|Of Sand Dunes and Carolina Bays||Cintos||2/22/11 1:37 PM|
The LiDAR imagery of the East Coast offers many instances where Carolina by landforms interact and intermingle with surficial sand dunes and dune fields. One question raised is: which came first? In many instances, the sand dunes look to be overriding the bays, suggesting the bays were in place prior to the advent of the dune development. In other cases, the bays seem to exist as cuts-outs in the dune field, as if they had been created after the dune field became inactive.
Dune fields arising from the broad channels of the major rivers are common. In our first example, the large bay is being in-filled by parabolic dunes from the west. The small bay it the lower left along the shoreline is seemingly unaffected.
In the Google-Earth formatted LiDAR image below, the Santee River, and the man-made Lake Marion which occupies its channel at present, pass west of a dune field. Contained in the field are several well-formed Carolina bays.
Our interpretation, borne out by observations in other areas which have transient dunes of lighter, loess dust from local drainage channels: If the bay has been broached by a drainage channel (or draped over an antecedent one), and clearly can hold no standing water, the loess infills. When the rim is contiguous, the entrained water will adsorb the light sandy influx and maintain it's ovoid perimeter. The divergent cases are demonstrated in the first image above.
The attached KMZ file will take you to the Lake Marion area and present the LiDAR and bay overlays for your inspection. By moving and adjusting the overlay around (Get Info on Mac or Properties on WinDoze), you can see how well the overlay maps to the bays.
To the south, along the eastern shore of the Savanna River in Jasper County, SC, a similar dune field developed, but with no Carolina bays in sight.
So which came first? Or did they develop together? What is clear is that they are independent natural processes with little in common. I am wary when experts suggest Carolina bays are just parabolic sand dunes.
Men occasionally stumble over the truth ... but most of them pick themselves up and hurry off as if nothing had happened.
...... Winston Churchill
|Re: Of Sand Dunes and Carolina Bays||Cintos||3/4/11 1:15 PM|
Transient sea levels since the time when Carolina bays were created have occasionally overwhelmed bays near the coastline. Sand dunes and sand ridges are very mobile along the Atlantic shoreline. That was the case with Blythe Bay in Wilmington, NC (See Previous POST), where a delta of sand was formed within the bay after the seaward edge of the rim was breached by higher sea levels at some point in the past.
Here we share our findings of bays across Roanoke Island, just inside the Outer Banks of North Carolina.
Are these "Carolina Bays"? Well, any classically educated geologist would say no, they are simply sand dunes. You be the judge. The land here is only a few meters above today's sea level, so we would expect (and see) that parts of some of the rims have been washed away. As seen at Blythe bay, the northwest end of one bay (9682) has been filled in partially with dunes, but the original rim can still be resolved in the LiDAR. No doubt a trained eye can see even more than the even dozen bays included in the attached KMZ file.
The imagery is 1/9 arc second LiDAR, processed in Global Mapper using USGS/North Carolina data.
The orientations (~127º from N) shown here were directly generated by our Bearing Calculator, offering an additional level of validation for our interpretation of these as true Carolina bays. These bays have eccentricities ~ 0.7, and are highlighted with the archetype bay_planform.png overlay.