Rocky Mountain, Colorado

Nowhere else in the United States can a visitor see so much alpine country with such ease. A mere two-hour drive from Denver, Trail Ridge Road takes visitors into the heart of Rocky Mountain National Park, traversing a ridge above 11,000 feet (3,353 meters) for 10 miles (16 kilometers). Along the way, tiny tundra flowers and other wild blooms contrast with sweeping vistas of towering summits; 78 of them exceed 12,000 feet (3,656 meters). Alpine lakes reflect the grandeur.

The summits form at least the third generation of mountains to rise in this region. The first probably protruded as islands above a shallow sea more than 135 million years ago, when dinosaurs reigned. Another range grew out of a later sea some 75 million years ago. Over the eons these summits eroded to rolling hills, which rose once again, although unevenly: Some portions sank along fault lines, helping create the striking texture of the current scenery.

Rock as old as that at the bottom of the Grand Canyon—nearly two billion years—caps the Rockies' summits. Within the last million years, glaciers, grinding boulders beneath them, carved deep canyons. Erosion later scoured the more jagged summits into their present profiles.

Rocky Mountain, though only about an eighth the size of Yellowstone, accommodates as many visitors—three million or so a year. Overcrowding worries park officials and conservationists, who cite distressed animals, trodden plants, and eroded trails. Condominium development is crowding the park's borders also, shrinking the habitats of elk and other wildlife and threatening to turn the park into an island of nature.