopes where rock layers have their edges
exposed. This is where one can find the edge of the escarpment.
Geology
Uplift of the Nashville Dome accompanied each orogenic episode
in Tennessee. As a result, the regions of the Eastern and Western
Highland Rims and the Central Basin all experienced periodic
increases in surface elevation during the Paleozoic and early
Mesozoic. At one time, the sandstones of the Cumberland Plateau
probably extended westward over these areas as well. Fractures,
resulting from uplift along the crest of the Nashville Dome,
however, made the sandstones and the underlying limestones more
susceptible to erosion. Consequently, the only remnants of these
sandstones in Middle Tennessee are preserved in features such as
Short Mountain. Isolated, resistant bedrock features like Short
Mountain are termed erosional remnants. [4]
Elsewhere in the Eastern Highland Rim, erosion has exposed
carbonate bedrock of Late Paleozoic age. These carbonate rocks
contain variable amounts of chert, and are often interbedded with
fine grained clastic rocks. As a result, these rocks are more
resistant to erosion than the underlying, purer limestones of the
Lower (Early) Paleozoic. Hence, the Eastern Highland Rim stands
above the Central Basin where Lower Paleozoic limestones crop out
and chemically erode quite rapidly. In addition, structural
fracturing would have been most intense over the top of the dome;
consequently, the Central Basin is more deeply eroded than the
adjacent Highland Rims. The geologic characteristics of the Western
Highland Rim closely parallel those of the Eastern Highland Rim,
resulting in very similar physiography as well. Elevations in the
Highlands Rim typically range from 600 to 1200 feet. Within the
Central Basin, the elevation rarely exceeds 800 feet, with 500 to
600 foot elevations more typical.[4]
As you drive north on US Highway 109 from Gallatin to Portland
(Sumner County) you will leave the Ordovician Period of rocks
(505-438 mya), briefly pass through the Devonian-Silurian Period
(438-360 mya), and finally reach the Mississippian geology
(360-320±). You have just passed approximately 180 million years of
geologic time, give or take a few million years. See the
“Generalized Geologic Map of Tennessee”
below.[5]
Notice the three geologic groups that occur in Sumner County in
the map above. One of the most important geological events of the
Ordovician period is the Taconic Orogeny, a series of mountain
building events located in what we now know to be the northeastern
United States. At the end of the Ordovician, there was also a huge
extinction of many forms of marine life due mainly to an increase
of glaciation and subsequent decrease in sea level. [6]
The rock types usually found in this period are limestone, shale,
dolomite, siltstone, sandstone and claystone.
The Silurian is a relatively brief period on the time scale.
During the Silurian, collisions between early continents that
included parts of modern North America, Africa, and Europe formed
long mountain chains. The earliest evidence of complex life on
land, in the form of small plants and animals such as centipedes,
is found in Silurian rocks, as well. [6] The rock types
usually found in this period are limestone, chert, shale, and
sandstone.
The fossil record of the Devonian is marked by an extinction
that killed 40% of all marine life took place. The causes for this
are uncertain, but possible factors may include glaciation,
atmospheric changes or meteor impacts. [6] The rock
types usually found in this period are limestone, chert, shale, and
sandstone.
The Mississippian is the first part of the time period
geologists originally referred to as the Carboniferous (the second,
later period is the Pennsylvanian) because of the presence of coal
in similarly-aged rocks around the world. Mississippian rocks are
well-exposed along the Mississippi River Valley (hence the period's
name). They are mainly characterized as fossil-rich limestones
formed in warm, shallow seas. The end of the Mississippian,
however, saw a decrease in sea level, causing a marine extinction.
[6] The rock types usually found in this period are
limestone, chert, shale, siltstone, sandstone, and dolomite.
Tasks
Take a drive on US Highway 109, preferably heading north from
Gallatin to Portland. Stop at these coordinates and note the
elevation: N36° 25.976, W086°
27.915. Now continue driving north to the coordinates
below and notice the road cuts. When you see the grayish-black
rocks you’ll know you’ve found a unique feature: the
uppermost layer of the Devonian System called the Chattanooga
Shale. This fissile carbonaceous shale has a thin sandstone layer
at the base and only averages about 20 feet in thickness. [NOTE:
Please be careful along this road at it is heavily traveled,
winding, and somewhat narrow for those who don’t drive it
often. DON’T STOP TO LOOK AT THESE ROCKS. You’ll pass
them once again on the way back down the escarpment.]
Proceed up the escarpment and find the water tower at these
coordinates: N36° 28.025, W086°
28.407. Note the elevation at the coordinates.
While here stroll over to the overlook and look south. You’ll
have a great view of the Nashville Basin. (Snap a photograph of
yourself at this promontory.)
Work your way to the west side of US Highway 109 and turn on
Music Mountain Road. These coordinates should help you find the
spot: N36° 28.042, W086° 28.627.
Note the elevation at the coordinates. Take notice of the
radio towers and the TBN television tower. (Those new to the area
or religious programming will learn that TBN stands for the Trinity
Broadcasting Network. A nice place to visit, especially at
Christmastime, is Trinity City in Hendersonville. The Christmas
lights are a pleasant attraction.)
Finally, proceed to these coordinates: N36° 27.449, W086° 28.318. Park your car in the
short driveway. [You’ll have just enough room to do a
three-point turn to get back on the road.] Step out and survey the
exposed rocks on the wall.
[NOTE: Use the two Google Earth screen shots below to orient
yourself.]
In the screen shot above you can tell the elevation changes as
you drive north on US Highway 109 out of the Nashville Basin and
come to the crest of the Highland Rim. (Green areas show cropland,
pastures, and residential development. Brown areas are wooded
lands.)
In the above screen shot you can tell you’ve reached the
crest of the ridge since there are several radio and television
towers, as well as a water tower. They are located here to take
advantage of the superior elevation (i.e. air waves and
gravitational water pressure).
To get credit for this EarthCache, sent an e-mail with the
correct answers to my account. Please don't place your answers on
the log. But do provide a documenting photograph of yourself at the
overlook.
Questions
Question #1: What is the elevation at your first stop
(N36° 25.976, W086° 27.915)?
Question #2: Was the elevation greater at the coordinates
for the water tower or for Music Mountain Road?
Question #3: Subtract the low elevation (at your initial
stop) from the high elevation and tell me the difference in total
elevation (within 10 feet).
Question #4: At the last set of coordinates (N36° 27.449,
W086° 28.318), assess the geology of the exposed rock wall. Tell me
what type of rock this is and what led you to this
determination.
Final Requirement: Include the photograph of yourself (GPSr
in hand) at the water tower overlook.
Sources
[1] Raitz, Karl.
Rock Fences and Preadaptation Geographical Review, Vol.
85, No. 1. (Jan., 1995), pp. 50-62.
[2] Tennessee Online Atlas – http://tnatlas.geog.utk
[3] Richat dome of Mauritania –
http://scijinks.jpl.nasa.gov/en/educators/gallery/landforms/richat_L.jpg
[4] Harris, Clay. Online publication titled “Short
Mtn.(2092’). www.mtsu.edu
[5] Generalized Geologic Map of Tennessee – http://www.state.tn.us/environment/tdg/bigmap.shtml
[6] National Atlas – www.nationalatlas.gov