Tuesday, January 3, 2012

The Delightful Details: Why I love the Grand Canyon

As my readers may know, I made it to the Grand Canyon as my last trip of 2011 where I saw a nice sunset, and a glorious sunrise. It's kind of a yearly tradition to stop at the canyon, as several family members live near by (in the southwest, 200 miles is 'near'; if you get that close, it is worth the diversion!). There are many things to love about a place like the Grand Canyon. For me, on this trip, it was the details. I don't think I had walked out to Powell Point in past visits, but it looks to be one of my favorite South Rim stops now. The point juts out well into the canyon, and the short trail has steep drop-offs on both sides. The view extends a great distance both east and west. It's easy to get lost in the grandeur of the view. This time I was looking closer, at some of the details. In the picture above, look at the center-right, in the deepest part of the canyon. See if you can pick out where I zoomed in on the picture below.
I zoomed closer, and concentrated on the scene below. It was a familiar one for me, as it is repeated many times in the Grand Canyon, as well as several other spots around the southwest. There are two main elements here, a light brown layer of sandstone on the top, about 200 feet thick, and a sheer cliff below made up of metamorphic rock and granitic intrusions. The story told in these rocks is part of what led me into geology; I walked across a surface like this during my first geology field trip in 1976 and was duly impressed. I wanted to know more.
The contact between the two rock sequences is a nonconformity, an erosional boundary between two contrasting rock types. Many people who see the Grand Canyon for the first time might think that the rocks represent some sort of continuous record of deposition, a complete encyclopedia of earth history. This couldn't be further from the truth. The succession of rocks in the Grand Canyon has been interrupted many times by uplift and erosion, and huge gaps occur in the sequence. There are more rocks missing than there are present. The gap shown in the picture above spans something like a billion years!

The sedimentary rock is called the Tapeats Sandstone. It is typical of the kind of sand that might accumulate along a shoreline, although in this case, the beach existed 525 million years ago. It would have been a strange environment to those of us in the world today...there was no life on land, just barren rock and sediment. There was life in the sea, but few species that would be familiar to us. The fossils most commonly found are trilobites, arthropods that are vaguely similar to pillbugs and horseshoe crabs. There were sponges, and some brachiopods (lampshells), and many strange and wonderful soft-bodied creatures who left little in the way of a fossil record (google 'Burgess Shale' if you want to know what they looked like).
The rock beneath is exceedingly complex, and a complete understanding of its origin is still to be determined by further research. We have discovered enough evidence to tell a dramatic story that included deposition of sediments and lavas on a sea floor around two billion years ago, the subsequent crushing and deformation of those rocks, and their rise from the deep crust to form the core of a gigantic mountain range around 1.8 to 1.7 billion years ago. This was followed by intense erosion for hundreds of millions of years, and then renewed mountain-building activity around a billion years ago. These mountains were also worn away until a nearly flat surface remained, interrupted here and there by low hills and ridges of resistant rock.

Then the land slowly sank, and sea transgressed across the landscape. The beach sands of the Tapeats buried all but the highest of the hills, which projected as modest islands. They weren't completely inundated until several tens of millions of years later. That's what I found incredible about the Grand Canyon on my first trip all those years ago. When you know what to look for, you can see these islands exposed in the canyon walls.

Many years ago, the first geologists to explore the secrets of the Grand Canyon called this "The Great Unconformity". I agree...
I place my hand on a billion year gap in time at Frenchman Mountain near Las Vegas. This is the same unconformity as the one seen in the deepest part of Grand Canyon

The rock samples pictured above are to be found along the Trail of Time between Verkamps Visitor Center and Yavapai Point on the South Rim, a recently completed exhibit. The sponsors and innovators did a great job of bringing alive the concept of geologic time.
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