We had a long, detailed discussion about this on an earlier FR thread about a year ago. It has to do with fluctuating water tables and prevailing wind direction. The long axes of the bays are perpendicular to the prevailing wind direction at a time when water tables were fluctuating greatly during eras of formation (we’re talking about seasonal fluctuation here). Wind-generated wave motion was primarily from SW to NE; maximum scouring along the shoreline occurred at the NW and SE ends of the wet areas, causing disaggregation and suspension of sands and silts in the soil. As water tables dropped seasonally during the summer and early fall, winds picked up the loose material and deposited it downwind. Picture this process happening over geologic time periods; it took a long time, but a lot of material was moved. This same process has been observed in other areas around the world.
Some scientists from the Savannah River Site solved this puzzle. If you have plenty of time on your hands, you can look up my posts going backwards until you find the thread....I cite the paper there.
Maybe I'll read the Savannah River paper in depth and see if they have a solution for a number of questions.
By the way, I grew up around these bays in NC. We had one about 55 acres, filled in as a pond, on our farm. So, I've had a long interest in them.
Thanks again for your reply and direction to the SR paper.