This may be the southernmost remaining marker of the Croton Aqueduct, which from 1842 to 1955 brought fresh water from upstate New York to Manhattan. A more substantial remnant of the aqueduct is the West 119th Street Gatehouse (shown below), which "functioned as the southern connection between the cast-iron inverted siphon pipes, laid beneath the nearly mile-wide Manhattan Valley to the north, and the masonry aqueduct that continued southward." The slate-roof structure is crowned by ironwork that may have some additional meaning, lost on me.
Croton Aqueduct Dp't., 1862
Eighth Ave. near the southeast corner with 40th St., Manhattan
