Photo by Michael H. Dickman
When rock layers are visible at roadcuts or mountains, the youngest rocks are at the top and the oldest rocks at the bottom. The rock layers form a continuous record of geologic activity. Those, at least, are the general rules. But sometimes there’s a jump in the ages of the rocks. That can happen if there was extensive erosion of existing rock, or simply no rock formation for some time. Such a gap is called an “unconformity.”
Unconformities of a few million years are not especially rare. However, at the Grand Canyon, Las Vegas, and other parts of the world, you can see one that’s called the “Great Unconformity.” At these places where Precambrian rocks from 1.7 billion years ago are exposed, the next layer of rocks is much newer. At Frenchman Mountain they are only about 520 million years old. That’s a gap of 1.2 billion years. The Earth is 4.5 billion years old, so that unconformity represents more than one quarter of the Earth’s history.
Photo by Michael H. Dickman
In this case, younger sandstone layers sit on top of older igneous and metamorphic rocks.
If you want to see that gap, the “Great Unconformity,” the good news is that it’s readily accessible from Lake Mead Boulevard. There’s a place to pull off the road, and a short interpretive trail to explore. It’s one mile east of the intersection of Lake Mead and Hollywood Boulevards. Look for the (vandalized) sign that says “Interpretive Site 500 ft.”
The bad news is that all the other signs on the interpretive trail have also been vandalized. But if you’re at all interested in geology, it’s still worth a visit. Before going, take a look at UNLV’s website about the Great Unconformity at Frenchman Mountain.
The first time I visited the “Great Uncomformity” there was considerably less litter than one finds up there now and there was a metal plaque that described the geology behind the location with an arrow pointing to the supposed “Great Uncomformity” — The only problem was that although the plaque was scientifically correct, whoever installed the plaque apparently had not been given directions on how to orient the arrow with the strata. The arrow actually pointed to an outcropping about 150 feet to the east of the uncomformity. I found the geologic marker quite puzzling until I found a reference online about the orientation error.
I’ve also visited the uncomformity that exists deep in Black Canyon on the Colorado River in the Grand Canyon.
Mark
Mark, that plaque is no longer there. Unfortunately very little information exists currently at the site itself. But the place is still interesting and there’s a good view of the valley and Las Vegas Strip from the trail. I hope that some day the plaque will be replaced, and that people will respect the efforts others have made to make our world more understandable.
I’m going to check that out next week. I love geologic history, but I didn’t realize we had such an easily accessible example of the great unconformity locally. I saw a show once that said that there were great mountains which rose and completely disappeared during that 1.2 billion year gap. Amazing!
Have you seen the 2001 edition of this book? Geologic Tours in the Las Vegas Area is a must-have guide if you like checking out local geologic history. I’ve used it extensively — It’s a very cool guide!
That book has one of best descriptions of the Uncomformity, I’ve found anyplace. To amplify Michael’s comments above — the contact is between Precambrian metamorphic and igneous rocks with tilted Cambrian Tapeats Sandstone. Joseph Tingley and his co-authors have done a masterful job of describing the strata in the area and it really is a cool thing to have in the car when you go driving in this area.
Mark
I’ve always thought it was NONconformity that was great. I’m happy to learn about great UNconformities, and that we have one right here in Las Vegas.
Great post
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