RocksTalk – Canyon Musings

Rocks are perhaps our oldest historians

Bright Angel Creek runs through Bright Angel Campground at the bottom of the Grand Canyon. I sat with my back against a boulder, cool water rushing around, forming a pool. Small fish tickled my ankles.

I scooped a handful of pebbles and small rocks, noticing their colors and textures. A purple and green rock, both rounded and jagged, had a raised image that looked like a buffalo and a flat marking that looked like a pronghorn.

“Where did you come from?” I asked the rock. “Has it been a long journey?” I looked at the canyon walls and all their colors. “It took me a few thousand miles by air and 2 days by foot to get here,” I said. “I suppose your story is quite different.” I thought of the Big Bang. “I guess we all come from stardust, so perhaps not so different after all.” I set the rock down where my hand had found it, grateful that I was a witness to its journey through time.

I looked again at the tall cliffs all around me. I saw the canyon differently. And I began to study it.

Like a lot of people, I started with memorizing the layers.

Know The Canyon’s History, Study Rocks Made By Time, Very slowly

The Yavapai Geology Museum sits at the Canyon edge on the South Rim. There I learned that the Grand Canyon Supergroup sat at an angle, sandwiched between horizontal layers above and the basement rocks below.

Something called the Great Unconformity had to do with younger flat layers sitting on top of the older Vishnu Schist & Zoroaster Granite. All this thinking called for ice cream.

Layered Paleozoic Rocks (millions of years)
Kaibab 270
Toroweap 275
Coconino 280
Hermit 285
Supai 290 – 320
Redwall 340
Muav 504
Bright Angel 506
Tapeats 508

Supergroup – Chuar
Kwagunt 729 – 753
Galeros 755 – 770
Nankoweap 775

Supergroup – Unkar
Cardenas Basalt 1104
unnamed diabase sills & dikes (Yi)
Dox (Yd) 1120
Shinumo 1130
Hakatai 1230
Bass 1255

Vishnu Basement
Granite Gorge Metamorphic Suite:
Vishnu Schist 1750
Brahma Schist 1750
Rama Schist & Gneiss 1751
Zoroaster Plutonic Complex:
Quartermaster Granite from later collisions causing crustal melts (Yg) 1375
Later granites and dike swarms – from collisions creating crustal melts (Xgr) 1662-1698
Granodiorite – from collisions of island arc magma chambers (Xgd) 1713-1740
Elves Chasm pluton – from Mojave microcontinent magma chambers (Xec) 1841

Rock story

There is a certain wildness in the lower rocks of the Grand Canyon. I wanted to understand what was happening. What did it mean that there is 1.2 billion years of missing geological time? Where did it go?

This question got me curious about parts of the canyon I had not hiked. I needed a better understanding of the Grand Canyon Supergroup rocks.

My safest route through the Supergroup would be down the North Kaibab Trail. I would have to step up my training and lighten my load.

After 6 months of intensive preparation, I started my fourth backpacking trip to the bottom and my first Rim to Rim, North to South.

After 5 miles of steep descent I was standing on the footbridge at Manzanita where the trail crosses Bright Angel Creek. I looked down creek. I should be looking directly at the Dox Formation, the uppermost exposure of the Supergroup along North Kaibab Trail.

Upstream, I could just make out a rock bed where Manzanita Creek empties into Bright Angel Creek. I didn’t know it yet, but creek confluences would turn out to be important, for just below them are the uppermost exposures of Supergroup layers.

From my perch at Manzanita I didn’t see anything that screamed Dox. I took photos and a video. In the glaring sun I could not check the images. Maybe I would get lucky and find it when I returned home.

The canyon was heating up. My hamstrings felt like jelly. I needed to press on. Eventually I hobbled into Cottonwood campground.

A few months after my return I went through my photos. At first I didn’t see it, but then I zoomed in: a small bit of tilted brown rock. Could it be?