With young black men used as bait, dogs were trained to track and pursue runaway slaves in the cobblestone courtyard of a Richmond slave jail. Hidden for more than a century, the courtyard of round, gray stones and other remnants of Lumpkin’s Slave Jail lay exposed yesterday in the corner of a Shockoe Bottom parking lot. Archaeologists have spent the past four months digging 8 to 15 feet down to uncover “an amazingly intact urban complex,“ which included brick foundation walls, said Matthew R. Laird, principal investigator with the James River Institute for Archaeology in Williamsburg.