Searching for Carolina Bays



Using the facilities and satellite imagery of Google Earth’s GIS, augmented with high resolution digital elevation mapping (DEM) data from the USGS, a survey was undertaken to catalogue the full extent of Carolina bays, indexed as localized “fields” of bays. This catalogue is available on line through the use of the Google Earth GIS and a Keyhole Markup Language (kml) file entitled Distal_Ejecta_Fields. This file contains placemarks with metadata annotations about the field, a "portrait" of the field, and a link to a set of kml specific to that field.


Example of Distal_Ejecta_Field Index in Google Earth

The field kml contains a default set of predicted arrival bearing lines and walk-back trajectories. A "Bearing Arrow" is provided, orientated according to the numerical bearing calculator's prediction for that specific latitude and longitude. Most also contain links to high resolution DEM data overlays, generated in Global Mapper GIS application as color ramp images.


Example of Field Elements in Google Earth

Finding Carolina bays in the Carolinas is easy, given their plentiful quantities and solid identification, but as the ejecta butterfly arc is walked northerly and southerly, the search becomes more challenging. Some of this is due to the more rounded shapes seen above Maryland and down in Georgia. An additional challenge is the increasingly rough terrain seen when moving inland from the costal plains. Our ejecta blanket sheet model suggests that the bays are present as defects (popped bubbles) in a thin layer of sandy ejecta. These can persist over the thousands of years of erosion only under special circumstances. If the landing area is relatively flat and moist, they will be easily stabilized as bays. If the area is level but very dry, the blanket will be reworked by the wind into a generic dune field, obliterating any bay formation.

When the landing field is in rough terrain, we propose it is quickly sloughed off in erosion. Any bay formations that are drapped over elevations that exceed the bay's rim hight can not hold moisture for stabilization. Thus in hilly areas further inland on the east coast, we have pursued the search for bays by identifying areas which exhibit level terrain. These can usually seen as plateaus in the digital elevation maps (DEMs) we are using. The plateaus could be indications of a plateau extent present when the ejecta landed, or be meerly the surviving remnants of a larger pleateau that has been invaded by erosional valleys. In the latter cse, we see that bays which once were stabilized were later compromised by encroaching erosional valleys.

Here is an example of the search methodology, as applied to interior Georgia. The USGS geographic information Viewer is first used to retrieve elevation data for an area. In the graphic below, we select the "Thompson" 100K block.



The retrieved data files are loaded into the Global Mapper GIS tool for display in a Color-Ramp image. The three circles highlight areas which look to be flat enough to suport bay formation and survival.


Color_Ramp Image of Thompson 100K block ( linked to a higher resolution version)

Next, we zoom into one of these areas to examine the surface details, in this case, the center circle above.


Zoom into Plateau Area

This higher resolution view reveals the presence of oval landform shapes in the smooth surface areas. Another level of zoom, below, brings out more of these details. The Global Mapper elevation profiling tool is used to identify the terrain and validate the bowl-shaped nature of a Carolina bay possibility.


Global Mapper Elevation Profile

Global Mapper can export the visible image as a set of coordinate indexed Google Earth kmz layers. These are imported into Google Earth, where they can are used to enhance the normal visual imagery. Once in place, each depression can be evaluated against the visual imagery of the location, to verify if a bay planform is present. If a correlation is found, the site can be tested with the Inferred Orientation Calculator, which will create a set of reference kml for Google Earth display and further correlation. You may note from the image above that several full-rimed ovals are present, and all exhibit roughly the same NNW to SSE orientation. Also apparent are indications of man-made drainage ditches cut into the center of several to drain them. Using Google Earth’s imagery, and in this case, historic black and white imagery (1999), the array of aligned bay landforms is readily apparent.


Aligned Bays in Wren Field GE Imagery (1999)

Using both visual and DEM imagery, a satisfactory correlation is made with the Calculator's predicted alignment.
Wrens, GA Field Detail with Bearing Arrow Overlay

The area is identified in Google Earth as being near the town of Wrens, GA. A folder of kml data is assembled for distribution as the "Wrens_GA" field. Similarly, bay suspects were located in each of the other "circled" areas in the Thompson 100K block, and a set of kml developed for each of the other two sites as well: Harts_GA and Hephzibah_GA.


Three Carolina bay Fields Identified in Thompson 100K USGS block

A similar process is used in identification of the bays in the state of Nebraska, as discussed in their chapter.