ROWLAND, N.C.—Brenda Schwab stopped her 16-year-old Ford pickup truck on Gaddy’s Mill Road and pointed to a chocolate-colored ridge about 30 feet long and 5 feet high, resting in a field and close to the road. A cold downpour had turned the air clammy. It smelled like a rancid potpourri seeping through the truck windows, singeing the occupants’ sinuses and glomming onto their raincoats. The dark r...