ZION A Geologic and Geographic Sketch of Zion National Park

A GEOLOGIC AND GEOGRAPHIC SKETCH OF ZION NATIONAL PARK

MAKING OF ZION CANYON

Few visitors to Zion Canyon fail to ask the question, "How was the canyon formed?" Many unfamiliar with geologic processes view the deep, flat-floored vertically walled trench as a great fissure in the earth's crust formed by some "mighty upheaval." (See Fig. 3.) Others comparing it perhaps with the Yosemite Valley, which has been deepened and widened by streams of ice, think of the canyon as the work of glaciers and point to the horizontal grooves and stretches of smooth walls as evidence. Still others think of the wind as the primary or at least a sufficient contributing cause. But the horizontal layers of rock have not been broken along the line of the canyon, (those on one wall correspond layer for layer with those on the opposite wall and would join perfectly if extended across); there is no evidence of glaciation; and large-scale features resulting from aeolian action are absent. Improbable as it may seem to the layman, the evidence clearly shows that Zion Canyon is chiefly the work of the Virgin River—the stream that now flows through it. The stream is directly responsible for the depth of the canyon and, in cooperation with other agencies, for its width. The down cutting was accomplished by processes still in progress. The bed of the channel is worn by rock grains carried by the stream which thus acts as a moving ribbon of sandpaper. In its passage downstream every fragment of sand that once was part of the cubic miles of sandstone removed in excavating the canyon has aided in cutting away rock. Though the scouring process is continuous and some rock waste is carried along each day, in flood seasons the work accomplished is greatly increased. Because the tributaries flow on bare rock and thus carry the rainfall quickly to the master stream, floods follow each shower. Thus many times each year the volume and velocity of the stream are increased many fold. Huge boulders are dragged along; the banks are undercut, and the walls are scoured 10 to 40 feet above low-water level. Running on the steep gradient of 50 to 70 feet to the mile, the Virgin River carries away from the park more than a million tons of rock waste. Even if the stream head deepened its trench but a small fraction of an inch each year, its life of hundreds of thousands of years is amply sufficient for the work accomplished. During its long life the Virgin River with its tributaries has been busy with two tasks-cutting profound gorges, and carrying to the Colorado the land waste supplied. In widening the canyons and producing other remarkable architectural features, it has played but an incidental part. At no time has the stream been as wide as Zion Canyon, probably never wider than at present, and though it has always meandered, it has not cut the embayments in the canyon walls by swinging from bank to bank. The stream has cut vertically downward, maintaining its original pattern of curves and straight stretches. At one time the canyon, where at present widest, was a gorge like the Narrows, (See Figs. 2, 4, 5.)

Figure 4. Zion Canyon looking down stream from Observation Point. On the left the east wall shows in turn the Great White Throne (foreground). Red Arch Mountain. the Twin Brothers, the East Temple, Bridge Mountain, the Watchman, and south Mountain (distance); on the right are Angels Landing, the Spearhead, Lady Mountain and one of the Patriarchs. At the Great White Throne the depth of the canyon is 2,447 feet: its width 1,200 feet. (N. P. S. photo by Grant.)

As the stream sunk its channel deeper and deeper into solid rock it left an increasingly large expanse of canyon wall exposed to the destructive action of atmospheric agencies. Frost in crevices, rain beating against the cliff faces, and acids in the air and water disintegrated and decomposed the rock, causing fragments and grains to fall to the stream below, which carried them to the mouth of the canyon and on to the Colorado River, eventually to the sea (now to Lake Mead above Boulder Dam). In this manner both walls of the originally narrow trench were worn backward,

So far, however, the walls have been eroded relatively little. Even where the canyon is widest they stand but a half mile from the river, and for long distances rise directly from the river's edge. This relation clearly shows that deepening of the trench has been much more rapid than widening. For cutting solid rock the Virgin River through Zion Canyon and farther upstream is very effective. It gathers water from a large area, flows swiftly on rock that disintegrates with exceptional ease, and its movement is not perceptibly retarded by soil or cover of vegetation. The main stream, branching headwards as Kolob Creek, Deep Creek, and Orderville Creek, is cutting so rapidly that its tributaries cannot keep pace. Because they drain small areas and flow only in response to showers, many branches are unable to cut channels as deep as the perennial master stream. From their mouths high on the canyon walls they descend from time to time as waterfalls. Like the Virgin itself these tributaries have done little to give the canyon its present form. Many of them leap from the rim barely wetting the walls behind them and scour their channels only in the short courses across the canyon floor. Scores of them after falling merely spread out as thin sheets of water that follow no definite runways. At first sight the alcoves in the canyon walls might seem to be the work of the streams from the plateau above, but the size and position of these re-entrants are not closely related to drainage areas tributary to them. Some fairly large side streams enter the canyon at places where the wall is but slightly indented. The evidence is clear that the canyon alcoves are the work of rain that falls directly into them and of underground water that seeps out through their walls. The process is sapping and undermining rather than excavation. The cliff walls of sandstone are undercut by the removal of soft shales beneath, and in response to gravity fragments fall off. The walls retreat and the rim develops curves and crenulations chiefly in consequence of erosion at their bases. Thus at Wiley Retreat, the Grotto, Weeping Rock, and other prominent alcoves part of the rain water that falls on the upland passes downward through porous rock until it reaches impervious beds and finally reaches the surface as springs and seeps. On emerging, these underground waters carve horizontal grooves in the canyon walls, leaving overhanging cliffs above and a slope below. Into the slope perennial streams from the spring are cutting channels that lead to the Virgin River. This method of erosion, in progress for thousands of years, is plainly in evidence today. The "spring line" in Zion Canyon is a conspicuous feature.

Figure 5. The Narrows just above the Temple of Sinawava. Through this defile, 2,000 feet deep and 20 by 50 feet wide, passes the Virgin River. the largest tributary to the Colorado in southern Utah.

In widening Zion Canyon and carving its architectural features, the work of the atmosphere and of ground water is greatly facilitated by the composition and structure of the sandstone that forms its walls. The weak cement that holds together the rounded grains of quartz is readily dissolved by rain that wets the walls and by water that seeps through the rock, thus loosening the tiny particles and permitting them to fall or to be swept from the cliff face by showers or by wind. The sandstone is reduced to sand, easily removed by streams. Features even more favorable for erosion are the cracks (joints) that traverse the canyon walls. Combined with the bedding planes—horizontal, oblique, and curved—these cracks determine the shape of the irregular chunks and huge slabs detached from the towering cliffs by frost, rain, tree roots, and ground water. Blocks embedded in the cliffs, blocks surrounded by open joints, blocks partly detached, blocks that rest precariously on some temporary support, and blocks that have fallen to the talus below indicate steps in the retreat of the canyon walls. That the process is continuous is shown by the scars on the wall, some almost obliterated, some considerably weathered, and others quite fresh. Each year many new blocks are pried off.

On some of the joint cracks slipping has taken place. The joint plane has become a "fault" — a plane of fracture along which the rock on one side has been raised higher than that on the other. Most of the faults in Zion National Park show a vertical displacement of a few inches or a few feet. Near the east entrance to the park a fault measures 20 feet; in Wild Cat Canyon, 105 feet; and east of Cougar Mountain, 210 feet.

The joint cracks so prominently displayed on the walls of Zion Canyon are not local features. They represent thousands of cracks that cut almost vertically through the rocks of the entire park. Along these cracks, especially where they are closely spaced and where faulting has occurred, the rock is broken, even shattered, and therefore readily eroded. Taking advantage of this condition, many streams have carved their valleys along lines of pronounced jointing. This guidance of streams by structures is clearly shown on the Zion National Park topographic map by the straight, parallel tributaries to the Parunuweap, Pine Creek, Orderville Gulch, the upper Virgin, and Wildcat Creek (a tributary to North Creek), developed along joints and faults trending north-northwest. The position of many alcoves and amphitheaters and the trend of many cliffs likewise reflect the control exercised by planes of weakness. Though West Temple, East Temple, and a few other lofty masses are capped by limestone—undissected remnants of the Kolob Terrace—and are thus protected from erosion, most of the "towers and temples of the Virgin"—such magnificent structures as the Watchman, Mountain of the Sun, Spearhead, Castle Dome, and Guardian Angel—are sculptured blocks of relatively unfractured rock that stand between zones of jointing. They owe their individuality to the headward growth of canyons in the fractured rock that surrounds them. In fact to a very large degree the sculpture of the park was predetermined by a network of joints and faults.

Figure 6. The Watchman, one of the many gigantic buttes in Zion National Park; rises 2,600 feet above the Virgin River at its base, Navajo sandstone at the top, below it in turn the Kayenta and Wingate (covered by talus), and the Chinle. including the "big ledge", and the variegated beds that contain petrified wood. South Mountain and Eagle Crags in left distance, beyond the Parunuweap. N. P. S. photo by Grant)

Downcutting and undermining of canyons are vigorously in process today and doubtless will continue for a long time. The bed of the chief tributaries to the Virgin could be sunk 1000 feet deeper and still have slopes sufficient to carry water to the Colorado River and on to the sea (now Lake Mead). Also the rock masses remaining between canyons are enormous. In other words, the scenery of Zion National Park is geologically young; its gigantic features mark but an early stage in the erosion of the lofty Markagunt Plateau. It has been developed during the few million years that make up the last chapter in a billion years of earth history. If continued without interruption, erosion will convert the present rugged landscape into plains near sea level.

This explanatory description of the features of Zion National Park and of the processes active in making them leaves unanswered the question, How did Zion Canyon originate? Why has this particular region so many profound canyons, lofty mesas, and vertical cliffs? The complete answer involves a knowledge of a long train of events in earth history and of forces and processes as yet not fully understood. In brief, the answer is, the rise of the land with respect to sea level. At a time long past, estimated as 13,000,000 years ago, all of southern Utah and adjoining regions began to rise. This upward movement, continued slowly and intermittently until the lands once near sea level attained an altitude exceeding 10,000 feet. This regional uplift introduced a long period of time during which the conditions have been favorable for erosion. On the original lowland, streams did little work. They flowed in broad shallow valleys of gentle gradient like those on the plateaus of Cedar Breaks National Monument and Bryce Canyon National Park. In consequence of the uplift, the paths of the streams were steepened and the streams became powerful agents of erosion. Their accelerated speed permitted them to cut trenches in solid rock and as the land rose progressively higher, to develop their trenches into the present profound canyons.

During the great uplift that brought the former low-lying lands of the Zion National Park region to a position two miles above sea level and gave the streams their power to erode, the beds of rock were broken into earth blocks many miles in length and width. One of these enormous blocks, the Markagunt fault block, includes Zion National Park. (See Fig, 1.) Its western border is the Hurricane fault which broke the beds of sandstone and limestone and raised the part on the east 1000 to 6000 feet above the part on the west. The escarpment thus formed, now much reduced by erosion, is the Hurricane Cliffs, a wall extending south from Cedar City and crossed by the Zion National Park road near La Verkin. The eastern border of the block is the Sevier fault along which, as along the Hurricane fault, the strata on the east was raised. In Parunuweap (Long) Valley south of Glendale the fault is marked by a red wall. At Mr. Carmel the limestone that caps Elkheart Cliffs stands 2000 feet above the same bed in the Virgin River below. The Markagunt block is tilted eastward; the strata descend across the park on a gradient of one to two per cent.