Our trip last week started in Las Vegas, Nevada, where we paid a visit to an exposure of the "Great Unconformity" which is also seen in the bottom of the Grand Canyon. A participant noted that he had seen Copperfield make an elephant disappear once, but that this week he had seen a geologist make a billion years disappear. That's what is incredible about the Grand Canyon. There are lots of interesting rocks to see, but the story they tell is that a whole lot more rocks are missing. Many times more rocks that we can see.
No part of the crust holds a complete story of the Earth. There simply hasn't been a spot that has been stable enough through the billions of years of geologic time to continually collect sediments. Instead, the story of every place one can ever visit on Earth is that it has been uplifted, eroded, deformed, and subsided numerous times. The Grand Canyon has been unusually stable compared to most, but still there are huge gaps in the story. These gaps where erosion took place are called unconformities. There are at least fourteen of them exposed in the Grand Canyon.
The most obvious kind of unconformity is called an angular unconformity and the eastern part of the canyon exposes one of the most famous examples in the world. It separates the tilted rocks of the Grand Canyon Supergroup from the flat-lying Paleozoic sediments that make up the most visible part of the canyon walls. Sediments more than 12,000 feet thick once covered the region. They accumulated between 1.25 billion and 700 million years ago, but were eventually broken and tilted by faulting. Almost all the rocks were eroded away except for isolated blocks such as those seen in the picture above.
|Photo by Mrs. Geotripper|
|The black lines are disconformities, places where erosion has removed rocks.|