A screw like this one at the bottom of each of the seven pilings (legs) of this lighthouse - allowing the leg to be screwed down into the soft bay bottom. Although it would have been easier to drive the pilings into the mud, screwing them in gave the lighthouse a much more stable foundation.
Ice tends to freeze around the the lighthouse pilings and as the tide rises up it often lifts the legs. The pilings screws eliminate this unwanted damage.
You would have seen the screwpile lighthouses like this one on the Chesapeake Bay, the Delaware Bay, and in North Carolina where the rivers and estuaries have soft bottoms and it is possible to screw the pilings down.
Ten inch solid wrought-iron pilings were screwed 25 feet into the bay floor to prevent shifting.