Zion Canyon, the best known example of a deep, narrow, vertically walled chasm readily accessible for observation, was made by the north fork of the Virgin River, the stream which now flows through it. Before this stream established its course there was no canyon. During the long period since its course was established the river has slowly deepened its channel and extended it headward until its original shallow valley has become a long narrow trench between towering walls. Though now deeply entrenched in the rocks of the Kolob Plateau, the river maintains substantially its original pattern. It flows in the same direction, and the curves and straight stretches of its present walls duplicate the meanders of the stream when it flowed some 5,000 feet above its present level.

For many thousands of years the Virgin River and its tributaries have been busy with two tasks, namely, deepening their channels and transporting material weathered from the canyon walls. At present the Virgin carries away from the park each year about 3,000,000 tons of ground-up rock at an average rate of 180 carloads a day. For such effective work the many-branched river seems incompetent. But though relatively small in volume, this stream system falls from 50 to 70 feet per mile (nine times the fall of the Colorado in Grand Canyon) and is at work on rock, chiefly sandstone, that disintegrates with exceptional ease. Many tributaries are on bare rock, little retarded by vegetation, and are fed by short but violent showers. Consequently, they are brought to flood stage not only seasonally but with each period of heavy rainfall. Because they flow only in response to showers, the smaller tributaries are unable to cut channels as deep as the perennial master stream. From their mouths high on the canyon walls, they descend as waterfalls.

Though they are primarily responsible for the depth and position of the canyons, the Virgin River itself and the other streams heading on the adjoining plateaus are only incidentally concerned with the detailed carving that makes Zion Canyon unique. The walls are retreating in consequence of ground water which emerges as springs and seeps, rain which falls directly into the canyon, water that spills over the rim, frost and tree roots which pry off slabs, and chemical agencies which weaken the rock by the removal of the cement about individual grains. Continuous sapping at or near the contact of the porous Navajo sandstone and the more impervious underlying beds has developed alcoves in the canyon walls at Wiley Retreat, the Stadium, Weeping Rock, Emerald Pool, Birch Creek, Oak Creek, and elsewhere.

In the development of the amazing variety of architectural features on the canyon walls, the composition and structure of the Navajo formation have served as controlling guides. In addition to the bedding planes—horizontal, oblique, and curved surfaces—the Navajo has developed parting planes (joints) that extend downward for short distances or pass through the formation from top to bottom. These bedding planes and joints determine the shape and size of the blocks that spall off from the towering cliff walls. Unlike that of humid regions, this type of erosion progresses from below upward; it causes the canyon to widen and still retain its vertical walls.