Existing comment:
Window on the Past:
The high gray ridge that fills the skyline across the valley in front of you is made of the oldest rocks exposed in the park. Fossils in these rocks place them in the Paleozoic Era, between 250 and 550 million years ago. The red rocks are younger, laid down in the Mesozoic Era, between 250 and 66 million years ago.
[The gray mountains under the "N" in "ON".] An Ancient Sea: 550 to 250 million years ago:
Almost 600 million years ago, the sea invaded this area. It advanced and retreated many times for another 400 million years. Most of the gray rocks that you see in front of you are Paleozoic limestones and dolomites made up of remains of the countless plants and animals that once flourished in this warm, shallow sea.
The gray mountains that you see here are composed of deposits laid down on this ancient ocean floor during the course of several hundred million years.
Fossils of the algae, trilobites, crinoids, corals, brachiopods and numerous other plants and animals that lived in this sea can be found today in these gray mountains. The drawing shows a 500 million year old reef constructed by a group of fossil spongelike organisms called archaeocyaths.
The sea is gone now. The massive deposits if left behind slowly wore away under the relentless forces of rain, snow, heat and wind. From the window to the Muddy Mountains across the valley is a distance of about three miles in space, but five hundred million years in time.
[The red hills on the right.] The Vanishing Sea: 250 to 66 million years ago:
The red sands that you see in front of you were laid down about 200 million years ago during the Mesozoic Era. Then the sea retreated for the last time, and this region became a gigantic plain of sand, silt, clay and gravel. Sluggish streams meandered across the land.
Wind and rain, heat and cold operated on the exposed ancient sediments, wearing them away and forming beds of mud, silt, sand, and gravel. For tens of millions of years, a desert even more severe than today's covered this region. Strong winds blew thousands of feet of sand from distant highlands, creating huge dunes. These ancient sand dunes, now "fossilized" and eroded into exotic shapes, are the spectacular red and white formations of the Valley of Fire.
The original sequence of all these deposits is shown in the geologic column on this panel. But -- and most important to understanding the view across the valley -- this sequence was changed about 100 million years ago, when older materials were shoved up on top of younger ones, creating the Muddy Mountains. At the same time the thick sandstone deposits in front of the Visitor Center warped upward and eroded, exposing the soft shales of the ancient sea bed underneath. These quickly wore away, creating the broad valley if front of you along which the park road now runs.
Geological Terms:
* Paleozoic Era: 550 million years to about 250 million years ago.
* Mesozoic Era: 250 million years to about 66 million years ago.
* Cenozoic Era: 66 million years ago to today.
Although the remnants of the Paleozoic thrust plate are visible in the hills, only Mesozoic rocks are present in the valley.
The upper plate of the Muddy Mountain Thrust has been eroded away in the Valley of Fire area.
Approximately 10,000 feet (3 km) of deposits have been lost to erosion over the past 70 million years.
[The little crevice under the second "W" in "WINDOW"] Muddy Mountain Thrust: 70 million years ago:
The mountains across the valley are evidence of a major movement of the earth's crust. During the Paleozoic Era, an oceanic plate slide eastward beneath the western edge of North America along the coast of California. The resulting compression in western North America caused thin slabs of gray Paleozoic rocks to be shoved many miles eastward, over the younger red Mesozoic rocks.
This Muddy Mountain Thrust Fault, shown here in cross section, reveals the reaction of local geologic deposits to the powerful pressure placed at the edge of the continent hundreds of miles to the west. Geologists now know this thrust is a continuation of the Keystone Thrust exposed in the Red Rock Canyon area west of Las Vegas.
The Muddy Mountain Thrust is a very complex fault. In some places in the Valley of Fire it is nearly horizontal, while elsewhere it is almost vertical. Across the valley you see gray dolomite in contact with red sandstone. This high-angle contact, a nearly vertical segment of the Muddy Mountain Thrust, is sometimes called the Arrowhead Fault.
Geological Terms:
* Fault: Break in the earth's crust along which movement occurs.
* Plates: Large pieces of the earth's crust which move in different directions from one another. Most mountain-building events occur at plate boundaries.
* Thrust Fault: A gently sloping break in the earth's crust caused by compression. In many cases, such as the Muddy Mountain Thrust, older rocks are pushed on top of younger rocks, reversing the normal order.
A Complex Fault: 70 million years ago:
Look out the window. Across the valley, the gray rocks you see are much older than the red ones, and until about 70 million years ago, the gray materials lay deep below them. Then compression caused by the interaction of plates off California caused the earth to move, here and the natural order was reversed. This thrust is a very complex fault which geologists still don't understand fully.
Erosion: Nature's Tool: 70 million years ago to now:
70 million years or more have passed since the Muddy Mountain Thrust Fault built the huge range across from the visitor center. In that time, approximately 10,000 feet (3km) of deposits have eroded away.
Aztec Upwarp:
During the same event that formed the Muddy Mountain Thrust, the Aztec Sandstone in front of the visitor center warped upward, then gradually wore away. The soft shales of the Mesozoic bed were exposed and eroded, creating the broad valley in front of you.
Only a Remnant:
The column on your right shows all of the geological deposits laid down by the ancient seas which created the Valley of Fire, after they were acted upon by the Muddy Mountain Thrust. To your left, the deposits are shown in sequence before this thrust forced the older materials on top of the younger ones. Nearly two miles of rock already have worn away.
Erosion:
Imagine this landscape completely covered by another 10,000 feet of rock. Such a layer is nearly as thick as the distance between the Visitor Center and the top of the mountains across the valley. After 70 million years of erosion nature is still not finished here,. What do you think the final picture will look like?
Erosion is the process of breaking own the removing materials. Agents of erosion may be mechanical (water, freezing, and thawing as snow and ice; wind; pressure from plant roots, animal burrows) or chemical (breakdown of rocks by changes in their chemistry, such as the oxidizing or "rusting" of iron in rocks).
The Valley Today and Tomorrow:
The geologic forces that act upon our earth are slow but irresistible. Park visitors will see only small changes, such as the crumbling of a cliff face. But all the time, mostly unnoticed, there will be pressures that build mountains and erosion to wear them down.
Hundreds of millions of years have witnessed the creation of the valley of Fire, one of the most geologically interesting and scenic areas in Nevada. Preservation of this magnificent area for future generations to appreciate in its natural state is a challenge to the sensitivity and responsibility of every visitor to this desert wonderland.
In this drawing of the Valley of Fire today, the Muddy Mountains across the valley are shown at the left, and the brilliant red remnants of ancient sand dunes behind the visitor center are located at the right. The major, open valley between them was carved out as erosion weathered away the Aztec Sandstone covering the soft shales beneath. |