It's part of the daft design that seems purposed to trap mud within the marina.
Here's a view to the E, with snow still on the hills (interestingly, not the highest part as it drifted down from the top). The foreshore covers a large area which was once a productive oil field, with Welsh coal being used to heat oil out of shale rocks. It's a contingency resource for hydrocarbons, and you can pick up rocks and burn them on beach barbecues. Bit smelly, though.
I live not too far from there and have looked at keeping the boat in the marina, but the sailing area is very unforgiving compared to most other areas.