This marker is on the east side of Fan Pier and is related to, but topically different from, the other markers there. It reads:
"South Boston, or Commonwealth, Flats was the name given to the tidal flats that existed at this site in the mid-1800s. The Boston Harbor Commissioners initially planned to fill the flats in order to channel the force, or scour, of the ebb tide into Boston Harbor.
"A heavy seawall was built around the curve at the mouth of the Fort Point Channel. Submarine divers constructed the foundations for these walls and placed the granite blocks, which were lowered into the water by steam derricks.
"To fill the area inside the seawalls, scows dumped clay and other material dredged from the harbor directly onto the flats being filled. Once the level of fill became higher than loaded scows could float at high water, the dredged claywas then dumped in front of the walls. From there it was lifted over the walls by a clamshell dredge, dumped on sloping platforms placed on the ballast behind the walls, and then slid down to the Flats.
"When the area near the walls had been filled, the clamshell dredge dumped the clay into small railroad cars that ran on tracks set on pile trestles. These cars were then pulled by a light locomotive to various locations on the flats. Work advanced quickly, and by the end of 1876 a large part of Fan Pier had been filled."
A note below indicates this text was from Gaining Ground, A History of Landmaking in Boston, by Nancy S. Seasholes. The text is accompanied by a diagram of seawall construction as well as a photo of dredging.