Watch Sunday Morning: Walden Pond: An endangered treasure - Full show on CBS

The National Trust for Historic Preservation has listed Walden Pond and Walden Woods, where Henry David Thoreau wrote his 1854 classic "Walden," one of "America's 11 Most Endangered Historic Places." Historian Douglas Brinkley reports on how the Concord, Mass., pond and its surrounding, spiritually-nourishing woods – a storied part of our national heritage – is under threat from nearby development.