The origin of the Carolina Bays remains a mystery more than eighty years after their discovery. This paper explores the hypothesis that the Carolina Bays were formed as the result of secondary impacts when a meteorite struck an ice sheet during the Pleistocene. The model describes the elliptical bays as conic sections from tilted conical craters formed by oblique impacts.
NOTE: The discussion presented in this web page has been published in the Elsevier peer-reviewed journal Geomorphology.
A. Zamora, A model for the geomorphology of the Carolina Bays, Geomorphology (2017), DOI 10.1016/j.geomorph.2017.01.019
The Carolina Bays are shallow lakes, wetlands and depressions, up to 10 km long that are found east of the Rocky Mountains and are concentrated mainly along the Atlantic seaboard. They are also known as Delmarva Bays, Maryland Basins or Nebraska Rainwater basins. The major axes of the ellipses generally point toward the Great Lakes. The bays were discovered in the 1930's from the first aerial photographs of the Atlantic coast. The rims of the bays are usually only one meter higher than the centers, making the bays virtually undetectable from the ground. The use of LiDAR, a laser imaging system, has been used to discover a great number of bays by emphasizing the small differences in elevation of the surface features. A web site for viewing Carolina Bay LiDAR images is available from Davias.
Bay Formation Theories
Many theories have been formulated to explain the origin of the Carolina Bays. The theories can be classified into those that propose forces within the Earth, such as wind or water currents, and those that propose impacts from an astronomical event, like a meteor shower. The proposals about impacts from meteorites or comets (Melton 1933, Firestone 2007; Kennett 2009a,b) have been opposed due to the lack of physical evidence resulting from a hyperspeed impact. This type of evidence is deemed essential to distinguish extraterrestrial impact structures from land features created by volcanism, erosion or other terrestrial processes (French and Koeberl, 2010).
Astronomers and geologists have rejected the theory that the Carolina Bays were the result of meteorite or comet impacts because the bays do not contain meteorite fragments or mineral crystals with the planar deformation features characteristic of hypervelocity impacts. (Pinter, et al. 2011).
The theories of bay creation by terrestrial processes include substrate dissolution, wind, ice, marine waves and currents that reduce the volume of karst-like depressions which are later modified by wind or ice-push processes (May and Warne, 1999). The bays have also been characterized as thermokarst or thaw lakes that are circular or elliptical in shape and are often aligned with the prevailing wind (Melosh 2011).
In general, the terrestrial process models do not apply well to the Carolina Bays because they do not provide a mechanism for the formation of elliptical bays with raised rims whose axes are aligned to radiate from a common point.
Carolina Bay characteristics
This section examines various physical characteristics of the Carolina Bays to try to determine mechanisms by which their features could have originated. Many of the features are best explained as a result of impacts, and it is for this reason that scientists persist in trying to develop an impact model that satisfies both astronomers and geologists.
These figures illustrate the great density of bays in the South Carolina coast. The surface is completely covered with bays except in the stream basins where they have disappeared through erosion. The structural preservation of the Carolina Bays for at least a dozen millennia may be partly due to the flat porous landscape which allows rain water to quickly filter underground thereby preventing erosion from lateral water flow. It is remarkable that all the bays are elliptical and aligned with their major axes from the northwest to the southeast radiating from one point. Another salient feature is that many of the ovals overlap, which implies a chronology of stratigraphic formation. Some bays are completely contained within others, while other bays only partially overlap. One important detail is the thickened rim on the southeast side of the bays. This is a characteristic of oblique impacts which tend to push surface material in the direction of the impact. It is also important to notice that large ovals may have small ovals within them, and that large ovals cover smaller ovals. This can be explained by noting that small bays created by small impacts would have been obliterated when overlaid by a larger impact.
Carolina Bays occur in unconsolidated ground close to the water table along the Atlantic seaboard. The bays are also found in Nebraska and Kansas in what once were the shores of the Western Interior Seaway of North America. The bays in the Midwestern states differ from the ones in the east coast in that their major axes are aligned from the northeast to the southwest. The fact that the Midwestern bays also radiate from the Great Lakes provides support for an impact origin of the bays. Fewer Carolina Bays can be observed in the Midwestern states because only the larger bays have endured the erosion by water and the accumulation of layers of wind-blown dust and silt.
The images illustrate the variety of bay configurations that preserve the basic elliptical shape with northwest/southeast alignment and the thickened southeast rim. In some cases, erosion or farming machinery have destroyed these features.
The Impact Hypothesis
Firestone (2009) and Davias (2010) have suggested the possibility that the Carolina Bays were created from impacts of ejecta from an asteroid or comet impact on the ice sheet that covered Canada. The lines drawn along the major axes of the bays in Nebraska, Kansas, Georgia, North Carolina, South Carolina and Virginia intersect around Michigan. The impactors that formed the bays have been suggested to be splashes of mud, ice-and-water slurry or just pieces of ice. However, to produce depressions like those of the Carolina Bays, the impacting body must resist deformation in order to penetrate the target surface. Ejecta such as slurries or mud merely form a layer on top of a firmer target surface without creating a depression (Gault and Greeley, 1978). Experiments with NASA's Ames Vertical Gun have demonstrated that a high speed impact on an ice sheet shatters the ice sheet ejecting large quantities of ice pieces, rather than just melting or vaporizing the ice (Schultz 2009).
Impact on an ice sheet ejects pieces of ice (Schultz 2009)
This paper examines the hypothesis that the Carolina Bays were made by impacts of material ejected at ballistic speeds from the North American ice sheet by a meteorite impact. The examination will try to determine if the characteristics of the bays could be reasonably explained by impacts from ice boulders with substantially slower speeds than meteorites, and whether experimental tests provide support for the hypothesis. The hypothesis also considers that the impacts were made on soft surfaces since the bays occur only on unconsolidated sediments and they are now shallow depressions instead of deep craters. No attempt will be made to establish the timeline of the meteorite impact, or to associate it with any mass extinction event.
Trajectory and Speed of Ejecta
If a meteorite impacted the ice sheet, the ejecta would have had to travel at least 1000 kilometers from central Michigan to North Carolina. We can use ballistic equations to calculate the speeds and angles necessary to launch a projectile this far. Speeds of 3 to 4 km/sec would be sufficient to reach from Michigan to the East Coast.
The ejected material would have traveled above the atmosphere for a portion of the trajectory. Liquid water in any ejected mud would have created clouds of ice crystals. The projectiles would have traveled without significant aerodynamic drag above the Earth's atmosphere. During reentry, the leading surfaces of the ice would have vaporized and acted as ablative heat shields. Aerodynamic stresses would have broken up some boulders to produce crater fields with many impacts. The breakup of the projectiles and mid-air collisions would have changed some of the trajectories and caused variations in the orientation of the resulting bays.
Point of impact
The Chippewa Basin, circled in red, was suggested as a possible extraterrestrial impact site by Firestone et al. (2006, p.267) because a seismic profile of the basin showed terrace faulting typical of an impact where large slabs of rock had cracked and slid downward. However, a USGS seismic stratigraphy analysis of Lake Michigan by Foster and Colman (1991) found an unconformity, but no evidence of faulting. The Michigan Basin that contains the Mississippian Aquifer is much older than the Carolina Bays and the thickening of the underlying Devonian, Silurian and Cambrian strata is not typical of an impact structure.
Davias and Gilbride (2010) integrated hundreds of LiDAR images of Carolina Bays and Nebraska Rainwater Basins with Google Earth. The great circle trajectories adjusted for the Coriolis effect converge at Saginaw Bay. This is more likely to be the point where the extraterrestrial object struck because ejecta always radiate from the point of impact.
Mass of Impactors
The size of the projectiles can be calculated from their speed, their density, and the diameter of the crater that they make. The University of Arizona calculator for computing projectile size from crater diameter (Melosh 1999) allows the input of the crater diameter and various initial conditions to determine the mass of the impacting object. Projectiles with a diameters of 25 to 180 meters traveling at ballistic speeds could account for the sizes of the Carolina Bays.
The energy of the extraterrestrial event can be estimated by adding the energy transferred to the ejecta. The amount of ejecta can also be calculated from the cumulative masses of the projectiles.
Oblique impacts create tilted conical cavities that appear as elliptical craters. The craters would be deeper at the terminal end due to the inclined incidence of the impact. The overturned flaps from the impact would create raised rims around the crater, particularly at the terminal end of the ellipse. When the ice impactor melted, the melt water would create a pool in the depression. Ice of terrestrial origin would not have any of the characteristics of an extraterrestrial origin, but it might be possible to find stones that were originally embedded in the ice.
Studies of the bays have not found evidence of disturbed strata, suggesting that no deep craters were created during their formation. However, unlike solid surfaces where the substrata are deformed by impacts (Melosh, 1989 p.79), the layers in unconsolidated surfaces are penetrated and parted by the impacting body; the original stratigraphy is later partially reassembled when the cavity is reduced by gravity. An impact of an ice projectile on a plastic surface in combination with the restoring effect of topographic relaxation would leave little evidence of disturbance.
Why conical craters?
It is reasonable to ask why the craters created by the impacts would have a conical shape. This is determined by cratering mechanics and the type of shock wave created by the impactor. Impacts on solid surfaces go through stages of compression, excavation and modification (Melosh, 1989). The transfer of energy from a hyperspeed impact is almost instantaneous so that even moderately oblique impacts create hemispherical shock waves and circular craters. Bottke et al. (2000) found that asteroid or comet impact angles of less than 12° from horizontal were required for the formation of elliptical-shaped craters on Mars, Venus, and the Moon. The modification stage is controlled mainly by gravitational forces that determine the final configuration of the crater. A collision with a plastic medium at a slower speed does not destroy the projectile. The excavation phase starts as the impactor continues to travel through the medium creating a conical shock wave until the projectile is stopped by friction. A plastic surface with low elasticity will retain the conical shape, and it will gradually be modified by gravity.
Eccentricity and time of emplacement
The size (major axis) of a bay is a function of the energy of the impact, but the eccentricity of the ellipse depends only on the angle of impact. For this reason, large and small bays have similar shapes, but the larger bays are made by larger projectiles.
The pattern of overlaps of the bays indicate the sequence in which the impacts occurred. In addition, using the formulas given above, the ratio of the minor to the major axes of the ellipses can be used to deduce the relative time of emplacement for bays which do not overlap.
The image above illustrates Bay A which is more elongated than Bay B. Bay A has axes of 637×1166 meters with a ratio of 0.546 which corresponds to an impact angle of 33.114°. Bay B has axes of 543×942 meters with a ratio of 0.576 which corresponds to an impact angle of 35.198°. Trajectories of projectiles originating from the same point and traveling the same distance at different launch angles are illustrated above. Assuming that the launch angles of the ice projectiles are the same as the impact angles calculated from the bays, the ballistic equations indicate that for projectiles traveling a distance of 1120 km, the projectile corresponding to A with the smaller launch angle and lower trajectory had a speed of 3.465 km/sec and a flight time of 386 seconds. The projectile for B with the higher trajectory had a speed of 3.415 km/sec and a flight time of 401 seconds. Thus, the projectile for A would have reached its destination 15 seconds before the projectile for B.
Gamble et al. (1977) suggested that the raised rims around the Carolina Bays might be secondary rims formed over time from eolian sand deposits which buried a primary rim at the edge of an initial depression. However, raised rims are a common characteristic of impact craters. The raised rims around an impact cavity are formed during the excavation phase as the shock wave of the impacting body penetrates the ground, displaces material laterally and ejects material above the surface. The overturned flaps are created during the remodeling phase when the ejected material falls back to the surface under the influence of gravity (Maxwell, 1977). On a plastic target, the ejected material acts like a breaking wave in which the base of the wave meets resistance from the surface while the crest continues its forward motion. The final overturned flaps usually display inverted stratigraphy. The following image indicates the locations of possible overturned flaps in several Carolina Bays.
Cratering mechanics are often studied using the CTH hydrocode software (Sandia), but the physical principles of this impact model can be tested easily with a simple experimental setup as illustrated by experiments on a mixture of sand and clay.
Projectiles which impact at a very shallow angle will ricochet, rather than penetrate the target. The craters left by these oblique impacts are also elliptical, but they differ from the conical craters made by projectiles that penetrate the surface.
Mechanism for Soil Fluidization
The impact model for the Carolina Bays proposes impacts of ice boulders on a plastic surface. Several factors could have contributed to the formation of a viscous target surface at the time when the bays were formed.
The Carolina Bays occur only in unconsolidated sediments in low-lying areas where the water table is within a few meters from the surface (Eimers et al. 2001). Saturated, cohesionless soils like these may liquefy and flow like liquids when subjected to monotonic, cyclic, or shock loading (Sladen et al. 1985). The soil can undergo large deformations typical of fluids when the shear resistance of the soil becomes less than the static, driving shear stress (Martin et al. 1975). Geologic flow processes caused by seismic vibration when water is not involved have been called "acoustic fluidization" (Melosh 1979) and "dynamic fluidization" (Richards et al. 1990).
The shock waves of a large impact would have been able to liquefy saturated soil, as commonly happens during earthquakes. In addition, the tremors produced by the multitude of impacts of huge ice boulders would also have contributed to the fluidization of the surface and speeded up topographic relaxation.
Hyperspeed impacts create different types of craters depending on the structure and properties of the target surface. Terrestrial craters with diameters of less than 4 km have a simple bowl shape, while larger craters typically have a central uplift from deeper strata or a multi-ring structure (French 1998). The uplift is created by the elastic properties of a solid surface which deforms and rebounds when impacted. By contrast, the Carolina Bays, even those with a major axis exceeding 10 km, have a uniform type of configuration due the way that impact cavities are formed on unconsolidated, saturated soils by projectiles at slower ballistic speeds.
A projectile at ballistic speed penetrates a viscous surface by parting the existing layers of the target during the excavation phase of the impact. The energy of the projectile is spent parting the material along its path. Viscous relaxation or Topographic relaxation driven by gravity levels the features made by the excavation and reduces the depth of the cavity.
Gravity reverses the sequence in which the cavity was created. The material that fills any level is the same material that was parted by the shock wave of the impact. Photographs of an experiment to verify this process are shown below.
Stratigraphic Restoration Experiment
The image above shows the crater made by an oblique impact on a target that had been prepared with an underlying red layer approximately two centimeters below the surface. The images show the gradual reduction in depth as the material flows to fill the cavity. The red layer remains at the same level and disappears from view when the impact structure reaches its final configuration.
The terrestrial models for thawed permafrost and other eolian and lacustrine processes lack a mechanism for explaining the regular elliptical shapes with raised rims of the Carolina Bays and their alignment toward a common radial point which are features more characteristic of impacts. The wide range of ages obtained for the Carolina Bays (e.g., Brooks et al., 2010) has been used to suggest that there were multiple periods of Carolina Bay formation by eolian processes and to refute the notion that a single impact event could have been responsible for the formation of the bays. However, if the bays were reconstituted by gravity, the material of the bays would have the same characteristics as before the impacts without affecting the age characteristics of the underlying material.
The new model explains many of the characteristics of the Carolina Bays, including the lack of shock metamorphism and meteorite fragments. The model postulates that oblique impacts on the soft surfaces of the coastline created slanted conical craters that became shallow elliptical pools through topographic relaxation of the substrata. The overturned flaps from the impacts became the raised rims of the bays. The model can be used to calculate the physical forces that may have been at play in the creation of the bays and provide some clues about the meteorite impact that created the ice boulders.
Proving that the Carolina Bays were created by impacts can be used as evidence that there was an extraterrestrial impact. The use of conic sections as a mathematical model for the impacts will facilitate the analysis of the bays and provide guidance for geological field studies. Overturned flaps with inverted stratigraphy in the raised rims at the terminal end of the ellipses are also characteristic of impacts and they could generally be identified from relatively shallow core samples or trenches. Geological exploration of the Carolina bays may settle the controversy about their origin and provide information about the site where the extraterrestrial impact occurred. Thus far, the strongest support for an extraterrestrial impact 12,900 years ago has been provided by analysis of Greenland Ice Sheet cores (Petaev, et al. 2013).
If the Carolina Bays were created by the proposed impacts, any fauna living in the central and eastern United States at that time would have seen a blinding flash followed by seismic shocks approximately four minutes later. It would have been almost impossible to avoid being hit by the barrage of enormous ice boulders seven minutes after the flash. Any survivors would have found their habitat destroyed. A local extinction event could have easily occurred under these circumstances, even without considering fires and other effects associated with the meteorite impact.