Found Along the Beach

Went for a nice walk along the beach with my husband this evening and spotted this arrangement of shells, stuck to the bottom of a structure.

Not sure what this structure is called, but it comes to a point with a triangular-shape on top, which you can just see in the water, when the tide is right in.

I think it used to indicate the end of an underground pipe.

I like the patterns on these shells and the arrangement of them.