Though it seems an unlikely candidate Rowlands (*p.16) gives the following description of a bay now filled with marsh, that has a beach near the landward side:

...at Mall-traeth marsh, about the middle place between tha sea and the farthest inland points of it, and very near the land, they find, by digging for coals, a perfect sea-shore with all its symptoms, as pebbles, shells, &c. under five or six yards of pure sand, as I have been credibly informed by an intelligent person concerned in those coal- works; and yet the surface there is very little above the level of the sea, as: appears by its frequent flowing up to that place; and if the channel was originally so deep there, so near the shore of it, any one may pro- portionably conjecture, not only how deep the whole bay was, but also how far the sea flowed up to the adjacent rising grounds towards Keint and Kefenny ; two brooks discharging themselves into that marsh.