By hello
Built between 1837 and 1842, the Old Croton Dam was the first substantial masonry dam in the United States and a core element of New York City's original Croton water-supply system. It was designed under engineer John B. Jervis. After the New Croton Dam was completed in 1906, the older dam was submerged beneath the reservoir; the structure still exists in place, though usually underwater. (en.wikipedia.org) Construction detail: granite ashlar foundation with a rubble core. (en.wikipedia.org) Why it remains: it survives as a nationally recognized historic engineering site rather than an active water-supply work. (en.wikipedia.org) Further information: https://www.parks.ny.gov/parks/oldcrotonaqueduct/details.aspx https://aqueduct.org/empire-city/croton-dam