What is the techniacl term for what caused the steepened stream gradient of the Virgin and other rivers?
The nine known exposed geologic formations in Zion National Park are part of a super-sequence of rock units called the Grand Staircase. Together, these formations represent about 150 million years of mostly Mesozoic-aged sedimentation in that part of North America. The formations exposed in the Zion area were deposited as sediment in very different environments:
The warm, shallow (sometimes advancing or retreating) sea of the Kaibab and Moenkopi formations;
Streams, ponds, and lakes of the Chinle, Moenave, and Kayenta formations;
The vast desert of the Navajo and Temple Cap formations; and
The dry near-shore environment of the Carmel Formation.
Uplift affected the entire region, known as the Colorado Plateaus, by slowly raising these formations more than 10,000 feet (3,000 m) higher than where they were deposited. This steepened the stream gradient of the ancestral Virgin and other rivers on the plateau.
The faster-moving streams took advantage of uplift-created joints in the rocks. Eventually, all Cenozoic-aged formations were removed and gorges were cut into the plateaus. Zion Canyon was cut by the North Fork of the Virgin River in this way. During the later part of this process, lava flows and cinder cones covered parts of the area.High water volume in wet seasons does most of the downcutting in the main canyon. These flood events are responsible for transporting most of the 3 million short tons (2.7 million metric tons) of rock and sediment that the Virgin River transports yearly. The Virgin cuts away its canyon faster than its tributaries can cut away their own streambeds, so tributaries end in waterfalls from hanging valleys where they meet the Virgin. The valley between the peaks of the Twin Brothers is a notable example of a hanging valley in the canyon.
Colorado Plateaus