Carolyn Pitts runs her palm over the hand-tooled sandstone exterior of an old textile mill. "The stonework is marvelous," she says. "It was obviously ! meant to be a real showpiece." Built in 1849 in Cannelton, Ind., alongside the Ohio River, the brooding, fortress-like structure with twin turrets and heavily bracketed cornice was abandoned in the 1950s. Now the roof is a wreck, and starlings nest inside.
A recommendation from Pitts could save the mill. As the sole architectural historian working for the National Park Service's history division, she decides what structures should be considered National Historic Landmarks. Pitts walks through...