The debris drifted in the currents round to the south side of the island and was gradually deposited under the water line for a stretch of over 10 kilometers.
Once it finds a suitable gap, it'll place the egg inside, and the corkscrew-like shape helps hold it in place, so the ocean current doesn't wash it away.
Over thousands and thousands of years, the paths of rivers sweep up and down the coast, dropping off piles of sand to be spread by waves and currents into smooth sandy beaches.