Georgia's Grand Canyons | Georgia Outdoors

Janisse Ray, Author of "Ecology of a Cracker Childhood": Half-Earth Educ...

Georgia's Grand Canyons | Georgia Outdoors

Sapelo Island | Georgia Outdoors

Georgia's Grand Canyons | Georgia Outdoors

The Sandhills of Georgia | Georgia Outdoors

Longleaf Pine Workshops Held Across Georgia

Secrets of the Longleaf Pine

As a follow-up to yesterday's post.

Longleaf for the Longrun

Oldest longleaf pines in the world - Boyd Tract - Weymouth Woods Sandhil...

LONGLEAF: THE HEART OF PINE

Janisse Ray's Blog.

Arbor Day Guide to Longleaf Pine.

Definition of an Ecological Fire Regime.

More Southern Woodlands Fire Regime info.

River-Recovered Timber info.

Wiregrass Ecosystem info.

More Longleaf Wiregrass Ecosystem info.

Longleaf Wiregrass Ecosystem flora.

(Some pertinent photos are planned for a subsequent post.)

My Own Exhumed Landscape Story in Georgia

 As a follow-up to this story on the Wichita Mountains as an "Exhumed Permian Landscape" is my own smaller-scaled "Exhumed post-Miocene Landscape" on the Georgia Coastal Plain (see "Youngest Formations" in this link).  

It was about 20 years ago, while I was working on the Georgia Geologic Survey STATEMAP Project, east of the Flint River, I was mapping a portion of Turkey Creek (with the permission of the Turkey Creek Hunting Club) on the Drayton USGS 7.5 minute quadrangle (Summerour, 1999).

In an area with an "Asymmetrical" creek valley (bluff on the south side, flood plain on the north) at this particular location, the bluff was on the "outside" of a meander.  Per this link's definition, looking downstream (southwest), this area had a local "Positive Asymmetry", i.e., bluff on the left, gentle slope (flood plain) on the right. 

At the base of the bluff, at creek's waterline (and just above) was a Middle Eocene Lisbon Formation "marl" (a sandy, calcareous, glauconitic clay), with numerous interesting small marine fossils.  

The matrix disaggregated easily in water revealing a mix of tiny Comatulid Crinoid centrodorsals (and other "pieces) (Microcrinus conoideus), small Brachiopods (Terebratulina sp.), small Echinoids (Echinocyamus parvus, Protoscutella conradi.), as well as other small fossils and microfossils (Summerour, 1999), some of which facilitated paleontological research (Oyen and Summerour, 2002) and 32 of which were donated to the Florida Museum of Natural History Invertebrate Paleontology collection.

On the bluff, above the "marl" was interpreted to be Late Eocene Clinchfield Formation marine sands, all the way to the top of the bluff (perhaps 30 feet (+/-)).  I don't recall if there was any thin or discontinuous Miocene Altamaha Formation at the very top of the bluff or not.

The Altamaha Formation is composed of clays, gravels, sands, all of which are interpreted to be fluvial.  The Altamaha Formation commonly comprises the surficial deposits of the Inner Coastal Plain.

During my solo mapping sessions and field-checking by colleagues, it was generally agreed that the north side (inside) of the meander was eroded Altamaha Formation, though it was significantly lower than the top of the bluff.  

This led to some time spent in deep contemplation sitting on the bluff, both alone and other times with a colleague, wondering "What the hell happened here?"  

I had dealt with smaller asymmetrical valleys in the area, including Turkey Creek, upstream and east of the town of Byromville.  In that and the other local cases, the side opposite the bluff was usually covered with terrace alluvial deposits and the topographic contrast was not as prominent.

It had been about 22 years since my Geomorphology course and I hadn't given much recent thought to "exhumed landscapes", though I had consulted Thornbury (1968) to make my case for isolated low "sand ridges" adjacent to some creeks being "non-paired, non-cyclical terraces in an asymmetrical valley" elsewhere in the study area.

Anyway, as I sat on the Late Eocene in the upper part of the bluff, I looked across Turkey Creek AND DOWN at the Miocene on the other side.  Though the proper word "exhumed" didn't jump out at me, I sensed that "something old was being uncovered", i.e., something from a previous cycle of erosion and deposition was exposed across the creek, rather than the "normal" flood plain deposits.  

As I couldn't properly articulate what I had seen (and I was on a schedule to finish and move elsewhere), I chose not to include that in my report. 

References:

Oyen, C. W. and Summerour, J. H., 2002  New records of comatulid crinoids from the Eocene of Southwestern Georgia; (abs.) Geological Society of America Abstracts with Programs, vol. 34, #2, p. A117.

Summerour, J. H., 1999  Geologic Atlas of the Byromville, Drayton, and Leslie, Georgia 7.5 minute quadrangles; Georgia Geologic Survey Digital Open-File Report 99-1, 42 pp., 4 pl.

What a Geologist Sees - Part 12

Figure 1.

Just a brief lesson on water well construction and aquifers and other stuff like that.

The upper photo is of a poorly constructed/ poorly maintained residential drinking water well, probably on the order of 120-150 feet deep. It was located on the inner Coastal Plain, a few miles south of Augusta, GA, in Burke County.

Though we (the Georgia Geologic Survey Tritium Project) never subjected this particular well to a detailed examination by geophysical logging, it probably "bottomed out" and was drawing water from one of several formations in the unconfined Upper Three Runs Aquifer.  

These could include the Late Eocene Utley Limestone Member of the Clinchfield Formation, Griffins Landing Member of the Dry Branch Formation, or the Irwinton Sand Member of the Dry Branch Formation. Overlying the Dry Branch Formation was the Late Eocene Tobacco Road Sand and that is what makes up the sandy soils seen surrounding the wellhead.

There are two areas of concern regarding the construction of this well. The first involves the apparent lack of a "grout seal".  When a drill rig drills a borehole, because of the sloughing of sediments, the borehole is rarely ever a perfect cylinder.  

To prevent the sloughing of sediments that would fill the well, usually well casing is installed consisting of Schedule 40 or Schedule 80 (for deeper wells) PVC pipe that is from 4 to 8 inches in diameter.  With this well, the rust on the exposed casing suggests a steel or cast iron casing (it has been almost 10 years since I worked this area). 

With the installation of the casing, the area between the borehole and the casing is called the "annular space" and it must be properly sealed to prevent surface pollutants from reaching the aquifer.  Around the "screen zone", where the water enters the well casing, coarse sand (sand pack or gravel pack) is usually introduced to keep the water flowing from the aquifer into screen zone. Above the "gravel pack" is where the first seal should be applied.  

Below the water table, pellets of bentonite clay are introduced by way of a small pipe from the surface.  Once wet, the bentonite pellets will swell and seal the annular space, if properly applied.  Above the water table and all the way to the surface, a thin slurry of concrete seals the rest of the annular space.  At the surface, the well owner is supposed to pour a concrete surface pad to act as the first line of defense against surface pollutants reaching the aquifer.

That is how it should be.   A closer look at the surface area (amid the debris) surrounding the wellhead (in the upper photo) suggests that the area slopes inward towards the well casing.  This is not good.  This suggests that rainwater has been washing the sandy soil into the ungrouted (or poorly grouted) annular space.  Along with the sand, whatever else might be on the ground surface (chicken poop, etc.) is susceptible to being washed down the annular space, possibly reaching the aquifer if there is no grouting at all.

If you are ever in the position of buying a piece of property and the wellhead looks like this, at least get the water tested by the county health department or walk away from the deal.  Due Diligence applies to things more than land titles and termites.

Figure 2.

In the photo above, these three GGS monitoring wells (one mile or so from the above well) exhibit properly installed surface pads to top off the annular space grouting. [There are three wells drilled into three separate aquifers at different depths to test for tritium among other things. 

Above-background (but below EPA MCL) levels of tritium were found in the shallow aquifer well (the same one serving the well in the upper photo). One goal of our study was to make sure that similar levels of tritium had not reached the deeper aquifers (which it had not).

If the residential well (drilled to serve a couple of trailers) had at least a surface pad, that would have offered some protection from surface pollutants.  Just a reminder, if you are going to purchase a homesite dependent on a water well, you should have it inspected before you commit to a purchase.