My 10-year-old daughter, Alex, and I follow the steep and rugged New Hance Trail on a nearly 5,000-vertical-foot march down into the Earth’s most-famous hole in the ground. The sky seems to levitate steadily higher above us, but it’s just a trick played on the eyes by the severe topography of the Grand Canyon: As we slowly descend deeper, burgundy rock walls creep higher, pushing the cerulean dome overhead farther away from us.

Geology factors into many units of the National Park System, but there are some parks that rise above all others if you have an interest in the geologic past...and present. What follows is a short list of some of the most geologically fascinating parks in the system, though we're sure you can add others.

Is the National Park System in danger of turning into a catchall system? Should a site dedicated to the nuclear arms race, another to union organizers, and another to First Ladies really fall under an agency that started out preserving spectacular vistas and landscapes, that showcases Yosemite, Yellowstone, and the Grand Canyon?

Railroads played a huge role in the establishment of the national parks in the early 20th century. Just how great their influence was will be discussed by Dr. Alfred Runte as part of the lecture series at Zion National Park this summer.