In this presentation, I consider the secondary impacts of glacier ice that made the Carolina Bays. I am trying to learn as much as possible about the physical mechanisms that made the mathematically elliptical basins.
In Newtonian mechanics, momentum is the product of the mass and velocity of an object. It is a vector quantity, possessing a magnitude and a direction. The total momentum remains constant in a closed system that does not exchange any matter with its surroundings and is not acted upon by external forces. Momentum is generally represented by the letter p in bold face to indicate that it is a vector. Sometimes a vector is indicated by placing an arrow over the variable. The inset shows a device called "Newton's cradle" that demonstrates conservation of momentum and energy.
The study of momentum is important because the Glacier Ice Impact Hypothesis proposes that the bays formed as inclined conical cavities from oblique impacts of glacier ice ejected in ballistic trajectories by a comet impact on the Laurentide Ice Sheet that covered North America during the ice age. This is a LiDAR image of Carolina Bays located about 4.5 kilometers or 2.8 miles southeast from Red Springs, North Carolina. Well-preserved Carolina Bays have a mathematically elliptical geometry.
This experimental model illustrates how oblique impacts of ice projectiles can create inclined conical cavities, but perhaps this mechanism does not scale to kilometer-size Carolina Bays. Glacier ice has a compressive strength of 3 to 15 megapascals, and it may fracture into small pieces upon impacting hard ground, including ground that was liquefied by seismic vibrations of previous impacts.
The elliptical geometry of the Carolina Bays can be verified by fitting them with ellipses by the least squares method. Points are selected along the perimeter of the basin while avoiding breaks in the rim caused by erosion, roads, and farming activity. The ellipse-fitting program calculates the length and the width of the basin and provides the azimuth, which is 132 degrees in this case.
All the Carolina Bays were emplaced contemporaneously during the ballistic sedimentation of ice boulders in the ejecta curtain produced by the extraterrestrial impact on the Laurentide Ice Sheet. The basin that was fitted with an ellipse was preceded by other impacts that are highlighted in this image.
This aerial image of the same area shows only a patchwork of farm fields, and it is very difficult to see any Carolina Bays. The images provided by Google Earth are typically from satellites, but sometimes they are obtained from airplanes.
This is the power of LiDAR, which employs laser pulses to measure distances with high accuracy. It is impossible to study the Carolina Bays properly without LiDAR. The colorized topography by Michael Davias helps to distinguish the raised rims of the basins relative to the centers and brings out features that are lost in grayscale LiDAR.
The azimuth of this Carolina Bay leads to the convergence point of 43.5 degrees north and 89.5 degrees west determined by Davias and Gilbride in 2009. The trajectory of the projectile has to be adjusted for the Coriolis Effect, which is the distance that the Earth rotates while the projectile is in flight. The launch point is about 120 kilometers to the east of the convergence point to compensate for the flight time of 449 seconds. The launch point of the ice projectile was in Lake Michigan, at a distance of 1,185 kilometers from the basin. The launch angle is calculated from the width-to-length ratio. Notice that the mass of the projectile is not included in the ballistic equation, so it does not influence the calculation for launch speed. All the basins at this location in North Carolina, regardless of their size, would have been made by projectiles of different sizes, but with initial speeds determined by the launch angle.
The sizes of the ice projectiles can be calculated using a program by Professor Jay Melosh and Ross Beyer that uses yield equations to correlate crater size to projectile diameter. A Carolina Bay with a diameter of 100 meters would have been made by an ice projectile with a diameter of 20 meters impacting at a 35-degree angle and traveling at 3.4 kilometers per second. In general, the ice projectiles that made the Carolina Bays correspond to projectile diameters that are one-fifth of the basin length.
A 100-meter basin looks just like a tiny dimple compared to the larger Carolina Bays. Most small basins on the East Coast have disappeared due to erosion. The large basin on the right has a length of 1750 meters, which is just over one mile. It would have been made by the impact of an ice projectile measuring about 350 meters in diameter, which is about the size of 3 and half football fields. That is a big chunk of ice!
The Carolina Bays have elliptical geometry, and since ellipses are conic sections, it is reasonable to propose that the bays originated as conical cavities that can be represented as elliptic cones. The volume of an elliptic cone is one third Pi times the semimajor axis times the semiminor axis times the height. However, the height of the elliptic cone corresponds to the depth of the cavity, which is unknown. The modern landscape shows that the centers of the Carolina Bays are flat. The leveling action is due to viscous relaxation at the time of emplacement of the bays when the seismic vibrations of adjacent impacts liquefied the soil.
Since there is no clue about the original depth of the conical cavities, the depth of the cavity is estimated to be twice the diameter of the projectile based on tabletop experiments as illustrated in the inset.
Dr. Michael Owen has been conducting hydrocode simulations under various initial conditions. His video from July 20, 2025 shows an ice projectile of approximately 20 meters in diameter with a speed of 1.8 kilometers per second impacting saturated unconsolidated soil with a density of 1700 kilograms per cubic meter at an angle of 35 degrees. The ice projectile gets crushed upon contact, but the cavity continues to expand from the momentum of the impact.
The first few frames of the simulation show what happens to the ice projectile. Frame 1 shows the initial contact when the projectile makes an indentation in the target. Frame 2 shows the projectile changing shape as the ice fragments. Frame 3 shows the fragments of the top portion of the projectile being diverted horizontally along the surface. The next three frames show the top portion of the projectile moving horizontally along the target surface as the cavity continues to expand.
The ice projectile does not fracture instantaneously. The calculation of maximum dynamic pressure indicates that the impact of an ice projectile on a target with density of 1,700 kilograms per cubic meter would far exceed the compressive strength of ice, and the ice projectile would start fracturing upon hitting the dense target surface. However, the speed of the ice projectile determines how much of the projectile can penetrate the liquefied ground before the ice projectile is completely fragmented.
Arakawa and two co-authors used a gas gun to fire cylindrical projectiles with a velocity of 3.6 kilometers per second at a cubic ice target. They observed that the speed of fracture propagation in water ice ranges from 3 km/s to 2.5 km/s. This propagation occurred in the shear damage zone after the precursor wave passed and the pressure fell below 240 MPa.
A 20-meter ice projectile like the one used in the hydrocode simulation would completely fracture in 6.7 milliseconds at a fracture propagation speed of 3000 meters per second.
This experimental impact on concrete illustrates what happens when an ice projectile does not penetrate the target. The projectile breaks up into many pieces, and the kinetic energy of the projectile is transferred to the ice pieces which scatter horizontally from the impact point.
I wanted to know how momentum is transferred by a fragmented spherical projectile. I calculated the volume of one-meter slices of a sphere with a diameter of 20 meters. The weight of each slice can be obtained by multiplying the volume by the density of ice, which is 917 kilograms per cubic meter. The graph shows the percentage of the total volume for each slice. As expected, the greatest volume and the greatest weight are along the center of the sphere.
This image illustrates the mass distribution of a spherical projectile consisting of many aggregated fragments. The calculation of conservation of momentum must take into consideration the effect of this mass distribution on the target material.
The book on Impact Cratering by Prof. Melosh illustrates the atmospheric entry and breakup of a large meteoroid. The image shows a crushed but closely grouped collection of fragments that produce a single crater. This is what we would expect of an ice projectile that breaks up when it hits the target.
Here, we calculate the distance traveled by a projectile with a velocity of 1,800 meters per second during the time that a 20-meter projectile is fractured. The fragmentation time of 6.7 milliseconds is obtained by dividing the projectile diameter by the speed of fracture propagation. A projectile traveling at 1800 meters per second travels about 12 meters in that time. Approximately half of the projectile will be imbedded in the target during this time.
After fragmentation, the upper portion of the projectile is no longer physically attached to the bottom of the projectile, and it can be diverted in a horizontal direction, as illustrated in the hydrocode simulation.
A projectile with a velocity of 3,400 meters per second, derived from the ballistic equations, would travel about 22 meters and embed itself fully in the liquefied target before the ice projectile fragmented completely. The crushed but closely grouped collection of fragments would transfer their momentum like a single projectile, as described by Prof. Melosh.
The idea that a comet crashed into Lake Michigan when it was frozen and pieces of ice were ejected to form the Carolina Bays sounds quite implausible, and many scientists object to the idea, but new research in 2024 found mysterious craters at the bottom of Lake Michigan. Were these craters made by fragments of the comet? Further investigation may provide some answers. Stay tuned.
