Monday, September 2, 2013

Into the Great Unknown: The Aftermath of Chaos...Finding Beauty in the Oldest Rocks of Grand Canyon

If you've been following the story so far, you will know that we reached day nine on our rafting journey through the Grand Canyon, and that day nine was a bit of a disaster for yours truly. Our raft had flipped in Crystal Rapid, the worst rapid on the river, and I took a very long swim in frigid turbulent water before being plucked out by my guardian angels (Barry, Bev and Jeff). We made it through eight more rapids that day without incident, and made camp at a narrow strip of sand called Hotauta.

As I noted in the previous post, my camera was badly damaged (I had a spare in storage), and my trip journal was soaked. I spent the evening carefully separating and drying the pages, hoping that I could recover most of them. I also had to deal with the psychological aftermath. Being in an unexpected ride down a violent river without my boat might be an expected part of river running (and certainly no one was saying it was an easy thing to do), but I'm a desk-bound professor most of the time. This was something very new for me.
The best thing I could imagine would be to have a peaceful and serene day on the river, and the Colorado seemed to understand this: there were ten rapids in our path on day 10, but only one of them, Walthenberg (6) was greater than 5 on the rapids rating scale. And it came early in the day. We had three excursions planned in the side canyons, and they sounded like wonderful places.
It seemed a good sign to have a big Swallowtail Butterfly land on my gear as we were loading the rafts. It seemed a bit disappointed that my bag wasn't the biggest darn flower ever, but it hung around long enough to get a picture.

We went through the first two rapids, Bass and Shinumo, and pulled out at Shinumo Creek.
The walk was short, but quite interesting, as the creek filled the canyon bottom. I was absolutely sure I was going to slip on the muddy rocks and destroy my remaining camera. But it was just beautiful, and the fifteen foot waterfall was a refreshing retreat from the hot sun.
We headed back onto the river, and soon passed the creatively named 113 Mile Rock, an outcrop of schist that practically blocked the river. It also marked the halfway point in our 226 mile trip (it was day 10 of 16, so our average daily mileage was going to increase).
Since Walthenberg Rapid, we had been traveling through the oldest rocks to be found anywhere in the Grand Canyon region: the Elves Chasm Gneiss, dated at 1.84 billion years. These rocks may represent the ancient crust on which the other metamorphic rocks were emplaced many tens of millions of years later. I hate to say it, but I was distracted that day and didn't realize that we were passing through these rocks until a day or two later when I retrieved my geologic map out of the luggage. But I managed to snap a number of pictures because the rocks were intriguing to look at whether I knew their age or not.
 In many places the rocks are intruded by pink dikes and veins of granite pegmatite, a rock with exceedingly large crystals of feldspar, quartz, and muscovite mica (above). In a few places I could make out darker intrusions that looked like basalt (which is youngest in the picture below: the black or the light colored intrusions?)
 Before Glen Canyon dam was built the Colorado River carried an incredible amount of silt and mud. According to some sources, when the river used to run at 100,000 cubic feet per second, half of what flowed down the river was sediment (Death in the Grand Canyon by Ghiglieri and Myers). The sediment gives the river the tools needed to sculpt the incredibly hard rock, and we passed numerous beautiful exposures of intricately shaped gneiss, schist and granite.
We were still in the Granite Gorge, but we noticed that the inner canyon wasn't as deep as it had been, and we started to see Tapeats Sandstone in the cliffs not too far above us.
As we neared Elves Chasm, the Tapeats was at river level, and we were treated to an exposure of the Monument Monocline in the sandstone layers. A monocline is a fold in the rocks that looks like a carpet thrown over a step: level horizontal rock, then a flexure as the rock bends downward, and then horizontal layers again. They tend to occur when a fault fractures harder rocks at depth, but only bends the sedimentary layers above.

The folds were an unexpected sight. So were the travertine deposits that were exposed along three miles of the river starting at Mile 116. Travertine is generally composed of calcium carbonate (the mineral calcite) which was leached out of the overlying Redwall, Temple Butte, and Muav formations and deposited by springs in the Bright Angel Shale. The travertine completely covered the Tapeats in places, including the area around Elves Chasm.
What is this "Elves Chasm" that I keep mentioning? It was our next stop, but that will be in the next post!