The day I made these it was high tide on the beach at Whitley Bay. It must have been a spring tide, because the sea was hitting the sea wall beneath the promenade. The waves would smack into the wall and then draw back, revealing the sand underneath for a moment.
With a 2 stop neutral density filter fitted to the lens, I leant over the sea wall and started taking long(ish) exposures - about 1/4 to a 1/3 of a second - as the waves receded. Back home, I looked over what I had made and the water seemed to resemble birds' wings.