Over hundreds of millions of years, the seas came in and the seas went out, covering and then exposing the changing landscape of western North America. Sediments were deposited, uplifted, and eroded in the eternal cycles of geologic time. The vertical cliffs and tumbling slopes of Grand Canyon expose the most complete stratigraphic record of Earth’s geologic history, a record more extensive than anywhere else on the planet. I have wanted to see these rocks from a raft on the Colorado River for as long as I can remember. I am finally living my dream.
Pointing at Rocks
(click on any pic to enlargenate)
Our pair of rafts glide serenely down canyon, down section. We have been on the river only a few hours. People chatter softly and point excitedly at what they see. With each passing river mile, singular layers of rocks become older by unknown increments of geologic time. How old is that layer? How did that get there? What is its tale to tell? We need to consider the chronology in a backwards framework, not the easiest way to figure out a progression of events leading to the present. Instead of thinking “This happened, and then something followed,” we must consider in reverse how these rocks were deposited. This happened, but something else happened earlier. It’s mind bending, and I love every minute.
Hermit Shale at river level
Sandstone, shale, limestone. It is the mantra of the sequence of deposition. You learn it in introductory geology class but sometimes it takes a while for the concept to gel in your mind.
Before the vast desert dunes of the Coconino Sandstone covered the landscape, an extensive river system existed which would one day become the Early (Lower) Permian red rocks of the 285-million year old Hermit Shale. Weathering and erosion of an elevated region called the Uncompahgre Uplift transported these river sediments across an arid landscape. Sandstones and some conglomerates found their place in the river channels where water flow energy was greatest. Lighter muds and silts were deposited later, in floodplains, as the energy of the river decreased or temporarily overflowed its banks. For the most part, though, silty sandstone and sandy mudstone are the most commonly found rock type in the Hermit.
This must be the Esplanade Sandstone at river level
The cliffs and splendidly sloping layers of the older Supai Group are generally understood to have been deposited along a broad coastal plain. The Esplanade Sandstone (Lower Permian), Wescogame Formation (Upper Pennsylvanian), Manakacha Formation and Watahomigi Formation (both Lower Pennsylvanian) – these layers comprise the Supai Group, from youngest to oldest, down slope, down section. I have no way at all to differential them. We do not stop but just drift by, the ledges and slopes keeping their secrets to themselves.
However, after I had come home and started researching the geology of the Canyon, I discover that rocks of the Manakacha Formation heralded an important change in deposition patterns from the previous 200 million years. Up until this time, the area was dominated by carbonates (limestones) and minor mudstones, indicating shallow sea conditions. The predominance of quartz sand in the Manakacha is early evidence of a major shift occurring towards desert and sand dune conditions across much of the western interior of the United States.
Somewhere in the Supai our guides steer the boats out of the channel to pause alongside an overhanging sandstone ledge. There is a rock hammer and a date etched into the sandstone by members of a US Geologic Survey expedition. In 1923 the group had descended the Colorado River, seeking to decide the fate of Grand Canyon. Fifty years after John Wesley Powell had journeyed these same waters, the USGS party was there to determine the best place to build a dam. Their dam was never built, but the group played a big role in what was known about the Canyon. Hoover and Glen Canyon dams were eventually built elsewhere along the river.
1923 USGS etching
I like the Esplanade Sandstone. Over the years, I have hiked on it for many wonderfully flat and scenic miles in the Toroweap area, in western Grand Canyon. It is a distinctive sandstone layer throughout the Canyon, thinner to the east while thicker to the west, sandwiched between the underlying Wescogame Formation and the overlying Hermit Shale. Cross-bedding in the sandstone suggests that the Esplanade is of eolian, or wind-blown, origin. The westward-thickening strata denotes the retreat of a shallow sea and the advance of desert sand dunes.
Geologic time and the layers of rocks peel themselves away as we continue downstream towards our first night’s camp site. We bid farewell to the Supai Group at river level. The Redwall is upon us.
The Redwall Limestone comes in at river level
I am fairly certain this spire has an off-color name but I forget what it is.
At river mile 23.5, the guides tie the boats down for the night at a sandy spot at the top of the Redwall Limestone. We jump off, ready to get busy doing our part to offload pretty much everything that is on board. It becomes second nature, after a while, to form the bucket brigade and heave dry bags, chairs, pads, utensils, stoves, tables, and what-have-you either off of or onto the boat. With 27 folks pitching in, the chore does not take long. The guides set up the kitchen while the rest of us look for a place to nest for the night. Kris and I find a spot at the edge of civilization to set up our tents. It’s Happy Hour! The dinner bell rings. It’s time to eat so we drag our chairs over to the dining area near the chefs. We discover that these guides really know how to cook a tasty camp meal.
First night’s camp
Nests for the night
Darkness comes slowly in the desert, even within the walls of the Canyon. I briefly wonder what time it is but then realize that I really don’t care. My watch and cell phone are back in Flagstaff, locked inside my car, tucked beneath the leftover field guides. I will see them all soon enough. After dinner I attempt a few notes in my journal. Words fail me, though, so I just sit and watch the growing darkness envelope our first camp on this early May nightfall alongside the Colorado River in Grand Canyon.