can anyone (Navy guys) identify this?

Back in the 1960s I was on an amphibious WW 2 ship (see below) and I was told that these things (or something similar) were used by the Nazis to prevent an amphibious ship from retracting from the beach. They were placed about 25 feet apart and if a ship ran up over one or more of them at high tide (they could not be seen then) they would prevent the ship from retracting from the beach because they would dig into the sand.

Benji