That isn't the last time they were "as high as they are now". They were higher 7000 to 5000 years ago, for example.
FIGURE 6.3 Late Quaternary fluctuations in sea level. Solid line is the generalized sea level curve (from Curray, 1965); dashed line is detailed curve (from Curray, 1960, 1961). Tree ring and Uranium/Thorium dates give greater age than the radiocarbon ages for these curves. Recent studies (Fairbanks, 1989; Bard, 1990) indicate the glacial maximum was 21,000 (230TH/234U) yearsBP with a sea level lowering of 121 ± 5 m.
And the original:
Your graph is probably a rolling 1000-year average or something like it. If you could view it year-by-year, it would be a lot more erratic.
One would assume there was/is some smoothing of chart lines along the way, but it doesn't even come close to explaining the elephant in the room and that is: Give or take a few feet, the last time sea levels were as high as they are now was ~120,000 years ago. ~70,000 years ago, sea levels were ~200 - 250 feet LOWER than they are today.
Now, if one assumes, based on THIS study, that South Carolina, and by extension surrounding areas experienced no appreciable uplift in the intervening years, where does that leave us as far as explaining the bay's formation by sea level fluctuations? To this layman, it doesn't seem possible.
I did some looking around to see if the La Palma "mega" tsunami could have generated enough havoc to be associated with the bays; doesn't appear so. Liquifaction caused by earthquake(s) that may have run the length of the Appalachians might be a possibility, but do we know if anything like that ever occurred???
One other point: You seem to have a habit of avoiding or possibly putting off some of my more interesting questions. I know we all get busy or otherwise distracted at times but it gets frustrating asking questions several times before hitting pay dirt. No offense.
FGS
I'm a soil scientist, not a geologist, and when the subject turns to tectonism, we're venturing out of my field of competence. That's why I wanted to direct you to Helaine Markewich.
I can tell you -- and in this case I am very competent to answer -- that the sediments in the portions of the Wando formation that I worked on in Darlington, Marlboro, and Dillon Counties, in South Carolina, were deposited in a tidal-marsh enviroment. Those sediments are silty and very uniform, and the geomorphic surface there has virtually no topographic relief. In that part of the world, there isn't any other means of deposition that could produce such sediments. A terrestrial riverine system wouldn't do it..there would be too much spatial variation in soil texture, and there would be oxbow/paleochannel scars, splay deposits, lag deposits, etc. So the only question is the date of deposition, and that question is best answered by a geologist.
OOPS! After reviewing Owen's map, I realize I was confusing the Wando and the Soccastee formations. Owens gives a date of 200,000 ka for the Socastee. Your blue-line graph shows sea level at that date being lower than it is today, so the same conundrum still applies. (The Wando in that area of the Pee Dee is riverine).
There is a marine (littoral) facies of the Socastee formation that stretches to about 30 km inland from the present coastline that exhibits bays. The area I referenced earlier does not exhibit them. I think Owens interprets that area as a backswamp; I do not, for reasons I posted previously. But let us not lose sight of the original purpose of this discussion. Extrapolating from your sea-level graph, the Socastee must have been exposed subaerially somewhere between 200,000 and 180,000 years ago. That upper portion of the Socastee (in the Pee Dee region) does not exhibit bays, while immediately adjacent areas of the older Duplin formation do. If meteor or cometary impact at any time in the last 180,000 years had caused the bays, then bays should exist on the aforementioned area; but they do not.
This morning I pulled some old soil surveys to look for pre-Holocene surfaces on the Coastal Plain of SC that do not harbor Carolina Bays, or upon which they are exceedingly rare. I found such areas to be extensive; the lower (seaward) half of Dorchester County, most of Georgetown County, etc. Other areas (such as the lower portion of Marion County, for example) that presumably are of the same geologic age, have bays that are well expressed. This gave me an idea. I don't know exactly how I'll do it, or how long it will take, but I think I'll construct a map of the CP of SC that shows those surfaces, with a data overlay showing the geologic formations and their ages. I think it would be revealing.