For the second time in as many months, workers excavating a subway tunnel on the southern tip of Manhattan struck what is believed to be a pre-Revolutionary stone wall.

City officials are still unsure of the exact date of the wall or its purpose, but agree that, like the first wall, it should be preserved. “It’s a historic wall of some kind,” the parks commissioner, Adrian Benepe, told the New York Times for yesterday’s editions.

The first wall was uncovered in November and workers stumbled on the second wall in late December while digging up Battery Park for the $400 million subway project to modernize the 1905-vintage South Ferry Station, which links the city’s subways to the Staten Island ferry terminal.

The Lower Manhattan Recovery Office said excavation of the subway tunnel could continue once the pieces of the two walls are cataloged, carefully removed, and stored in crates, the Times said. The work is expected to be completed by early February.

The mortared stone wall found in November is more than 40 feet long and 7 feet wide and reaches down to bedrock.

“All of the remnants gathered so far indicate that this wall is an important part of New York City history,” Mr. Benepe said at the time. Officials said the wall appeared to be part of a gun emplacement connected with one of the several forts, dating from the early 17th century, that gave Battery Park its name.