During the late Pleistocene Epoch, 12,900 years ago, a meteorite hit the Laurentide ice sheet in the Great Lakes area. The extraterrestrial impact ejected a barrage of glacier ice chunks in ballistic trajectories. The great energy of the ice impacts produced seismic vibrations that liquefied unconsolidated soil and made it possible for subsequent impacts to create inclined conical cavities that were modified by geologic processes into the shallow elliptical depressions known as the Carolina Bays. The width-to-length ratios of the Carolina Bays correspond to cones inclined at approximately 35 degrees, which is consistent with the ballistic trajectories.
On level terrain, the inclined conical cavities were modified by topographic relaxation into shallow depressions with the prototypical elliptical shape shown in Figure 1. However, on sloping terrain, the conical impact cavities were deformed when the liquefied terrain flowed downhill like a mud slide.
The elliptical bays occur mostly in the relatively flat areas of the Atlantic coastal plain, but the bays in inclined territory tend to be deformed. Figure 2 shows a location near the border of South Carolina and Georgia along the Savannah river that has deformed bays (Lat. 32.904774, Lon. -81.380051). The inclination of the terrain is shown by the elevation color gradient. The green center of the lower bays is at an elevation of 43 meters above sea level. The yellow represents an elevation of 48 meters, and the orchid pink represents 55 meters above sea level.
Figure 3 shows bays that have been distorted from an elliptical shape. The terrain where the distorted bays are found is at the boundary between the flat coastal plain and more elevated terrain with uneven ground that has preserved fewer Carolina Bays and has more dendritic stream erosion. The top part of the image shows the pink colors corresponding to a higher elevation.
The distortion of the bays can be explained by soil movement from higher ground to lower ground, as illustrated in Figure 4. The image shows the distorted bay superimposed by an ellipse that is presumed to correspond to the shape of the original bay, and the red arrow shows the direction of possible flow of material. The same mechanism would apply to all the adjacent bays.
The following images show an experiment to test whether viscous relaxation of the conical impact cavities in an inclined terrain can produce distorted bays.
Figure 5a shows the conical impact cavity viewed from the angle of entry.
The impact surface was prepared by mixing equal parts of clay and sand with enough water to have the consistency of bricklayer's mortar. The surface was covered with a thin layer of colored sand to enhance contrast. Figure 5 shows an inclined conical impact cavity made by an ice projectile fired with a slingshot. Viewed from the top, the conical impact cavity has an elliptical shape with raised rims.
Topographic relaxation decreases the depth of the impact cavity from the bottom up. This process is accelerated by shaking the container. The elliptical shape of the cavity is deformed by the downhill flow while the container is inclined as shown on Figure 6.
The target material flows from higher elevation to lower elevation while the depth of the impact cavity is reduced by gravitational forces. The resulting shape in Figure 7 is a distorted ellipse with a portion modified by the flow of surface material. The following image shows the extent of the deformation from an elliptical shape.
The experiment demonstrates that the proposed mechanism of bay deformation by soil movement from higher ground to lower ground is plausible. Viscous relaxation driven by gravity decreases the depth of impact cavities made on viscous ground, and the cavities can undergo further modifications by lateral movement of the terrain on inclined surfaces. Figure 8 shows a distortion analogous to Figure 4. Ground movement can shift the position of a bay, and the final shape of the bay depends on the direction of the mud flow and the relative motion of the deeper and shallower layers of the terrain. In general, it is necessary to take into consideration the characteristics of the terrain in order to understand the shapes of the Carolina Bays.
The distortion of the bay is caused by soil flowing downhill after the formation of the conical cavity, but the soil flows faster on the uphill side due to the steeper gradient into the cavity. This causes the elliptical shape to be flattened on the end adjacent to the higher elevation.