The Cape Fear Arch displays a collage of mystery
Put together by events in geologic history.
A hidden rock ridge beneath the Coastal Plain,
Important existing conditions occur in a chain.
The Arch is dormant and quite massive,
But above it things are dynamic and not passive.
The Arch causes a bad aquifer to rise and leak.
Brackish water and gas at the land surface can peek.
Preface
This essay contains important statements with scientific backing but without full cited references and without complete analyses of various factors. More conventional scientific studies and documents must come later. While approaching the age of 92 and after 60 years of intermittent study and thoughts about the complex hydrogeology in the region of the Cape Fear Arch, I do not have the energy and capability to pursue further studies. This essay should provide some guidance for further research. Illustrations are not available to facilitate better understanding of the essay.
A strange dome of ancient brackish ground water in sand and clay formations lies near land surface above the basement rocks in an area called the Cape Fear Arch. The main area of focus is in New Hanover and Brunswick Counties in North Carolina. The geologic features in this area have resulted in an unusual set of conditions, which collectively appear to be unique. In hydrologic terms, this area is a major discharge zone of high-pressured brackish ground water along a part of the Atlantic coast. Knowledge of this setting is essential to prevent possible troublesome reactions to human activities concerning ground water. Also, knowledge needs to be gained to determine if leakage of natural gases in the area contributes to global warming. For example, it can be said that understanding and managing this intrusive brackish and gas-prone aquifer and its adjacent potable water system should be an issue for true regional management.
A noteworthy combination of sequential geologic phenomena since Cretaceous time has occurred above the Cape Fear Arch in North Carolina. The linked conditions have elements of human importance that even include, in a lighthearted way, the senses of seeing, hearing, smelling, tasting, and feeling. Moreover, certain myths, such as those of the Maco light and of the Guns of Seneca in southeastern North Carolina, likely have important scientific interpretations.
Having scientific and historical adherence, the total story stretches through geologic and human time. It is sketched here in a narrative, abbreviated way. Elements of the story will need to be studied in detail and to be fully integrated. Most conditions expressed can be well documented but are incompletely interpreted. Some explanations are suggested here and are open to critical questions. Nature has left us with a specific setting, showing that human interests can go hand-in-hand with picturesque geologic phenomena.
A part of the story about the Cape Fear Arch
Driving from Raleigh or from Myrtle Beach to Wilmington a traveler does not put eyes on the Cape Fear Arch, or the Great Carolina Ridge, as it was earlier called by geologists. Yet, the Arch rises vividly when visualizing the underground landscape. Puzzling discoveries are yet to be made about the Cape Fear Arch. The Cape Fear Arch is a basement rock structure beneath the general area of the lower Cape Fear River. The basement is at a shallower depth than at similar geologic coastal places in South Carolina and Georgia. Draping over the Arch are nearly flat sand and clay formations of Cretaceous age that are slightly tilted coastward. They extend, and are equivalent in age and character, to formations beneath the general area of the South Carolina coast, where they are more deeply buried beneath younger formations. Thus, the basal Cretaceous formations on the Arch are a shallow segment of the regional Coastal Plain sediments of the Atlantic Coastal Plain. All formations dip coastward and tend to thicken coastward far out to sea.
The occurrence and flow of water in this regional artesian system both now and in the geologic past are only partly known. All aquifer systems tend to have a recharge zone at a high elevation, which leads to a transmission zone and then to some type of lower discharge zone, which may be close or far away. We cannot trace each drop of water, but ground-water levels can be mapped so that the general behavior of ground-water flow is displayed.
With focus on the regional basal Coastal Plain beds, the general flow of water in the composite sand aquifer system tends to extend from a recharge area in central to south-central Georgia, where the pertinent formations lie about 300 feet above sea level. The natural early-stage flow in the confined system is southeastward and coastward to a position about 1800 feet beneath the Savannah and Charleston area where no natural deep discharge zone is available. This slow flow began long ago in Cretaceous time. The fresh water flow is blocked further southeastward by dense salty formation water already anciently stored in deeper and coastward parts of the aquifer system. Still trying to find a discharge zone, the fresh water in the aquifer turns northeastward beneath the general South Carolina coastal area. In this buried northeast travel it abrades against the seaward salty water of the confined aquifer with which it becomes brackishly mixed. It is still seeking a discharge area, although quite far away toward the Wilmington area.
North of Myrtle Beach the aquifer and its pressurized water has been pushed upward on the southern bedrock flank of the Cape Fear Arch. The aquifer becomes closer to land surface in an area northwest of Wilmington.
While making ground-water studies in southeastern North Carolina in 1954, I discovered a broad dome of brackish ground water occurring in a large area west of Wilmington. A search for its apex indicated that it reaches land surface as a seepage spring in a small creek between the Cape Fear and Black River a few miles above their confluence.
The unusual natural occurrence of brackish ground water inland at the land surface above the level of high tide has presumably not been reported elsewhere in the Atlantic Coastal Plain. A semblance of an ancient salt pit marked the locality where brackish water seeps from surface sands into a small stream. From the salt pit the natives evaporated the brackish water to get salt for human use during early days. Both the water and channel sand emit an odor of hydrogen sulfide, and white to gray slimy sulphur-smelling deposits occur along the bottom of the channel. Only bits of the puzzle were recognized then.
Many people are familiar with two mysterious phenomena that may be related to the story told here. One is the legend of the Maco Light and the other is the Guns of Seneca.
The Maco light. The legend, and part ghost story, of the Maco light dates back to 1867, when, as the ghost story tells, one evening a conductor on the last car of a train found that his car had been uncoupled and was about to be crashed by an on-coming train from the rear. He waved his lantern to alert the oncoming train, but the wreck decapitated him. It was reported that he continued to wave the lantern, until the lighted lantern rolled over and finally came to rest in an upright position. Shortly after the accident, the Maco light began to be noticed along the main track. It has been appearing there since and has become a popular curiosity for warm summer night visitors. It is reported that almost no one fails to see the light. Following the Charleston earthquake of 1886 the light stopped but returned sometime later. In 1889 the light was reported to have been seen by President Grover Cleveland. (This Maco light account was compiled by Troy Taylor, in Ghosts of the Prairie). Disregarding the ghost story related to the conductor and lantern, focus is on the likelihood of people seeing the light and possible implications. The light had to be observable before the ghost story of the lantern would be fabricated. Several theories have been proposed about the cause of this curious condition. The most reasonable explanation is that it comes from gas rising from a large marsh nearby. This explanation should get support from the fact that additional gases are rising into surface strata from deeper beds in this region. Does the light come from this combination of gases? Further study is needed
The Guns of Seneca. Mysterious booming noises off the coast of Brunswick and New Hanover Counties have been heard at various intervals by many people for more than a century. They have been called Guns of Seneca for being similar to the legendary booming noises on Seneca Lake in New York caused by Indians wanting to annoy immigrant Europeans with booming rifle blasts during early American times. They are also called sea farts by some local people. Some reports of shaking ground have been made, but they have not been scientifically related to specific earthquakes. I suggest that the booms may be the result of explosions of gases that have accumulated from aquifer leakage near the sea bottom five or more miles offshore. The gases are known to seep upward from the basal aquifer system on the Cape Fear Arch onshore. The gases leaking out of the offshore part of the aquifer would be trapped beneath the dense sea water until sufficient pressure would be disturbed at various times for the inversion to rectify and for an explosion to occur. Such explosions are known from trapped gases in some volcanic lakes under similar trapped conditions. Aquifer water less dense than sea water is also being trapped and might cause the explosion. There is probably a threshold position off shore before which and beyond which the pressure conditions would not be favorable for an explosion to occur. Not related directly to this situation but similar in principle is the theory that in the geologic past, massive explosions at certain times from accumulated gas hydrates on the sea floor have upturned and reached the atmosphere in sufficient volume to cause much life on earth to be exterminated. Gas hydrates are farther out in the Atlantic beneath deeper water. The sea farts in the region deserve scientific focus.
The basal aquifer system is more highly pressurized than might normally be expected. During my studies more than 50 years go, I realized that flowing wells over the Cape Fear Arch were more common than elsewhere on the Coastal Plain, which indicates substantial fluid pressure in the basal formations. Several decades ago an effort was made to dispose of some chemical wastes in the deep aquifer in an area several miles northwest of Wilmington. This plan failed because the pressure in the deeper aquifer was too great to accept the wastes satisfactorily. Also, mapped water levels in the basal aquifer showed the flow of water is not southeastward and down dip in the formation as is common elsewhere but up dip and northwestward. This strange and anomalous condition indicates a strong tendency for salty formation water to move inland from seaward positions on the flank and top of the Cape Fear Arch.
Where does this high fluid pressure come from? Some of the water has a tendency to come from a high intake recharge area in inland Georgia and South Carolina, before making a u-turn beneath the South Carolina coast, as indicated earlier. Also, a likely probability is that a relic and preserved pressure system has been maintained for many thousands of years when sea level and aquifer water levels were higher. Moreover, there has been an overall and sporadic inland movement of sea level and aquifer salt water zone from low positions a few tens of miles off shore from the present stand during Pleistocene and Holocene times. This long term encroachment of the sea on land has serious consequences. The rising of sea level would result in briny aquifer water offshore pushing the landward, aquifer water back inland under high pressure. The seaward high pressure would be maintained and increased inland because movement is toward the narrow part of the wedged aquifer system. Thus, this continual rising of sea level is a serious impediment in maintaining fresh water supplies in this coastal area. The indirect effects of rising sea level may not be fully evaluated by many hydrologists who study fresh and salt water relations in coastal areas for water supplies.
The discharge of water and gas has not been steady at certain places. The discharge zone would have moved outward and southeastward as the sea retreated and would have moved inland, on the Arch, as sea level moved inland. Some water is constantly leaking upward into overlying beds in its overall travel. The absence of the Maco light immediately after the 1886 Charleston earthquake suggests that leakage of gas and water to the Cape Fear Arch was temporarily lessened as minor faulting and upward leakage of water lessened the deep water pressure in the large aquifer area beneath the coastal area.
How extensive is the spread of the gases now? Although some escape of gas may appear concentrated in the vicinity of the salty water seepage spring and also along some spots in the Cape Fear River channels, the outward leakage is likely widely dispersed through creeks, marshes, and some Carolina Bays. Could the aggregate gas discharge tend to increase global warming? It is almost certain that gas and fresh and brackish water have seeped outward to varying degrees and sporadically through hundreds of thousands of years as sea level advanced and retreated. The significant aquifer discharge has been active for ages. Whether we call it the Cape Fear Arch or The Great Carolina Ridge, its presence causes a major aquifer system to be upturned so that confined water and gas can discharge in ways to leave many unsolved mysteries.
In a geologic sense, one can see underground. We can see the Cape Fear Arch, at least in a picturesque manner. The human senses come to mind in the Cape Fear region also. One can see the slimy sulfur deposits in a stream channel and also the light at Maco. One can perhaps hear the booms of the Guns of Seneca, smell the hydrogen sulfide odor, taste the brackish and sulfide water, and feel the ground shaking from booms of the Guns of Seneca. In an artful and facetious sense, where else on earth can all human senses be similarly recorded?
The sight of the Grand Canyon receives much publicity,
But we can see the Cape Fear Arch in “virtual” reality.
Both are spectacular in their own way,
Underground landscapes are pictures to display.
An undesirable evil aquifer has a wide spread.
Seemingly proud of its high pressure head,
Salty water and gas would like to burst out
And harm us somehow round about.
We must learn how the aquifer will behave
So that parts of the environment we can save.
Here we may need to leave some of the aquifer alone
Or pursue actions only with the proper tone.
Comment on references
A more scientific document would have called for more references. The purpose of this reference is to indicate the location of the cited salt-water and gas seepage spring.
LeGrand, H. E. 1955, Brackish water and its structural implication in the Great
Carolina Ridge, NC: Am Assoc. Petroleum Geologists Bull.,
v. 39, no.10, p. 2020-2027.
