In defence of the realm

These huge, six foot high, cubes of concrete were cast in 1940 when we were living in fear of being invaded by Nazi Germany. They run along the beach in a continuous line for 12 miles from the mouth of our local river, the Ythan, all the way to Aberdeen They were designed to prevent tanks and other vehicles from landing the the beach.
Sometimes they disappear under the sand only to appear again, as if by magic, after storms and high tides.