While walking through the sand dunes at Ynyslas, I spotted in the side profile of an eroded piece of the dunes, a band of shells and other large material running through the dunes. I suspect that these shells could have been deposited during a large flood/storm event where the ocean flooded these dunes, causing the material to be deposited. This could be quite a recent deposition or may stretch back in time much further. Above the large material deposit the dune is entirely sand, which could have been deposited by the wind, a process which could have occurred quite quickly as sand gets caught in the grasses growing from the dunes.
Another feature of note here is the angle of repose of the dry sand, seen along the base of the profile. The sand held in the vertical profile is likely more compressed, wetter and stabilised by the plants growing in the dunes.
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