This presentation is about Carolina Bay deformations and irregularities.
The Carolina Bays are shallow elliptical depressions along the East Coast of the
United States whose major axis points toward the Great Lakes. The bays have
raised rims and they are found on sandy soil close to the water table.
The size of the bays varies from about 100 meters to several kilometers in size.
The Carolina Bays completely cover the landscape
except for places where water has washed away evidence of their existence.
The geometrical shape of the Carolina Bays resulted from the mechanism by which
they were formed and by the geologic and topological environment where they were
emplaced. The archetype shape of the Carolina Bays is elliptical but the
geometry can be deformed by wind and water erosion, by ground movement, and by
bioturbation, which includes soil disturbance by plant roots, burrowing
animals, and human activity such as farming, damming, logging, urban
development, and drainage of wetlands.
The Carolina Bays are not thermokarst.
Some geologists have proposed that the Carolina Bays have a similar
origin as thermokarst lakes. Karst depressions are formed by dissolution
of minerals or by melting of permafrost that produce sinkholes.
However, thermokarst lakes do not have the regular elliptical structure of the
Carolina Bays, and their alignment is toward lower terrain for drainage, rather
than to a common focal point.
The regular shape and orientation of the
Carolina Bays was what first attracted the attention of geologists.
The Carolina Bays are enormous. Several block-long buildings can easily
fit inside some of the bays. LiDAR, a laser imaging technology, enhances
visualization of slight elevations in the terrain and provides excellent images of
the bays. The Carolina Bays have been described as either oval or elliptical.
Melton and Schreiber in 1933 and Prouty in 1952 characterized the bays as
elliptical but they did not perform a geometrical analysis to verify that the
bays were mathematical ellipses.
Before we can talk about deformations or irregularities of the Carolina Bays,
it should be mentioned that well-preserved Carolina Bays are perfect ellipses with
an average width the length ratio of 0.58 ± 0.05.
This corresponds to cones inclined at approximately 35 degrees. An ellipse with
the same width to length ratio as a Carolina Bay fits the bay exactly after
scaling and rotating the ellipse. This makes it possible to model the bays as
mathematical conic sections and to use ballistic equations and power laws
relating energy to crater size to derive their conditions of formation.
The Carolina Bays have been preserved because the porous ground allows rain
to pass quickly to underground aquifers where water flows without disturbing the
surface features.
Nebraska also has elliptical structures called
rainwater basins with the same geometrical aspect ratio as the Carolina Bays.
The Nebraska basins also point toward the Great Lakes but their orientation is almost
perpendicular to the Carolina Bays.
The Nebraska Rainwater Basins are
terrain that is 600 to 700 meters above sea level.
Many basins have been greatly degraded by wind and water erosion but ellipses
can be fitted for the basins that have maintained well-defined borders.
The projections of the major axes of the Nebraska Rainwater Basins and the
Carolina Bays intersect at Saginaw Bay in Michigan.
The Nebraska Rainwater Basins have the same elliptical morphology as the Carolina
Bays. The Nebraska basins occur on abandoned Platte River fluvial deposits.
Unlike the Carolina Bays that can drain by subterranean aquifers toward the
Atlantic Ocean, Nebraska's rugged terrain allows water
to flow on the surface and this has contributed to the destruction of the
elliptical features. Most Nebraska Rainwater Basins smaller than
four kilometers have been completely obliterated. It is impossible to measure
accurately the width and length of the basins when large portions of the
elliptical rims have been washed away or covered by wind-blown dust.
The Glacier Ice Impact Hypothesis,
published in 2017, proposed that the Carolina Bays and the Nebraska
Rainwater Basins were made by secondary impacts of glacier ice ejected by a
meteorite impact on the Laurentide Ice Sheet
approximately twelve thousand nine hundred years ago. The impacts of the
ejected ice chunks with energies of thirteen kilotons to three megatons
liquefied unconsolidated ground, and the ice projectiles created conical cavities
inclined at about 35 degrees that were remodeled into shallow elliptical bays
by viscous relaxation. The extraterrestrial impact and the
saturation bombardment by the glacier ice boulders caused an extinction event and
a global winter. The date of the extraterrestrial impact is deduced from
the disappearance of the North American megafauna and the onset of the Younger
Dryas cooling event.
The hypothesis that ice impacts could create conical
cavities was tested with a physical model to evaluate impact conditions and
viscous relaxation. A target was prepared with a mixture of pottery clay, sand and
water, covered with a light layer of colored sand. A slingshot was used to
fire ice projectiles toward the target. The experiments revealed that the
overturned flanges around the conical impact cavities become the raised rims
of the bays and that adjacent conical cavities become overlapping bays.
The overlaps follow the geological principle of superposition in which newer
features cover older features. These images illustrate the way in which
viscous relaxation decreases the depth of the conical craters. The experimental
container is shaken to promote liquefaction and speed up viscous
relaxation. During the formation of the Carolina Bays, the seismic vibrations of
nearby impacts would have speeded up viscous relaxation.
In this image, Carolina Bays completely cover
the landscape near Branchville, South
Carolina. These bays have poorly defined rims, and water channels flow through
many bays. The degradation of the bays makes it difficult to see complete
ellipses and to count the number of bays. Human activity is responsible for much
of the erosion.
This is a satellite image of the same
area as the previous slide. We can see rough outlines of some of the biggest
bays, but the small bays are completely obscured by a patchwork of farmed fields.
Drainage of wetlands and yearly plowing of the soil since the land was settled
by European farmers has gradually destroyed these Carolina bays.
The bays near maps of Virginia, in the Delmarva Peninsula, are circular rather than
elliptical. Thirty-five million years ago, a meteorite struck the Chesapeake Bay and
created a tsunami that washed away the loose surface sand of the Delmarva
Peninsula. The shallow depth of unconsolidated material can explain why
the bays in Delmarva are circular. An elliptical Carolina Bay with a major
axis of 400 meters requires a layer of unconsolidated soil 140 meters deep
to allow the formation of a conical cavity inclined at 35 degrees. Conical
cavities cannot be made on solid ground. Ice projectiles impacting a hard layer
disintegrate explosively and create hemispherical shock waves and circular
craters like those illustrated here.
Many Carolina Bays have a thicker rim on the
southeast side. The raised rims of the Carolina Bay correspond to overturned
flaps around the crater. The thicker rims toward the distal end of the bays were
formed by a ploughing action in the direction in which the projectile was
traveling.
From the time that the bays were discovered, many geologists
suggested that the Carolina Bays were formed by wind and water processes
because the bays did not have shock metamorphism, meteorite fragments or
other evidence of extraterrestrial impacts. Tests of the bays by Brooks and
his colleagues using optically stimulated luminescence produced a wide
range of dates. The authors concluded that the Carolina Bays were created over
thousands of years. The dates for Big Bay in South Carolina showed that the
concentric sand rims representing previous shorelines became progressively
younger toward the bay interior and that a windblown sand sheet originating from
the Wateree River on the west was moving over the bay 74,000 years ago and it was
later modified 33,000 to 29,000 years ago. This research was done
before LiDAR images became widely available and before geometric analysis
revealed that most Carolina Bays are perfect ellipses. The eolian/lacustrine
hypothesis for the creation of Carolina Bays claims that active shorelines and
eolian deposition shaped the bays according to alternating
warm and cold periods in the Earth's paleoclimate. However, this explanation
cannot possibly apply to the Nebraska Rainwater Basins that have the same
geometry as the Carolina Bays but are 600 to 700 meters above sea level on
terrain that has not been close to any sea for 60 million years.
The eolian and lacustrine origin of the Carolina Bays was
bolstered by Kaczorowski’s 1977 thesis describing wind and water experiments
that modified circular pools on sandy soil. Kaczorowski’s experiments did not
produce the elliptical geometry characteristic of the Carolina Bays, but
his thesis has been widely referenced in publications proposing the
terrestrial origin of the bays. In contrast to the wind-and-water experiments, oblique
impacts on viscous ground naturally produced inclined conical cavities that
become shallow elliptical conic sections through viscous relaxation. The
geometrical relationship between cones and ellipses was well known to the
Ancient Greeks. Just in the basis of geometry, the elliptical Carolina Bays
are more likely to have originated from inclined conical craters than from
variable wind and water currents. The eolian and lacustrine hypotheses have
never explained the detailed physical mechanism by which wind and water
currents could produce perfect ellipses either experimentally or in Nebraska and
in the East Coast.
Optically Stimulated Luminescence, or OSL, is a method for
dating sedimentary layers. Radioactive isotopes of uranium, thorium, rubidium and
potassium that occur naturally in the soil produce ionizing radiation that
causes electrons to become trapped in defects of buried quartz crystals.
Exposure to sunlight or intense heat release the trapped electrons.
Luminescence dating estimates the time since grains of quartz were last exposed to
daylight by detecting their subsequent response to light of specific
wavelengths. The problem with using OSL on the Carolina Bays is that the bays
were not created by sedimentary processes. The bays were produced by
impacts that form cavities on liquefied ground. The stratigraphy of the bays was
subsequently restored by viscous relaxation
without exposing the subsurface to light. Thus, the OSL dates of the
subsurface of the bays corresponds to the age of the terrain and not the date
of bay formation.
The subsurface of the Carolina Bays appears undisturbed as contrasted with
craters created by impacts on hard surfaces. This can be explained by the
behavior of viscous targets after plastic deformation. A projectile at
ballistic speed penetrates the viscous surface by parting the layers of the
target during the excavation phase. Experiments have demonstrated that
viscous relaxation partially reconstitutes the stratigraphy because
hydrostatic pressure increases with depth and promotes faster laminar flow
at the bottom of the cavity. The cavity is filled from the bottom up, but due to
friction the interior of the bay never rises to the original level of the
surface.
This LiDAR image shows Big Bay with more geographical context than
illustrated by Brooks. From an impact perspective, what Brooks described as an
eolian sand sheet could be debris deposited by a tsunami from an impact on
the Wateree River. Notice that the small Bay at the top of
the image, circled in red, is over the sand sheet. This means that the small bay
was emplaced after the sand sheet. Big Bay and the companion bay below it are
covered with the sand sheet. This means that the sand sheet came after Big Bay was formed.
But if the sand sheet is eolian,
why wasn't the small bay covered with sand also?
These features of the terrain can be explained by the
following sequence: two big ice boulders created Big Bay and its lower companion bay.
Seconds later another big boulder impacted the Wateree River. The splash
from the impact created a tsunami that dredged the sandy deposits that had
accumulated along the riverbank for 74,000 years. The sandy
soil was carried inland by the tsunami for six kilometers and a sand sheet was
deposited on a portion of Big Bay and its companion bay.
A few seconds later a small ice boulder came down to create a small bay on top
of the sand sheet.
If the sand sheet had an eolian origin, one would expect that
there should be more sand sheets all along the river, but this 11-kilometer
section of the Wateree River has the only sand sheet in the region.
Another clue indicative of a tsunami is that this is the only stretch of the
river where the banks do not have sandy deposits or Carolina Bays within three
to five kilometers of the river.
Some bays are deformed when the terrain moves.
This image shows a large bay that has been stretched by uneven ground
movement toward the creek. The bay has a wavy water channel, which indicates that
the portion of the bay closer to the creek is more saturated with water than
the portion away from the creek. This causes the bay to deform in the
direction of water flow. Large bays are more likely to deform than small bays
because a small bay can be carried intact by a section of moving ground,
whereas the large bay may straddle several sections of ground that move at
different speeds and in different directions.
Catfish Bay in South Carolina is another example of a bay that has been stretched
by ground movement. The bay is in a trough that drains to the northwest into the
Great Pee Dee River and to the southeast into the Little Pee Dee River. Consequently, the
original elliptical shape of Catfish Bay has been stretched in both directions.
Notice that the smaller bays on the rims of the trough have retained their
elliptical shape.
These non elliptical Carolina Bays are found close to the banks of the Savannah
River. They are examples of bays that were distorted by the topography of the
terrain when they were formed. The color gradient indicates elevations. The orchid
color in the center corresponds to an elevation of 58 meters above sea level
and the green color toward the bottom corresponds to an elevation of 43 meters
above sea level. All of these bays have a flattened shape toward higher terrain.
The distorted bays have an elliptical end and a flattened end. The shape of the
bays can be explained by noting that the flattened ends of the ellipses are
adjacent to areas of higher elevation, and that mudslides could have changed
the shapes of the bays. The elliptical end can be fitted with an ellipse
because, mathematically, only five points are necessary to fit an ellipse to an
elliptical curve.
The experimental model can be used to confirm the mechanism of
bay deformation. An elliptical impact cavity is modified when the terrain is
inclined. 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. The resulting structure has an
elliptical end and a flattened end similar to the bays near the Savannah
River.
Some Carolina Bays appear to have multiple or concentric sand rims.
Proponents of eolian and lacustrine mechanisms claim that the multiple rims
are evidence of bay evolution over long periods of time, but it should be noted
that these terrestrial mechanisms cannot explain how the overlapping bays are
created or the reason why the bays have elliptical geometry with specific
width-to-length ratios.
Experiments that demonstrate the transformation of
conical craters into shallow bays sometimes produce bays with double rims.
This image shows an internal V-shaped double rim caused by uneven laminar
flow during viscous relaxation. Such internal rims are created when the
centripetal laminar mud flow that fills an impact cavity from the bottom up
encounters resistance as the liquefaction of the soil decreases.
The overlap of the bays corresponds to the sequence in which
were formed. The principle of superposition indicates that the small
way in the center was created first followed by the large bay and finally by
the intermediate size bay. This presentation demonstrated that the
Carolina Bays are typically elliptical because they originated from inclined
conical cavities. The deviations of the bays from the elliptical geometry can be
explained by the characteristics of the terrain on which the conical cavities
were emplaced and also by the action of terrestrial mechanisms, such as erosion.
The clues presented by the terrain may be interpreted in different ways and
sometimes it is difficult to determine the mechanism that formed a specific
feature. We have to keep in mind that the passage of the meteorite through the
atmosphere ignited fires that produced a lot of smoke.
Also, the vapor trails from the reentry of the ejected ice boulders created
black clouds that completely blocked the light of the Sun. The impacts of the
giant ice boulders that made the Carolina Bays probably occurred in
absolute darkness. The dates obtained for various Carolina Bay features by
Optically Stimulated Luminescence show ranges of thousands of years, but it is
wrong to apply a dating technology intended for sedimentary processes to a
landscape created overnight by violent impacts. Reliance on these OSL dates will
continue to cast doubt on the impact origin of the Carolina Bays, but the
geometrical conic sections will eventually prove more convincing because
they enhance our understanding of the mechanisms that formed the landscape.