Zion National Park:
A Tweak of History and a Tweak of Geology

by D. S. Grover

Lithosphere (February 1996); Fallbrook Gem and Mineral Society, Inc.; Fallbrook, CA


[Ed. Note: This article won third place in the adult article competition in the California Federation of Mineralogical Societies in 1996.]


Zion National Park, comprising 146,551 acres, is the site of a beautiful canyon cut by the Virgin River into the vermillion and white cliffs of southwestern Utah. It is entered between two great blocks of stone, the West Temple and the Watchman. Beyond the impressive gateway is the large canyon.

Against the east wall of the canyon stands the great White Throne, a flat-topped dome. In vivid contrast to the Throne is the dull red of Angels Landing directly opposite. The auto road ends in a great parklike amphitheater -- the Temple of Senowana. Beyond, a trail leads to the Narrows where the canyon is 2,000 feet deep and, at one place, only 20 feet wide.

A Mormon scout, the first white man to see Zion, discovered the canyon in 1858. He reported it to the church, and the leaders were so impressed by the majesty and grandeur of the rocks, colors, and peaks that they named all of the features of the canyon as they related to the religious connotations of the Mormon Church.

The earliest rocks of Zion were born when mud, silt, and sand settled in ancient swamps and along streams. Later, massive dunes, formed from windblown sand, covered the muddy underlayers. These raw materials eventually formed the rocks we see today -- the slope-forming shale and the cliff-forming sandstone. The land mass, as it accumulated layers of sediment thousands of feet thick, began to sink. The sinking occurred at about the same rate as the materials piled up, so the elevation remained nearly constant. Eventually, the land sank below sea level and was flooded by a shallow inland sea. Water seeped down between the grains of sand and other particles, carrying minerals with it. Together with the tremendous force of compression, these minerals cemented the sand grains together, turning them into stone.

Much later, forces deep in the earth caused the land mass to rise. This slow, steady uplift occurred over a broad area -- namely the entire Colorado Plateau, of which Zion is a small part. Portions of the plateau were raised over 10,000 feet. The uplift is still occurring today. As the uplift occurred, rivers and streams which once meandered slowly across flat plains began to pick up speed. Tumbling rapidly off the edge of the plateau, they began to carve into the rock layers. The Virgin River carries sediment continuously, and hauls masses of mud, sand, and boulders during floods. Over time, the north fork of the Virgin River has carved this great chasm called Zion Canyon.

Zion Canyon is famous for its majestic 2,000-foot-high vertical walls of Navajo Sandstone. This formation resulted from the massive sand dunes present in the area during the Jurassic period, about 170 million years ago. During this period, dinosaurs flourished elsewhere. The desert climate in this area supported only sparse plant and animal life; thus, few fossils are found in the Navajo layer.

East of the Zion-Mt. Carmel Tunnel lies the white upper layer of the Navajo Sandstone. Without iron oxide, the rock is white rather than red, is weaker due to fewer cementing materials, and erodes easily into domes and buttes. Alternating horizontal lines in the rock, called "cross-bedding," reveal the exposed layers of ancient sand dunes. Often called "slickrock," this weak rock often has a layer of loose sand on the surface. Watch your step.

Although the rocks are fairly old ("middle age" by geologic standards) the canyon itself is "quite recent." Geologists believe the uplift and canyon carving began no earlier than 10 to 12 million years ago. By this time dinosaurs were long gone, mammals were well established, and the Rocky Mountains had already formed.

The cliffs of Zion are stained and streaked with a variety of bright hues. Iron oxide causes shades of red, orange, green, and brown. Lichens and moss growing on the rock appear white, gray-green, black, and bright orange. Dull black streaking may occur where organic matter has been deposited by water runoff. Shiny black surfaces, called desert varnish, contain iron and/or manganese.

Autumn color displays begin in September in the high country and peak in Zion usually in late October. Winters are fairly mild. Winter storms often bring rain or light snow to Zion, but heavier snow to the higher elevations. Clear days may become quite warm, reaching 60 degrees or more. Nights are in the 20 - 40 degree range. Summer highs in July and August are generally 90 to 100 degrees.

References

Anonymous;
Compton's Encyclopedia.

National Park Service;
The Sentinel; periodical; United States Department of the Interior.


Copyright © 1996 by Fallbrook Gem and Mineral Society, Inc.

The preceding article was originally published in the February 1996 issue of Lithosphere, the official bulletin of the Fallbrook [California] Gem and Mineral Society, Inc; Richard Busch (Editor).

Permission to reproduce and distribute this material, in whole or in part, for non-commercial purposes, is hereby granted provided the sense or meaning of the material is not changed and the author's notice of copyright is retained.


Last updated: 18 September 2002
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