Saturday, August 24, 2013

Into the Great Unknown: Looking for the Rivers Within the Rivers of Marble Canyon

The Grand Canyon has many moods. So many of my pictures show a canyon bathed in bright sunlight and vivid colors, but twilight often revealed softer tones. The picture above is the view from my campsite at Buck Farm Creek at dusk on the third night of the trip.

We continued our journey into the Great Unknown of the Colorado River. On day four we were expecting to make for Nankoweap Creek about 11 miles downstream. For me it was the chance to find the "river" within the river. That's not some kind of philosophical quest. It was a geological thing, to be explained shortly. But before that, we did some exploring with the extra time afforded by a short distance on the river.

As we often did, we explored the area around our camp while taking advantage of the shade. We headed up Buck Farm Canyon just to see what could be seen. There were a lot of large fallen boulders of Redwall Limestone littering the lower slopes.
Buck Farm quickly narrowed as we climbed up the canyon. We had reached deeper into the crust and were now walking on ledges of Muav Limestone, which in most places underlies the Redwall Limestone. It seems simple to say that one layer sits on top of another, but there is just a bit of time difference between the two layers: the Muav dates from the late Cambrian Period, while the Redwall is Mississippian in age. More than 150 million years elapsed between the deposition of the two layers. That's the same time period that separates the Jurassic (and her many dinosaurs) and our own current era. What happened here?
We climbed over the dry talus slopes and eventually discovered a verdant canyon and even a small clear trickling stream. It was a nice little explore, but now the sun was shining down on us, furiously hot. We headed back to the river.
It was another day remarkably free of large rapids, with only President Harding Rapid (4) standing in our way. As we drifted down the river, more gigantic caverns could be seen in the Redwall cliffs. I think the alcove below is called the Royal Arches.
We also continued to see evidence of recent mass wasting activity along the river. The rockfall seen in the picture below shows a truer color of the Redwall Limestone. It's gray on fresh surfaces, but is often stained by iron oxides leaching out of the overlying Supai Group.
Mile 43 revealed an odd sight. A thousand years ago, the Ancestral Puebloans constructed a foot bridge across ledges of Muav Limestone. From river level we could see no sign of a trail or possible route on either side of the bridge, but there it was. We joked that it was a ruse designed to catch their enemies. They would climb up there to see where it went, where they could be picked off at random.
Some of the Cambrian layers in the Grand Canyon are greenish in color, especially the Bright Angel Shale. The color comes from a mineral called glauconite. The Bright Angel is composed of what used to be ocean-floor muds, and generally forms slopes. Where freshly carved by the river, it formed the ledges seen below.
At Mile 47, we reached Saddle Canyon. I kind of awoke from my entrancement and prepared to stroll up the gentle tributary valley. I asked about the cut in talus slopes to the left of the canyon. "Oh, that's the trail" they said, "the canyon itself is far too steep to go climbing in". Oh geesh, this wasn't going to be at all that easy! We started climbing the incredibly steep talus slope, up through several hundred feet of Bright Angel and Muav layers.
 As always, though, the view of the river was wonderful.
 The "trail" leveled briefly. It was wonderful not stepping up and over the large boulders.
The stream valley rose to meet the trail, and suddenly we were in a merry little paradise. It's amazing what a little bit of water in the desert can do. Life was everywhere.
We reached the small waterfall and the lusciously cool pool at the base. My picture doesn't have people in it, but I can assure you that every one of us submerged ourselves under the water.
At this time of year I didn't expect to find many flowers, but in the cool moist micro-climate of Saddle Canyon there were some beautiful columbines, one of my favorite flowers.
 I spoke of a river within a river in the title. I was looking for something I had never seen before: the Temple Butte Formation. The Temple Butte is Devonian in age, around 400 million years, which puts the formation between the overlying Redwall and the underlying Muav Limestone. But it isn't a continuous layer. The Muav Limestone was exposed to erosion and eventually developed a series of river channels that ultimately filled with limestone as sea level rose in Devonian time. In other areas, the limestone covered the whole landscape, but after its deposition, the Temple Butte in eastern Grand Canyon was eroded again, leaving only lenses of the formation in the 400 million year old river channels. There are ancient rivers in the depths of the modern river canyon.
Walking out of Saddle Canyon, I spied a really nice example of the purplish Temple Butte between the Redwall and the Muav. If you can't quite visualize what I've been describing, I annotated the picture below. I missed the formation on my previous trips in the canyon, but once I saw it and knew what I was seeing, I began seeing it everywhere!
We continued to float down the river for four miles to our camp at Little Nankoweap Creek. The canyon was beautiful beyond words. Pete let me row again, and I even traversed two gigantic rapids. Okay, they were riffles. Okay, they weren't even that...they were piffles. But I did manage to navigate the raft into camp!


Celia Lewis said...

Another beautiful day ... clear photos, and I am very much enjoying your expertise/teaching as you describe the layers all around you. Awe-inspiring. The 'bridge' remnant was a definite surprise.

Gaelyn said...

The Zuni believe that bridge was made by their ancestors. It is such a joy to see this part of the river through your excited eyes.

intaminag said...

That Temple Butte info is awesome. It's amazing to see the level of understanding required to fully reveal the history of these old rocks--their intricacy is astounding.

DSchwartz78 said...

Out of curiosity, many years later, did you look to the other side of the river to see if the fossilized channel continued?

Garry Hayes said...

I did not on that day (it was so blazing hot!), but others have carefully mapped all the exposures, and produced paleogeographic maps showing the locations of the rivers and delta channels. I'm sorry I can't lay my hands on the references, but they should be googleable.