* The discovery of the marker for a couple who died more than 150 years ago intrigued construction workers. Research showed that the man was one of Dred Scott's lawyers.

As if timed for the season of celebrating all departed souls comes a new urban mystery: the sudden appearance of a marble tombstone at a St. Louis building site in the 3000 block of Eads Avenue, where new homes are being built in an area called St. Vincent Park.

And as if this unexpected and unexplained occurrence weren't enough, the tombstone, a four-sided column that's about three feet tall, turns out to have marked the graves of David N. Hall, one of the attorneys in the early stages of the Dred Scott case, and his wife, Sarah C. Smith Hall.

The opinion of the Supreme Court in 1857 that Scott, a Missouri-born slave, was not a U.S. citizen and had no right to file a suit, was one of the triggers of the Civil War.

Scott, who claimed to be free because he had lived for seven years in states where slavery was banned, was given his freedom by his owners' sons shortly after the decision. He enjoyed his freedom for only a few months, dying in 1858.

Hall and his partner, Alexander P. Field, represented Dred and Harriet Scott in earlier phases of the case at the Old Courthouse in St. Louis, according to Kristin Zapalac, a historian with the Missouri Department of Natural Resources. Zapalac did the research on the stone after she got an anonymous phone call about the grave marker.

She has also learned that Hall and his partner were the attorneys of record in another freedom suit, this one filed in 1842 by Pierre, "a man of color," against Gabriel Chouteau, a member of one of the city's founding families. Knowing that Hall was in St. Louis for 10 years, this recent find means he was involved in freedom suits since shortly after his arrival in the city, Zapalac said.

Zapalac was called to the site where the Hall marker was found because she works in the Department of Natural Resources' historic preservation office, which has to be notified - along with the police - if there's a possibility of unmarked human burials.

In this case, there was only a tombstone, but Zapalac was asked to do further research to determine if Hall could possibly have been buried there.

She soon determined that there was no graveyard at the Eads site and that the couple had been buried in nearby Christ Church Cemetery, at Caroline and Ohio streets, about two blocks to the north and east.

That cemetery was later closed. In 1884 the Post-Dispatch carried an advertisement that bodies there were being moved to New Wesleyan Cemetery, then in University City.

Since the cemetery appeared to have been closed in 1859, Zapalac thinks it's likely that the Halls' bodies were moved. Why the headstone didn't accompany them is part of the mystery.

One of the first to spy the stone was David Cameron, the site superintendent for C.F. Vatterott, the developer of St. Vincent Park.

"Why this showed up on my job site, I don't know," he said.

When the stone was discovered on Oct. 8, "I thought, oh my, what did we do," he said.

He said it was in a pile of other stones, including rough blocks of marble that look as if they were to be made into grave markers. He thinks it was all dumped at the construction site recently.

As the days went by, everyone involved got more and more interested in the mysterious grave marker. A plumber and a painter on the job did their own research, and each day came to work with more information about the Halls, including the family of Sarah Smith Hall. Her father, they learned, was a prominent physician in Massachusetts.

Cameron said he rather surprised himself with his own interest in the history of the tombstone. "I build new houses, and if something's in the way, I want to get it out of the way," he said. "I have a schedule to meet. But this was worth stopping for."

From her research, Zapalac learned that the Halls were both from Sutton, Mass. They lived near 15th and Pine streets and died young. She was 26 when she died in 1849. He died two years later, at 33. They had been married two years and had no children. "Consumption" was listed as the cause of death for both.

Apparently Hall was sentimental, said Zapalac, because he kept all his wife's clothes. In his will, written just two days before he died, he gave them to his sister in Massachusetts.

The will also ordered that a headstone be carved.

Hall was well-known enough that his death merited an obituary in the Missouri Republican, and the local criminal court adjourned "in token of respect to the deceased."

The stone is now in the custody of the Missouri Historical Society. Andrew Walker, senior curator there, said the stone was important for the society to have, although there were no immediate plans for its display.

"It's rare that you find such a substantive artifact from which you can build larger historical narratives: for David Hall, for Dred Scott and for African-Americans in St. Louis," he said.