Friday, April 29, 2011

The Virgin River Staircase

The North and East Forks of the Virgin River rise on the south face of the Markagunt Plateau, in southwest Utah, amidst colorful outcrops of the Claron Formation. From these early Tertiary sediments, the tributaries begin to flow back in time, entering Zion National Park through Cretaceous Dakota Sandstone before carving magnificent canyons of Jurassic Navajo Sandstone; the North Fork has produced the more famous Zion Canyon while the East Fork sculpted Parunaweep Canyon.

The Forks join west of Zion, where the Virgin River begins a journey across Triassic redbeds, interrupted by Quaternary basalt flows along the Hurricane Fault. Beyond St. George, the river dips into the northwest corner of Arizona, where it has carved the spectacular Virgin River Gorge through Permian limestone and older, metamorphosed Paleozoic strata. West of the Grand Wash Fault, which marks the west edge of the Colorado Plateau, the canyon widens and the river enters the stark landscape of the Mojave Desert. After flowing into Nevada, the Virgin River curves to the south and joins the Colorado River within Lake Mead.

Western rivers are appealing to the naturalist in many ways. Producing ribbons of vegetation through arid landscapes, they also reveal the complex geology that underlies and explains the rugged, surface topography. Though relatively short, the Virgin River of Utah-Arizona-Nevada has bestowed its gifts in spectacular fashion.