This week, the National Park Service marks its hundredth birthday. Hailed by filmmaker Ken Burns as “America’s best idea,” there are today 59 national parks spanning every region of the country—from the mountains of Glacier National Park in Montana to the cactus forests of the Saguaro National Park in Arizona. [See eight amazing parks you've never heard of.]

As a child Mark Woods, author of Lassoing the Sun: A Year in America’s National Parks, used to go on camping trips with his parents in the parks. And so, after his mother was diagnosed with cancer, he decided to set off on a yearlong journey through the national parks to reconnect with the past and