Not a dam. Or, it's a natural dam. I suspect a big piece of the phenomena is sea levels massively shrinking, and then expanding ... massively, but slowly. 11000

Not a dam. Or, it's a natural dam. I suspect a big piece of the phenomena is sea levels massively shrinking, and then expanding ... massively, but slowly. 11000 years is plenty of time for, say, an ice age cycle (even a minor one). After all, if a large % of the water on earth suddenly collected at the poles, it's not as though every water body's shoreline would contract at the same width, like a photoshop "contract selection" command. Undersea geography, depth, etc would play a factor.