To stand on the South Rim and gaze into the Grand Canyon is to behold an awesome immensity of time. The serpentine Colorado River has relentlessly incised a 280-mile-long chasm that in some places stretches 18 miles wide and more than a mile deep. Visitors to Grand Canyon National Park will encounter an exhibit titled the Trail of Time, and learn that scientists believe the canyon is about 6 million years old — relatively young by geological standards.