Tuesday, August 20, 2013

Into the Great Unknown: Whodunnit? A Mountain Range Goes Missing

The new day, our second on the grand river, began with a startling sunrise. Sunrises and sunsets in the Grand Canyon are not like other places. One never sees the sun rise above or dip below the horizon, because in the deep canyons, there is no such thing as a horizon. Like Plato's cave, we can only comprehend the sun rising over the Earth's horizon by imagining it from the reflections coming off the adjacent cliffs. If the sun hits camp at sunrise or sunset, someone has made a big miscalculation in choosing a campsite. Staying out of the sun for as long as possible is a continuing strategy during a summer rafting trip on the Colorado.

There was a foreshadowing of today's mystery in the previous post. On our continuing journey into the Great Unknown, rafting the Colorado River through the Grand Canyon, we had passed through three geological formations, the Kaibab, the Toroweap, and Coconino. The first two formed in a coastal environment, while the Coconino developed in a sand dune dominated desert. But we ended our first day on the river in the midst of the Hermit Formation, a striking red-brown layer composed of siltstone and mudstone. The bright red color in such rocks (and the presence of sedimentary structures like ripplemarks) is strongly indicative of deposition in a river floodplain environment.

As we drew deeper into the canyon, we encountered successively older rocks, and they revealed more and more redbeds. Now there were ledges and cliffs of sandstone and other coarser sediments, indicative of a higher energy environment, such as the river channels of large floodplains and desert sand dunes. Clearly a major geologic event had taken place, one that produced a vast volume of sediments over a wide region. For a region of the Earth's crust that had remained remarkably stable for nearly a billion years, this event stood out.

We had entered the Supai Gorge, named for the four formations of the Pennsylvanian-Permian aged Supai Group. Their names are tongue-twisters: the Watahomigi, Manakacha, and Wescogame formations and the Esplanade Sandstone. Where did all these sediments come from, and how do we know? The sediments had to come from a mountain range, but any such mountain ranges no longer exist, having been eroded to rubble many millions of years ago.

Ripplemarks and crossbedding in the sediments show that the rivers were flowing in a general southwest (today's southwest, anyway) direction, meaning the mountains lay to the east or northeast. In southwest Colorado, there are younger rocks, similar to the Mesozoic rocks found around Zion and Capitol Reef in southern Utah. But beneath them where the Paleozoic rocks should be found, there are only ancient Proterozoic metamorphic rocks and intruded granites. These ancient rocks are the roots of the Uncompahgre Uplift, otherwise known as the Ancestral Rocky Mountains.

The origin of the Ancestral Rockies remains somewhat of an enigma to geologists. They coincide in time with the collision of South America and Africa with the southeastern part of North America, an event that ultimately produced the supercontinent Pangea. But the Ancestral Rockies are hundreds of miles removed from the collision zone, so geologists infer a sort of chain reaction in the crust along ancient fault systems. In any case, the rise and destruction of a mountain range in Pennsylvanian and Permian time has provided us today with one of the most scenic and colorful layers in the Grand Canyon.

I came to love two bird species who make their home in the canyon. One of them was the blue heron. We would see them all through the canyon, gracefully soaring over the water, or stepping carefully in the shallows, looking for fish and other prey.

We continued to descend deeper into the canyon. Today's route would take us through a part of the canyon called the "Roaring Twenties", a series of rapids spaced on average only a half mile apart. After the relative paucity of rapids on the previous day, it was a real roller coaster. Remembering the scale of 1 to 10 for rapids in the Grand Canyon, we faced House Rock Rapid (7), North Canyon Rapid (5), Twentyone Mile Rapid (5), Twentythree Mile Rapid (4), Twentythree and One-Half Rapid (4), Georgie Rapid (6), Twentyfour and One-Half Rapid (6), Twentyfive Mile Rapid (6), Cave Springs Rapid (5), Tiger Wash Rapid (5), and Twentynine Mile Rapid (2).

All the boats made it through the rapids without incident, until (I think) Tiger Wash. As we slipped over the tongue of the rapid, I thought I saw one of the boats go vertical, and someone tumbled out. I thought it was my nephew at first, but soon found that it was the boat with my brother and his wife. Somehow they had stayed in the boat, but unbeknownst to them, their oarsman was missing. They heard him say something along the lines of "your oarsman is NOT in the boat". They looked behind in surprise as he clawed his way back onto the raft just in time to get hung up on a boulder. Our raft was closest, so Pete rowed back into the eddy, and we started to climb up the rocks to see if we could assist, but they managed to back off the rock, and we were able to continue on.
Looking up the canyon, we could see that the monsoon storms were not finished. The towering cumulus clouds were wreaking havoc upstream, but the day remained sunny and clear for us. Tonight I would be sleeping under the stars.
We passed another stunning example of mass wasting just before hitting the Roaring Twenties. Boulder Narrows is a huge single chunk of Esplanade Sandstone that practically dams the river (below). According to the river guide, it is the single largest boulder to be found in the river through the Grand Canyon.

The canyon became deeper and more vertical.

At lunch, I was able to concentrate on smaller things, like the wonderfully colored and patterned river boulders.
The Supai Group is one of the thickest units in the Grand Canyon sequence at more than a thousand feet, so it was with us for quite a few miles. I had not been this close to the redbeds in a long time, and the ledges and cliffs made for a scenic stretch along the river. The combination of scenery, geology and rapids made for a memorable day.
There was an otherworldly appearance to the cliffs that rose above us, straight out of the river.

At mile 23, a new layer appeared, hard ledges of gray limestone. We had reached the Mississippian-aged Redwall Limestone. There wasn't a bit of red in it, at first, but that would change before long. I'll save the detailed description for the next post, because today's exposures were just a preliminary tease.

The Redwall Limestone is a true cliff-forming rock, and in places the rock hung over the river.
After the excitement of eleven major rapids and a near miss in the flip department, Pete was a bit tired. We were past the rapids of the Roaring Twenties, so I took to the oars for a couple of miles. I'm not saying I'm a natural oarsman, but the several miles I rowed probably amounted to about a half mile of actual river. I had a devil of a time sticking with the current and staying out of the eddies. Plus, you can see I didn't put on my gloves, so I also got a blister (whine, whine). It was fun!

In the next post: the incredible Redwall Limestone of Marble Canyon.

3 comments:

Gaelyn said...

OMG, that boulder is HUGE. And I thought the lava plug above Lava falls rapid was big. Glad you stayed in the boat even if you got blisters.

intaminag said...

Awesome shots, as always!

feipeng wu said...

Thanks for sharing.
Beautiful landscape.