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The Cedar Mesa Project

A Brief History of the Geology of the Grand Gulch Area

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"The canyon country of southern Utah and northern Arizona - the Colorado Plateau - is something special. Something strange, marvelous, full of wonders. As far as I know there is no other region on earth much like it, or even remotely like it. Nowhere else have we had this lucky combination of vast sedimentary rock formations exposed to a desert climate, a great plateau carved by major rivers - the Green, the San Juan, the Colorado - into such a surreal land of form and color."
Abbey, Edward, The Journey Home, E. P. Dutton, New York 1977, p. 85.

Grand Gulch drains most of the west side of Cedar Mesa which is a portion of a much larger anticline called the Monument Upwarp. This north-trending structure extends a distance of some 90 miles (144 km) from near Kayenta, Arizona to the confluence of the Green and Colorado Rivers. The eastern margin is marked by the Comb Ridge monocline while the western flank gradually dips into the Henry Mountains Basin. On Cedar Mesa the crest of the Monument Upwarp is mainly aligned with Highway U-261.

On Cedar Mesa the predominant rock is, logically enough, Cedar Mesa sandstone with Grand Gulch being almost entirely within this layer. This light-colored rock layer, up to 1,200 feet (365 meters) thick, was formed from beach and sandbar sand that was left by a sea invading from the northwest during the Permian Era. This sandstone erodes to form vertical cliffs, alcoves, arches, and bridges. Other exposures of this sandstone form the Maze District of Canyonlands National Park and the giant spans of Natural Bridges National Monument.

You may have noticed the bold streaking on the rock. This is called ''desert varnish". It is a thin deposit of an iron-manganese solution which runs down the rock with rain or melted snow. The longer the process has been going on, the darker the streaks. No one knows exactly how the process works, but it requires a hot, dry climate, the proper minerals, and plenty of time. A dark coat of desert varnish might take 2,000 years to form.

The three natural bridges in the Monument are the product of erosion on the Cedar Mesa Sandstone, mostly by the actions of water. As White Canyon was carved down into the sandstone, it developed bends, or meanders. These meanders, with time, become more and more twisted, bending back upon themselves, until just a thin fin of stone separates them. The soft siltstone layers form a weak spot in the canyon wall which flash-flood waters periodically pound against. Eventually, water cuts all the way through the meander, forming a natural bridge. At first the bridge is thick and massive, but erosion attacks it on all sides, the bridge gets thinner, and some day it will collapse.

The reddish brown layers directly above the Cedar Mesa Sandstone are know as the Organ Rock Shale and the Moenkopi formations. These two formations are difficult to distinguish from one another because they are both comprised of fine grained mudstone, siltstone, and marine shales. The Organ Rock Shale tends to form gentle slopes while the Moenkopi usually forms walls and towers. Ripple rock is one of the more distinctive features of the Moenkopi formation.

Above the Moenkopi formation is a whitish strata known as the Shinarump member of the Chinle formation. It is comprised mostly of stream deposited shale, sand, and gravel. It also contains uranium, petrified wood, and both plant and animal fossils of a wetter climate.

The red section above the Shinarump is known as the Moss Back member, yet another member of the Chinle formation. Made up of siltstone, sandstone, and conglomerate, it was laid down by streams. A good example of this strata may be seen just under the top of the Bear's Ears, below the red cliffs.

Above the Moss Back formation is Wingate Sandstone. A very impressive stratum of reddish color, it is composed mainly of small, rounded quartz grains. It is cross-bedded, and cliff-forming. The top of Sundial Butte and Bear's Ears are capped with Wingate Sandstone.

Other strata of more recent geological time were deposited, then eroded long before the arrival of man on the scene. The area is very stable seismically, but the constant, patient actions of erosion - ice, water, wind, plant life - are at work wearing the scenery away.

The erosion of the area is an on-going process. There are at least two sites in the monument where there is strong evidence of natural bridges formed, eroded, and fallen. Likewise, there are other sites in the area of potential future bridges - Natural Bridges in the making.

In the last mile of Grand Gulch, the reddish Halgaito Shale is exposed. This layer was also formed from deposits left on coastal plains. However, the sediments came largely from the Uncompahgre Uplift to the northeast.

Extracted from: BLM-UT-GI-89-007-4333 and Natural Bridges National Bridges Geology pamphlet

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URL of this page: http://bcn.boulder.co.us/environment/cacv/cacvgeol.htm
Revised '9-Jun-2001,11:10:14'
Copyright ©1996, 1999 SCCS.