ZION A Geologic and Geographic Sketch of Zion National Park

A GEOLOGIC AND GEOGRAPHIC SKETCH OF ZION NATIONAL PARK

THE ORIGIN OF ZION VALLEY
By
RUSSEL K. GRATER
Park Naturalist

To many visitors there is a marked similarity in shape between the sheer-walled gorge of Zion Canyon and the Yosemite Valley in Yosemite National Park, though the colors of the two areas are decidedly different. In both areas the canyon walls rise sharply from a flat valley floor, culminating in a series of high peaks and domes on the rim. So pronounced is this similarity in appearance that many people feel that their origins must be somewhat the same.

It is easy to show, however, that while glaciation was largely responsible for Yosemite Valley, no glaciers occurred at Zion. What appears to be "ice lines" on the walls of Zion Canyon are in reality only dune deposits of the Navajo formation, cross-sectioned by the cutting of the Virgin River and etched out through centuries of weathering. The sheer walls, so characteristic of glaciation, have been largely due to the extensive fracture system of the region, plus the action of natural arches created by the seeps at the base of the cliffs. Working together these two factors have aided in keeping the walls relatively vertical.

Figure 13—Zion Valley near Mt. Majestic. The characteristic flatness of the valley floor is well illustrated in this sector.

The origin of the flat valley floor of Zion brings to light one of the most interesting chapters in the history of the canyon. From field studies completed in 1943, the probable events leading up to the present-day valley can be reconstructed.

A few thousand years ago the Virgin River had cut down to the Springdale member of the Chinle formation in the vicinity of the Court of the Patriarchs and through it to the southward. Rising to the west towered the huge bulk of Sentinel Mountain. Now Sentinel Mountain, like the rest of the plateau, was sitting on a tilt to the eastward of approximately 2 degrees. Running through the mountain itself was a series of fractures, one of them extending far to the northward through Phantom Valley and Wildcat Canyon. Quoting now from "Landslide in Zion Canyon, Zion National Park, Utah (1)": "Then came a tremendous slide that blocked the entire canyon. From the formations involved it seems probable that water, fed into the fractures, filtered downward until the shale beds on top of the Springdale member were thoroughly saturated, creating a "greased skid" for the great mass of overlying rocks. As the slide came down into the canyon it fanned out, filling in the gorge from the present location of the Zion Canyon-Mount Carmel Road junction to the west side of the Court of the Patriarchs. The crest of the slide was directly opposite the Sentinel and shoved a great mass of debris high up on the slope beneath the Twin Brothers and Mount Spry. That the crest of the slide was at least 500 feet above the river-level is shown by the fact that the river was then around the 4,100-foot level, and the present high points of the slide on opposite sides of the river are 4,712 feet and 4,703 feet, respectively. Erosion has undoubtedly lowered the crest, although not to any great extent. The blocking of the river formed a large lake that filled the canyon probably back as far as the Narrows at the foot of the Mountain of Mystery. In this still water, silt and clay muds carried by the old Virgin River were dropped, forming extensive beds of clay underlain with oozy muck. In the meantime, the impounded waters finally began to pour over the top of the slide barrier. The lowest point along this barrier was on the east side of the canyon below the toe of the slide. Probably the initial erosion was rapid, as the waters had a tremendous fall and had only loose debris to move. It is believed that the maximum lake elevation was maintained for only a comparatively short time and so left no mark of its position. After this, the lowering of the lake level by removal of the slide debris was probably slower, allowing extensive deposition to continue in and around such still-water areas as Birch Creek and Emerald Pool canyons as well as behind the rock dam itself. The final draining of the lake and the exposure of the clay deposits was probably a relatively show process, as the deeper the stream ate into the slide debris, the more its gradient was reduced and the slower it cut. This allowed for partial consolidation of the underlying clays in the lake fill, and their protection against rapid erosion by a heavy vegetation cover that came in over the valley floor. Today, at least 76 feet of this lake fill have been removed at its lower end, yet extensive deposits remain that are virtually untouched.

Figure 14.—Sentinel Mountain. From its highly shattered face came the slide whose debris can be seen below.—Grater Photo.

There has been much speculation as to how rapidly the slide debris has been removed and will be removed in the future. Some conception of this can be obtained by records kept since the park was established in 1918. Since that date there have been two major slides involving the old slide debris. Both slides, incidentally, occurred because the saturation by water of the shale beds overlying the Springdale member of the Chinle allowed the entire mass to move. No figures are available as to the amount of material that came down and blocked off the river in 1923, but the 1941 slide was estimated to have comprised at least 150,000 cubic yards of rock debris. Thus, in twenty years, the river has removed a great quantity of the old slide. Using this as a gauge, the rate of removal and erosion has been rapid, and at most only a few thousand years can have elapsed since the slide occurred."

The great valleys of Zion and Yosemite thus have one thing in common in their origins. In both instances the rivers were dammed up by rock debris, forming lakes which were finally filled with silt, creating the flat valley floors of today. In Yosemite this natural dam was the result of glacial action; in Zion it was due to a mammoth rock slide.