A worthwhile trip for those who enjoy the road less traveled, the North Rim, or "other side" of Grand Canyon is visited by only 10% of all Grand Canyon visitors. The centerpiece of the park is the Grand Canyon, a gorge of the Colorado River. The Grand Canyon is 277 miles (446 km) long, up to 18 miles (29 km) wide and attains a depth of over a mile (6,000 feet or 1,800 metres). Nearly two billion years of the Earth's geological history have been exposed as the Colorado River and its tributaries cut their channels through layer after layer of rock while the Colorado Plateau was uplifted. Designated as an UNESCO World Heritage Site in 1979.