The Grand Canyon is one of the world’s most awesome erosional features. It is 277 miles (446 km) long, including the 60 miles (96 km) of Marble Canyon upstream. The depth of the main segment of the Grand Canyon varies between 3,000 and 6,000 feet (900 and 1,800 m), with the rim-to-rim width between 4 and 18 miles (6 and 29 km). Its origin has plagued geologists since the time of John Wesley Powell’s first courageous voyage down the Colorado River in 1869. Despite an increase in knowledge about its geology, evolutionary geologists have yet been unable to explain the canyon.