Despite the staining, the remains of this groyne (groin) were remarkably clean – I suspect that it was a combination of being sand blasted when the wind got up and the equivalent water/sand action when the tide was in.
For those unaware, groynes are constructed as barriers to prevent the movement of sediment. In the case of these ones, they would be for preventing long shore drift, which is, if I recall correctly, the movement of sand/shingle along the beach/shore through tidal movements of the sea.
I can vouch for this action, as I used to do a bit of beach fishing and if you cast a heavish weight, say 5 oz + hooks, out directly in front of your, say 80 yards, you could watch the line move as the tide pulled the line/weight down the coast. In strong tides, leaving it long enough, would mean you would be pulling the weight/hooks out of the breakers very soon – in some tides it was impossible to ‘hold bottom’.