Geology
Glen Canyon was carved by differential erosion from the Colorado River over an estimated 5 million years. The Colorado Plateau, through which the canyon cuts, arose some 11 million years ago. Within that plateau lie layers of rock from over 300 million years ago to the relatively recent volcanic activity. Pennsylvanian and Permian formations can be seen in Cataract Canyon and San Juan Canyon. The Moenkopi Formation, which dates from 230 million years ago (Triassic Period), and the Chinle Formation are found at Lees Ferry and the Rincon. Both formations are the result of the ancient inland sea that covered the area. Once the sea drained, windblown sand invaded the area, creating what is known as Wingate Sandstone. The more recent (Jurassic Period) formations include Kayenta Sandstone, which produces the trademark blue-black "desert varnish" that streaks down many walls of the canyons. Above this is Navajo Sandstone, the result of more compressed sand dunes. Many of the arches, including Rainbow Bridge, lie at this transition point. This period also includes light yellow Entrada Formations, and the dark brown, almost purple Carmel Formation. These latter two can be seen on the tops of mesas around Wahweap, and the crown of Castle Rock and Tower Butte. Above these layers lie the Straight Cliffs Sandstone and conglomerate shales that make up the Kaiparowits Plateau and San Rafael Swell to the north of the lake.
The confluences of the Escalante, Dirty Devil and San Juan Rivers with the Colorado lie within Lake Powell. The slower flow of the San Juan river has produced goosenecks where 5 miles (8.0 km) of river are contained within 1-mile (1.6 km) on a straight line.
Read more about this topic: Lake Powell