Carolina Bay Planforms
The recognition of the existence of "Carolina bays" was driven by their distinct and persistent geometric planforms and their sheer numbers, in combination with sharing a common alignment in any one area. The visual photography available since the identification in the 1930's only told a part of the story.
With the availability of today's LiDAR (Laser Imaging and Range Detection) systems, the true extent and planforms of these enigmatic landforms are brought to life in a stunning manner. Interpreting these ovoid landforms as being derivatives of well-understood wind and water-shaped dunes cannot be supported by evidence of similar processes elsewhere in the world.
Carolina bays surrounding Rex, NC area - click for higher resolution image
The Color-Ramp shading of the above LiDAR-derived image (USGS Datum) demonstrates on of the most commonly overlooked characteristics of the bays: the blanket is continuous across the pre-existing terrain, and the bays show no differential based on emplacement elevation. The above view covers ~ 600 square kilometers, and significant elevation changes.
The primary planform seen as a bay in this discussion is the oval. Their characteristics have been reviewed extensively by other workers. Most prominent, we feel, are the sharply defined closed ridge which surrounds the structure, the persistent repetition of a common shape within a locale, the common alignment of those shapes, a predisposition to possess a higher, thicker ridge on one of the major axis ends of the shape, and finally the vast diversity of sizes they are generated in. Dune fields often generate structures with some of these characteristics, but never all of them simultaneously.
Numerous studies have examined the bays' planforms. Most extensive was Douglas Johnson's work, as presented in his book "The Origin of the Carolina Bays", Volume IV of Columbia University's , Columba Geomorphic Studies series. His examination of bays, both in person and in aerial photographs, noted three planforms: The elliptical/oval, egg shaped elliptical/ovoid, and random. In the ovoid form, we see a simple development of the down-range tip of the bay, where it narrows more than in other bays, although we do see the "narrowing" concept at work in the vast majority of bays.
Johnson offered a drawing in his book that includes references to both Oval and Ovoid, along with measured bearings for a variety of ares.
Oval and Ovid bays. Linked to higher resolution image
Here is a pen drawing included by Johnson, which details his understanding of how bays are included in the coarse sand stratum as depressions, without causing observable distortion to the stratum, which would be expected from a direct impact and excavation scenario. See extracts from the book discussion this aspect in the Origins_Book Section.
FIGURE 18: Typical cross profile of sandy rim from inner side next bay to outer side bordering plain. Vertical scale greatly exaggerated.
Altitude of broad flat-topped rim usually varying from 2 or 3 up to 5 or 6 feet; breadth from 150 to 800 feet or more.
Two other drawings are available, both of which demonstrate the same inclusion principal. First, in R.B. Daniel's drawing of bays formed within the Goldsboro Ridge, followed by the cross-section provided from the Midlothian bays. We see these three depictions as supporting our ejecta blanket hypothesis, where the bay depressions are imperfections in the surface of the blanket created during emplacement. The entire bulk of the sand stratum in which the bays are contained is considered to be ejecta.
from Guidebook To The Geology Of The Upland Gravels Near Midlothian, Virginia By Bruce K. Goodwin and Gerald H. Johnson
Evaluation of the profile of the bays has identified them as having ages of 7,000 to 50,00 years. The graphic below illustrates the findings from the carbon dating of core samples from the Flamingo bay, by Brooks, Taylor & Grant. A linear extrapolation would put the bottom sediments dated at 12,000 yr B.P.
The paper concluded that:
- Significant modification of the bays, including rim development and basin infilling, occurred during the Holocene
- ponds on the early Holocene landscape were larger and more permanent than at present
- early Holocene climate, as indicated by both depositional processes and human activity, was not characterized by prolonged periods of extremely dry conditions; and
- fluvial-centric models of terminal Pleistocene-early Holocene human adaptations require revision to include intensive use of isolated upland ponds.
An evaluation of Carolina bay geology is offered in an extended excerpt from the South Carolina Maps and Aerial Photographic System's web site at Clemson University. We encourage the reader to review this document for interesting historical and geologic content. The following excerpt we find particularly informative:
Carolina Bays have characteristic soil assemblages which are the result of the very moist conditions commonly found in these environments and which can be distinguished easily from one another and from surrounding soil types on aerial photographs. Wet soil generally appears darker due to the greater accumulations of black organic matter. However, when winter cover crops have been planted, wetter soils usually support more vigorous plant growth and appear a deeper red or pink on infrared photographs than drier soils. Soil mapping surveys, such as those run by the United States Department of Agriculture, commonly draw boundary lines, delineating different soil types, directly on aerial photographs while working in the field. These surveys look at factors such as landscape position, shades of bare soil, types of vegetation growing on the soil, and water drainage patterns commonly found on that soil. Although not all soils can be so easily determined, the unique soils of the Carolina Bays can usually be separated and identified on a variety of remotely sensed images. Three distinct soil types are found in most of the larger Carolina Bays:
- PONZER - This is often
the dominant soil in large Bays, and it is also found in
the center of smaller ones. Due to the lack of oxygen
caused by water saturation, which slows decomposition,
this soil is almost all organic matter. The soil microbes
which would normally cause complete decomposition need
oxygen to break down the leaves and other plant litter
that fall to the soil surface. Over the years, an organic
rich "A" horizon layer accumulated that is several feet
thick. This soil appears dark in an aerial photo. While
some pine trees can grow in it, they cannot compete well
with better adapted deciduous vegetation. When drained,
this is a highly productive agricultural soil.
- RUTLEDGE - This soil is
found along the inside of the boundary of large Bays and
occupies most or all of the area of smaller Bays. It is
slightly higher in elevation than Ponzer soils and is
therefore slightly drier. It also has a high organic
matter content but is much more sandy. It also more
easily supports loblolly pine trees. Rutledge soils also
make productive agricultural land when drained. The land
appears dark in an aerial photo, but not as dark as
Ponzer. In Infrared aerial photos (taken in winter),
Rutledge soils will appear much redder than Ponzer due to
the abundance of evergreen trees such as pines.
- RIMINI - This soil is sometimes found on the sandy rims of Carolina Bays. It is a rather unusual soil in that its subsurface "B" horizon layer is full of organic acids combined with aluminum atoms that leached from overlying horizons. While the surface color can be bright enough to appear almost white, the color of the "B" horizon layer is often brown or black, like topsoil, but it is found about four feet below the soil surface. [Soils of this type are usually found in northern regions, like New England, Northern Michigan, Minnesota, and Canada.] For several reasons, including acidity and possible aluminum toxicity, this is not a good soil for plant growth and is only sparsely covered by scrubby pines, blackjack oak, and turkey oak. This is not an extensive soil, and is almost never used for agriculture. It appears very light in an aerial photo due to the high sand content and dryness of the soil surface.
We posit that relatively shallow basins were created as surface features during the energetic deflation of steam inclusions in the ejecta blanket; effectively “popped bubbles”. The assortment of bay sizes and geospatial distributions within bay clusters is a direct result of a physical fractal processes, as the frothy slurry spread downrange. An assumed high-temperature and high-pressure emplacement created stratum that, while unconsolidated, has maintained its structural integrity.
Frothy Foam Fractal Distribution Experiment
Our hypothesis that the ovoid shape represents a blemish in a distal ejecta sheet leads to a corollary principal that the inferred alignment of arrival is displayed in the planform as a momentum artifact. We propose that the alignment is along the major axis, with the higher ridge being at the down-range end. To measure and capture this inferred alignment, we employ a "Bearing Arrow" with a graticule as an overlay in the Google Earth GIS visualization tool. The overlay is manually rotated so that it aligns with the user's interpretation of the bay's orientation. Since the bays are rarely perfect ellipsoids, the interpretation is better qualified by comparison with numerous companion bays as a "best fit".
Ancestral Drainage Channels
Douglas Johnson, in his 1942 book "The Origins Of The Carolina Bays", made numerous observations about common bay planforms. Almost exclusively, Carolina bay formations "rest" within an anomalous sand stratum. It is neither stratified nor laminated, but rather shows a hummocky, turbulated appearance. While there is a "rim", it stands only a very short distance above the surrounding pediment, and that pediment consists of the exact same sand. It is known that the basins do not distort either the surrounding sand nor the underling strata, but rather simply exist within that strata of course sand. Johnson made numerous observations which we feel supports our ejecta layer hypotheses. I have taken the liberty of including some of his text on our web site at http://cintos.org/SaginawManifold/Distal_Ejecta/Planforms/Origin_Book
With regard to Dr. Johnson's observations about "OUTLET CHANNELS FREQUENTLY TRAVERSE RIM BARRIERS", we have identified a few locales in which we see the bay formations as clearly overlying ancestral drainage channels, and those channels mask up through the bay stratum.
Ancestral Drainage Channel exiting at highest rim elevation
Ancestral Drainage Channel exiting at highest rim elevation
Here is a basin in Nebraska which is disected by an ancestral drainage channel. The presentation uses a Google Earth embeded gadget.
Round bays in North and South
While the bays of North and South Carolina are seen as elongated ellipsoids, many of the bays of Maryland and New Jersey to the north, and Georgia to the south, do not present the elongation to assist in deducing an inferred orientation. In fact, it has long been reported by others that bays in the far south and those in the north trend towards a round shape. What is left, significantly, is a predisposition for having a segment of the enclosing ring be fatter & higher than the opposing side. If we continue to deduce the alignment to be from the shallow side to the fat "lip", then we may be able to continue with the process.
Here are three examples of what look to be fields of "Squashed" bays, where the momentum during emplacement was more downward than laterally (higher loft angle?).
Bays Reworked by Wind
Here are three LiDAR graphics depicting bays being over-ridden by parabolic dunes. Elevation and proximity to large waterways seem to drive these particular dune fields. In areas with low rainfall, we propose that the ejecta sand in the bays can be reworked into common dune formations. As seen below, often the original bay forms are visible as a palimpsest after the rework.
Overprinting Dunes in New Jersey
Overprinting Dunes in Southern Maryland
Overprinting Dunes in North Carolina
The bays in Nebraska represent yet another planform. Their orientation is, of course, reversed so as to present an orientation towards the northeast. While there is a great deal of research covering the "Carolina bays" on the eastern seaboard of the US, little attention has been paid to the significant quantity of oval-shaped landforms in the eastern areas of Nebraska. These, too, are aligned with each other. Significantly, the inferred alignment is considerably different than that of the orientation of the extensive sand dunes in the area. As in the east, the blanket is continuous across the pre-existing terrain, and the bays show no differential based on emplacement elevation.
Basins in Nebraska
While not nearly as extensive as those in the east, the identification of these bays to be critical to the impact site triangulation and correlation with their eastern-US brethren. Here are two Global Mapper graphics which detail the elevaion profile across the two axis of one of these bays.
We interpret the placement some of the Nebraska bays as being indicative of them being "pedestals" landforms, which owe their existence to the bowl-shaped interior's capability of retaining moisture. Over the millennia since their emplacement, the majority of the ejecta blanket has been subjected to wind and water erosion, whereas these areas have been stabilized. The analogy is that of tire tracks in snow, outlasting the surrounding snow, rising above the road surface. A similar process is implicated in the pedestal craters on Mars. Numerous landforms in nearby areas suggest they were at one time 'bays", but have been compromised by encroaching erosion. Such a fate is likely in the future for the Garfield Township bay shown above, which is beginning to be invaded by a stream. Additional visual examples of the Nebraska bays are provided in the Nebraska bays section of this site.