For three days in July, 1863, two great armies collided on the fields at Gettysburg. It remains the bloodiest battle in U.S. history. And improbably, 155 years later, there are still living witnesses to that moment in time: The trees. Across 6,000 acres of Gettysburg National Military Park, rangers have documented at least a dozen "witness trees" that were alive during the battle – some scarred by bullets and cannonballs – that uniquely bring America's bloody past into the present. Mark Strassmann reports.