Dreams of Summer and Southwest Travels: Grand Stories Exposed in a Canyon

No one place on Earth can ever tell the whole story of the Earth. But there are lots of places that tell part of the story. That's the fact that makes geology one of the most fascinating sciences there is. It's an incredible detective story that must be pieced together from disparate bits and fragments that must be correlated and organized into a coherent narrative. Some places tell more of the story than others, and some places do it in a fashion that is awe-inspiring. Grand Canyon is one of those places. It is one of the greatest places on the entire planet to teach the basic principles of geology. The geology is in your face, exposed perfectly, largely free of soil and vegetation, and yet the canyon is still a place of mystery and unsolved questions.

We are in the depths of winter this week, so I've been traveling back through the photo archives for a look at warmer times, our field studies trip to the Colorado Plateau last summer. We spent two days on the South Rim of Grand Canyon, which barely gave us time to scratch the surface of the fascinating place.

For me, a nice moment came while staring into the depths of the canyon from Lipan Point. The view, seen in the first picture, reveals a labyrinth of tributary canyons, but the Colorado River is visible in the center of the photo. I zoomed in (photo two). What's remarkable about this one spot is that it is one of very few flat open areas on the floor of the Grand Canyon. It's called the Unkar Delta, and I have wonderful memories of an exploration there two years ago.

The "delta" (really kind of an alluvial fan) formed as debris poured onto the canyon floor from Unkar Creek. It is one of the few arable spots in the depths of the canyon, and the Ancestral Puebloan people utilized the soils to grow food there for more than three hundred years, from about 850 to 1200 AD. We stopped there during my one and only rafting trip down the Colorado River 813 years later in 2013. I wrote about the spot in my blog series "Into the Great Unknown". It was around 112 degrees as I explored the ruins that dot the delta, while the rafters scouted Unkar Rapid. The rapid is one of the first of the big rapids one encounters in the canyon, rated as high as 7 out of 10 on the difficulty scale (we made it through without incident other than getting wet, which was a relief in the sweltering heat).

The deep maroon color of the canyon walls at Unkar Delta reveals an interesting period of the Earth's history. The rock is part of a layer called the Dox Formation, which was deposited in estuaries, tidal flats, and deltas during the Mesoproterozoic era, just over 1.1 billion years ago. The rock is mostly composed of easily eroded shale and siltstone, which explains the open aspect of this part of the canyon. The Dox also contains fossils.

Life existed on Earth a billion years ago, but it was only of the simplest forms, algae and bacteria. The algae grew on pebbles in tidal flats, and as the tides ebbed and flowed, mud stuck to the algae-covered surfaces eventually forming layered structures called stromatolites. The stromatolites seen in parts of the Dox formation are among the oldest fossils found in the American west.

The Dox is part of a larger sequence of tilted layers in the Grand Canyon called the Grand Canyon Supergroup. The Supergroup is upwards of 12,000 feet thick, more than twice the depth of the canyon itself. How do the rocks fit in the canyon? They've been tilted and subsequently eroded. The exposures can be followed for a number of miles along the river in the depths of the canyon, but they have otherwise been covered by 4,000 feet of later (Paleozoic) sediments deposited between 540 and 250 million years ago. The rocks are fascinating to study, but the only way to do it is to hike to the bottom of the canyon, or raft the river (I've had the privilege of doing both). They can easily be seen from viewpoints in the eastern part of the canyon on both the north and south rims.

The Grand Canyon region exposes more than sedimentary rocks. There are numerous faults and folds throughout, including the unique monoclines, folds that are draped over step faults. One can be seen as the dark ridge in the middle of the photograph below.  Volcanism is a part of the story of the Grand Canyon as well. The peaks on the skyline in the same picture are the San Francisco Peaks, a group of cinder cones and a very large stratovolcano. Humphreys Peak, the high point on the rim, is the highest mountain in Arizona at 12,633 feet (3,851 meters). Prior to extensive erosion, the peaks may have exceeded 16,000 feet. The origin of the this massive volcano is somewhat enigmatic. The volcanic field is youngest on the eastern side, suggesting a possible origin as a hot spot in the mantle.

The volcanoes are more easily explained in the western Grand Canyon, far away from the tourist haunts. Crustal stretching has caused extensional faults to form (normal faults), allowing magma to rise from the underlying magma. Some of the lava flows spilled over the rim into the canyon, forming gigantic lava dams that produced giant lakes that extended hundreds of miles upstream.

The Paleozoic rocks (541-251 million years) of the Grand Canyon are mostly horizontal layers, and include limestone, sandstone, siltstone, and shale recording the transgression and regression of shallow seas across the region. A mountain-building episode took place in the region around 300 million years ago. The bright red and brown sediments of the Supai Group record the erosion of the Ancestral Rocky Mountains, and deposition in floodplains and deltas in the Grand Canyon region.

The origin of the canyon itself, and the carving of the rocks by the Colorado River perhaps are the biggest mystery for those who study the geology of the region. The river carved the canyon, but it's not entirely clear which Colorado River did the work! Different parts of the river system formed at different times, some many tens of millions of years ago, and other parts only 4 million years ago. It's a complex story, far beyond the scope of this short post, but I highly recommend the book by Wayne Ranney on the subject, available here: https://www.grandcanyon.org/shop/online-store/geology/carving-grand-canyon-evidence-theories-and-mystery-2nd-edition-wayne.

We took a break at Desert View at the east end of the canyon, and headed down the highway. There was a lot more to the Colorado Plateau than the Grand Canyon, as spectacular as it is.

Into the Great Unknown: In the Depths of Grand Canyon, There are Three More Grand Canyons...Checking out the Supergroup

It was now day six of my journey down the Colorado River through the Grand Canyon, a trip I've been calling Into the Great Unknown, in honor of John Wesley Powell and his crew, who first navigated the river in 1869. We had floated down 65 miles from Lee's Ferry, and had now worked our way through 4,000 feet of Paleozoic sedimentary rocks that told a story of transgressing seas, tropical reefs, the growth of mountain ranges in strange places, vast sand seas, and once again a transgressing sea. It is an incredible story encompassing 300 million years, but it turns out these layers are only a small part of the whole story of Grand Canyon.

On this day we reached the hidden depths of the canyon that reveal a sequence of rocks three times as thick as the Paleozoic section. These rocks record another 550 million years of history in the time prior to the Paleozoic Era.  The rocks are known collectively as the Grand Canyon Supergroup and they range in age from 1.25 billion years to 700 million years, part of the time period we call the Proterozoic Eon.

We set out from our camp at Carbon Creek and floated past exposures of the Dox Formation, which formed in river floodplains and coastal complexes. It is more than 1,000 feet thick.

Dark cliffs appeared on both sides of the river. We had reached the lava flows and intrusions of the Cardenas Lavas which were erupted just over a billion years ago on top of the Dox Formation.

How does one fit 12,000 feet of sediments and lava flows into the bottom 1,000 feet of the Grand Canyon? The rocks are tilted! In the picture below, the Tapeats Sandstone can be seen as a horizontal layer at the top of the cliff. Beneath the Tapeats are tilted layers of black Cardenas Lavas and reddish Dox Formation. The eroded surface that forms the boundary between the two sequences is called an angular unconformity. The next question might be "how did they get tilted?".

Moments later we had an answer: the rocks of the Grand Canyon Supergroup were tilted by major faults that formed when the North American continent was breaking away from Australia and Antarctica in latest Proterozoic time. We passed one of these major fault lines, the Butte Fault, near mile 66. The sedimentary layers have been dragged to a nearly vertical orientation (below).

The character of the canyon was changing once again (as I've mentioned previously, there is no single Grand Canyon; it was different every day and practically every hour). For nearly a week we had been in a deep gorge with vertical walls hundreds or thousands of feet high. Now the canyon was much wider and more open.

We saw one of the truly rare evidences of human activity from our vantage point on the river. In the picture below one can barely make out the Desert View Watchtower (just left of center on the rim). The tower is three or four stories high, and yet was barely visible among the high cliffs. The tower is one of the main tourist centers on the South Rim of Grand Canyon National Park. I thought of how many times I had stood at Desert View looking down and wondering what it would be like to float down the river. This week it was finally happening, after a wait of forty years!

We reached Tanner Rapid and pulled over to hike up to some petroglyphs on boulders that had tumbled down the slopes of the Dox Sandstone. The openness of the canyon was refreshingly different.

To many people, the canyon was not the "Great Unknown". It was their home.

I always wonder what was being said by the individuals who chipped these messages in the rock.

It was a hot day. I had the outdoor lead for the thermometer in the river and the indoor lead on the boat. 113 degrees! Yikes. We decided against a hike at lunch near Tanner for lack of shade. We went a bit farther down the river where we needed to scout Unkar Rapid.

The Unkar Delta was one of the largest settled areas along the Colorado River in Grand Canyon. The sites were occupied between 850 and 1200 AD, some 350 years! They were partly excavated in the late 1960s, and the park service has constructed a trail that visits many of the sites.

Despite the heat, I hiked the whole loop. It was hard to think of this barren area as a possible "home", but it was probable that the inhabitants moved higher up in the canyon in the hottest times of year (and conversely this would be far more comfortable a home in winter).

Meanwhile, the oarsmen were scouting Unkar Rapids (6 on a scale of 10). It looked rather ferocious, but these rafters know their craft, and the run was splashy and fun.

Ron in the other raft was pointing towards the North Rim. I looked up and saw for just a moment the arch of Angel's Window, a popular viewpoint on the Walhalla Plateau.

I've stood near Angel's Window many times, looking at the Colorado River framed in the arch (the picture below is from a trip in 2012). Look at the size of the people on the arch and consider how small they are from a river point of view! When Coronado's troops saw the Grand Canyon for the first time in 1540, they thought the river was only six feet across. They were only off by 200-300 feet! The Grand Canyon can have a huge effect on one's sense of scale.

We pulled into our camp at Upper Nevills. The river was wide and calm at the campsite, and there were nice rock ledges that made cleaning and laundry convenient. I also for the first time decided to actually test my personal flotation device and did a dunk in the river. At 52-53 degrees, the river was COLD! I commented in my journal that I was glad I hadn't been involuntarily dunked in the river yet (literary note: this sentence is an example of foreshadowing...).

Tomorrow was a big day. We were facing some of the monster rapids, including Hance (8), Sockdolager (7), and Grapevine (7). I hit the sack.