By Steve Tawa

PHILADELPHIA (CBS) – They may conjure up ghastly images, 19th century graveyard headstones near the Betsy Ross Bridge, but a historian says they found their way to the river’s edge in the Bridesburg section of Philadelphia.

Historian Tom Keels says they’re headstones culled from the old Monument Cemetery, founded in 1837 at Broad and Berks Streets, in the heart of what is now the Temple University campus.

“When it was first founded, this was way out in the country,” says Keels.

The central obelisk, designed by famous Philadelphia engraver John Sartain, being dismantled in 1956. (Credit: Tom Keels)

But by the mid 1950′s, the cemetery was neglected and Temple bought it to build a parking lot and athletic fields.

Delaware River Port Authority’s Tim Ireland says the bodies of the deceased were moved elsewhere and the grave markers were deliberately left behind.

“These are not stones that are looking for loved ones. These are basically just pieces of granite that were left behind,” Ireland explains.

Keels believes that a contractor who did work at the cemetery also was involved in a mid-1950′s city-run drainage control project where the Frankford Creek meets the Delaware River.

He says that’s how the heavy grave markers made their way to the shoreline, to be used as riprap to prevent erosion.