Gray clouds linger in the sky like battered marshmallows as we motor our way through river mile 60 on the Colorado River. My feet are soggy raisins inside my wet neoprene booties and rubber river shoes. My nose drips like a sunburnt popsicle in July. It has been raining intermittently sideways for the past ten river miles and my waterproof gear seems to have sprung a few leaks. My core is a shuddering block of slush. As instructed by our flip-flop-wearing guides, I am wearing my bathing suit under multiple layers of thermal wear. Now they tell us it is time to go swimming. Really? The last thing I want to do is plunge my hypothermia-verging self into that frigid river. Deep inside my cold-addled brain I imagine a toasty sauna or (ooohh!) hot spring materializing somewhere nearby.
The Tapeats Sandstone at river level (click on any pic to enlargenate)
Good luck with that, eh? This isn’t Yellowstone, after all. It is early May in Grand Canyon with not a hot spring or sauna in sight. Fortunately, my attention is diverted from the cold. The 540-million-year-old Tapeats Sandstone has begun to poke its head above the water line.
An interesting geologic relationship exists between the layers of the Tonto Group through which we have recently descended – first the Muav, then the Bright Angel, and now the Tapeats. Together these three interrelated formations comprise the classic sequence of sandstones, shales, and limestones, revealing a textbook example of the cycle of transgression, or advance, of a shallow sea across an ancient continent.
How do we know this stuff?
Hundreds of millions of years prior to deposition of the Tonto Group sequence of sediments, Precambrian mountains in the area of our future Grand Canyon had been uplifted to heights rivaling the Andes, only to bevel down to their metamorphic crystalline roots by the forces of weathering, erosion, and gravity. Today, we know these roots as the metamorphic schists and gneisses of Granite Gorge, the deepest part of the Canyon.
Around 540 million years ago during Early Cambrian times, these roots lay exposed on the surface. They had weathered, eroded, and gravitated into a barren flatness broken only by remnant blocks of crystalline basement or other uplifted blocks of the Grand Canyon Supergroup (Wait. What? Who said anything about a Supergroup?). This stable core of the North America continent, or craton, lay above sea level. There is no evidence of vegetation that would have slowed erosion. It would be millions of years before trees, ferns, and flowering plants appear on the scene. It was upon this barren, beveled landscape that the Sauk marine transgression moved in from the west, a shallow sea encroaching onto the continent from the deeper continental shelf and slopes offshore.
Geology is full of concepts and models indicating a particular environment. One of these is called the facies model, which states that as we move farther away from land and deeper into a body of water, sediments go from coarse to fine, larger to smaller. The coarsest-grained sediments such as sandstones are deposited in a nearshore environment along the beach or strandline, while finer-grained sediments such as mud are deposited a bit farther offshore. Farthest from land are the finest-grained sediments, which are the carbonates, or limestones. It is significant that, during a transgression or advance of the sea across the continent, these sediment layers also migrate inland.
Tapeats Sandstone, the lowest layer in the Tonto Group, is at water level
Within the Tonto Group, the earliest (lowest and oldest) Tapeats Sandstone represents shoreline sediments of the Sauk Sea, deposited along the western margin of the beveled craton. Over time, as the sea slowly transgressed eastward across the continent and became deeper over the Grand Canyon region, the strandline of the Tapeats also migrated eastward. Deeper water, finer-grained muds of the Bright Angel Shale were deposited on top of these older sandstones. As the sea continued its transgression across the continent, the water eventually became deep enough for Muav Limestone carbonates to be deposited above the Bright Angel Shale.
Thirty million years
So that is how we know “this stuff,” this fascinating story told by the rocks of the Tonto Group. Sandstone, shale, limestone. Tapeats Sandstone, Bright Angel Shale, Muav Limestone. Older beneath younger. A textbook transgressive sequence.
There is one more thing to consider with these rocks. They are time transgressive. The same formation can be a different age, depending on when and where it was deposited. As the Sauk sea moved eastward through time, so did its sediments. Sands, muds, and carbonates in eastern Grand Canyon would have been laid down at a later time than those in western Grand Canyon. The Tapeats, Bright Angel, and Muav would each look similar from one location to the next, but their ages would differ significantly the farther east they go.
This cycle did not happen overnight. It took the Cambrian sea up to 30 million years to finish what it had started, to inch its way back and forth over what would one day become Arizona. The contacts, the boundaries, are not often clear, but the outline is there.
Stop for a spell at the Little Colorado
Anyone for a swim?
The Little Colorado joins the Colorado River on top of the Tapeats Sandstone. The guides bump our boats into the bank and tie down just upstream of the confluence. We disembark onto the muddy shore, thrashing through the underbrush in the drizzly rain. I huddle briefly with some of my compatriots beneath a sandstone ledge. When I notice that my camera is not slung around my neck I make a mad dash back to the boat to get it, and soon realize that the rain has stopped and that the sun is trying to break through the clouds.
The Little Colorado River
The Little Colorado is a vivid swath of turquoise reflecting suspended particles of calcium carbonate. It puts me in mind of the steaming terraces of Mammoth Hot Springs in Yellowstone except for the absence of steam. I hope for warm but know better. I dip my finger into the blue milkiness and am slightly disappointed. This water is stone cold.
The Little Colorado cuts through the Tapeats Sandstone
There is no hurry so I take my time. It’s nice to get out of the boat and go for a walkabout. Random rays of sunshine do their best to fend off the chill. After a while I convince myself that I am sort of warm. Actually, it is so beautiful alongside the river that I forget I was ever uncomfortable.
Seeking respite from the crowd of rowdy rafters, I walk along the sandstone ledges and have a pleasant conversation with the voices in my head.
Does anyone else notice there is no elevation or other identifying label on this benchmark?
It is no wonder the guides bring us here.
View downstream from the riffles on the Little Colorado
We reach the spot where the bathing suits make their appearance. The idea is to put your life jacket on upside down, around your waist, and float down the riffles feet first. Let’s be honest here – it looks like you are wearing a gigantic orange diaper. The idea of going full-body into that cold water is less appealing than a root canal for some of us. The rest fearlessly take the plunge, including river buddy Kris. She is last down the chute in this video:
Kris and I chat quietly as we wander slowly back to the boats, admiring the turquoise glow of the Little Colorado. The wind has picked up again, and the clouds have moved back in to lend a gray cast to the sky. It doesn’t really matter, though. We both feel extraordinarily lucky to be in this place.
Kris takes her time on the Tapeats
We reach the rafts, scramble on, and hunker down against the chilly wind. The guides start the motors, back us up into the channel, and point the rigs downstream once again. We have not seen the end of the Tapeats at river level, however. It will show up again later in the trip. For now, though, just around a few river bends and rapids, rocks of the Grand Canyon Supergroup and The Great Unconformity await our arrival.