Keystone Thrust

From my last posting about Mt. Charleston, you saw this picture of badly twisted gray limestone near the summit.  You were surely disturbed by such grossly contorted rocks, but none of you commented on it.  I fully understand your being so upset that no words seemed adequate.  I will attempt to alleviate your dismay.  

Mt. Charleston is in the Spring Mountains about forty miles northwest of Las Vegas.  On the eastern edge of the mountains is a place called Red Rock Canyon, a misnomer, since many colors of rocks appear there, not just red.  The old name was Cottonwood Canyon, and the cottonwoods in this picture typify the name.

Red Rock Canyon

It has been my practice to study the geology of an area before visiting it.  Geology is one of several careers I wish I had pursued, and pursue it now as an amateur.  It is with pleasure and girlish giddiness that I present the answers to some of your most pressing questions, based on the Mt. Charleston hike, on yesterday’s visit to Red Rock Canyon, and on research.

The question that puzzled geologists studying Mt. Charleston was: why is limestone way up here, far above the sandstone at Red Rock Canyon, which is much younger?  Shouldn’t the younger rocks be on top?  Furthermore, in Red Rock Canyon they found the old limestone resting directly on top of the younger sandstone.  In this picture you see gray limestone at the bottom, a young sandstone layer sandwiched, then more of the gray limestone on top.  It is not the way rocks are suppose to be stacked.  

The limestone is pretty easy to date in this region, there being so much of it in so many places.  It formed in the bottoms of oceans and contains fossils of extinct creatures that can be linked in an evolutionary chain from Cambrian to Permian (500 to 250 million years ago).  Back then, there was only one continent—Pangaea.  Everyone seemed happy then, until suddenly, almost all of the many species went extinct—no one knows why. 

Then came the Mesozic Era when California was still under water and Nevada, raised up on a gamble, looked something like the picture.  But the important thing happened below the crust of the earth: the Pacific plate began sliding under the North American plate (subduction), and that was a big deal.

It raised the Sierra Nevada Mountains, and raised Nevada too.  Desert winds blew sand into massive dunes, on which early dinosaurs, Grallators, left their tracks.  It was like today’s Sahara Desert.

The dunes hardened and became white sandstone.  But they did not stay white, for as water percolated through them, it deposited the brilliant reds and other colors seen today at Red Rock Canyon.

But it was subduction, mentioned above, in its relentless pushing, that finally explained the older rocks being on top of the younger ones.  The Pacific plate did not slide smoothly under the North American plate.  No, it pushed hard against it and broke it in many places.  The faults were nearer to horizontal than vertical, and older rocks were thrust above younger ones as shown in these four diagrams.  

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As layers were pushed ever higher, even higher than Mt. Charleston, erosion steadily cut them down.  In the picture at the right you see a diagram showing how rocks in the picture above, the one with the red stripe through it, came to rest in their odd positions.  We call it the Keystone Thrust.   It was all this pushing and bending that contorted and lifted the rocks near the summit of Mt. Charleston seen in the first picture.

These rugged mountains along the Colorado River near Boulder Dam (picture at left) moved here only recently.  They were a volcano, thirteen million years ago, and rested on top of Wilson Ridge, which you see, looking in the opposite direction, (picture on  right).  It took them only that long to get here.  They moved along a thrust fault, like the Keystone Thrust, at about 1/16 of an inch per year.  I thought the Wilsons were faster than that.