Sunday, September 16, 2018

2018 Grand Canyon Association Field Institute 10-Day Colorado River Trip

Every year for the last six years I have served as a geologic interpreter on a 10-day motorized river trip for the Grand Canyon Association Field Institute. The trip is meant to share the geologic wonders of the great gorge with GCA members who support the mission of the Association. By their nature, GCA members come to the trip already in love with the landscape of Grand Canyon but may want to delve deeper into its fascinating origin.

This year I did not take any of my own photographs. So what you will see here are a selection of photos I am using from two participants, Patrick and Barbara McGuffey. I thank them both for allowing me to use their photos in narrating this posting.

We often stop on the east side of Navajo Bridge (completed in 1929) which is now used as a viewpoint of the river 400 feet below. The new bridge (barely visible on the far left) was completed in 1995, making the old bridge accessible for pedestrians. I remember how for twenty years we drove across this narrow (18 feet) patch of concrete.

View upstream from Navajo Bridge. At this location, the river has incised down through the Kaibab Limestone (upper cliff) and the Toroweap Formation (lower, talus covered slope). Directly beneath the bridge (not seen here) are the first exposures of the Coconino Sandstone, only about 20 feet or so thick in this location. The Coconino thickens to the south and thins to the north.

While on Navajo Bridge, someone in our group spied California condor #54, roosting on one the bridge struts. What a treat.

We began our river trip - with a hike! This is a downstream view of the Colorado River at Lees Ferry. We are standing on the Shinarump Conglomerate Member of the Chinle Formation (foreground). The same stratum is also visible across the river as the short, shadowed cliff in the photo center. The tilt in the strata is attributable to the Echo Cliffs monocline, which is over 100 miles long and passes beneath this area. The deep red Moenkopi Formation lies beneath the Shinarump cliff down to the rivers' edge, while the slopes above that cliff are composed of the remainder of the Chinle Fm. and the Moenave and Kayenta formations. All of this is capped by the Navajo Sandstone on the far left. Note the large debris fan entering the river at right center. This is from the Paria River, which enters the Colorado on the far right. It is one of only five streams that enter the Grand Canyon from outside of it.

Just a little more than 12 miles into the canyon, the river has sliced into the Kaibab, Toroweap, Coconino, Hermit and Supai** formations. The top of the Supai is within the cross-bedded sandstone at river level on the right center. (** I have increasingly been a proponent of returning to the original name of the Supai Group back to Supai Formation. The individual formations within the Group are difficult to differentiate and unnecessarily confuse the stratigraphic column for educators and students. We'll see if this attempt will take hold with other interpreters in the canyon).

A Common side-blotched lizard (Uta stansburiana) perches on a rock in the mouth of North Canyon.

Sacred datura (Datura wrightii) was blooming just about everywhere we stopped on this trip. I have noticed it blooming frequently at the end of the monsoon season. Guests have mentioned to me that they appreciate my occasional musings into the extra-geologic aspects of the canyon's natural history. I attribute my interest in the plants and reptiles of the canyon to my years serving as a backcountry ranger and trail guide. However, if I don't get many questions, my tendency is to revert to exclusive geologic commentary.

A typical river camp with tables in the kitchen and chairs for guests to enjoy the view.

Hiking in the Grand Canyon is a different beast and river guides are often at a loss to describe the nature of hikes away from the river. Most hikes do not involve a lot of miles but there is extremely varied terrain that is often in and out of water. Short and steep rocky sections are the norm. Sometimes, these short but difficult stretches turn away the otherwise interested hiker. But if one perseveres through no more than 5 or 10 or 15 minutes of "pain" (typically) the rewards are great.

Our head guide with Arizona Raft Adventures (AzRA), Laura Fallon, showed me this fossil in the Supai on the hike up North Canyon. There are many root casts to be seen in this canyon but she showed me this one that was described to her by another geologist as possibly being a horse-tail (Equisetum). These plants have been around since the Devonian some 375 million years ago (375 Ma) and the Supai here is no older than 316 Ma. It was segmented like Equisetum.

The "narrows" in North Canyon. Note the exfoliated shape in the textures of the canyon walls. As the rocks were progressively removed by countless flash floods, the pressure that once kept them tightly packed was also removed. What follows are fractures that develop parallel to the open space.

Slicing downwards into the Mississippian Redwall Limestone, a dissected cavern system is exposed. Directly opposite this cave on the opposite bank is another cave with similar dimensions. The idea is that the cave system was formed before the river exposed it. Note the lens-shaped light patch on the far right. This is a cave that is choked with Redwall Ls. rubble.

Stanton's Cave was named for Robert Brewster Stanton who was the first person after J.W. Powell to complete a Grand Canyon river trip. In 1889 three members in his party were drowned before they were 25 miles into the Grand Canyon. So the next year in 1890, Stanton returned to finish the surveying through the entire canyon for the proposed Denver, Colorado Canyons, and Pacific Railroad. Thankfully, it never was built.

Note the dark and light-colored banding in the Redwall near South Canyon. Our assistant raft guide, Elisabeth McGuirk, shared with me a paper she obtained from Dr. Gary Gianniny of Ft. Lewis College in Durango. Colorado. He made numerous thin sections of the rock, (where slices of rock only 3/1000s of an inch thick are studied under polarized and unpolarized light to determine their make-up). He found that the dark bands were composed very porous dolomite mudstone layers having up to 35% porosity (average 18%) with the lighter bands being less porous and composed of crinoidal and oolitic calcite grainstone. His work shows that the Redwall trends from deeper water settings below and grades upwards into shallower environments.

We saw many Desert bighorn sheep (Ovis canadensis nelsonii) on this trip, as the rams were busy assembling their autumn harems. Here a big ram (center) leads four ewes up the slope.

Our green water trip ended when we encountered the Little Colorado River at River Mile 62. Recent rains had turned the LCR a chocolate brown color and later on our trip became even more muddy, or as we say, sediment rich.

Just downstream from the LCR.

Looking up nearly 1,000 feet to the contact of the dark Precambrian crystalline rocks (below) with the light brown, Cambrian Tapeats Sandstone capping it. In the Grand Canyon, this is known as The Great Unconformity. I typically tell guests to strain their necks to look up and see the contact so far above their heads, since the contact will eventually return back to river level in a day or two. This is an educational technique I use to explain warping in the layers.

Near River Mile 108, pink intrusive dikes of the Zoroaster Granite penetrate the dark Vishnu Schist. The Great Unconformity caps it all.

This double rainbow signaled the end of a two-day period of summer rain we enjoyed while traveling down the river. It was spectacular.

The Powell Plateau receives morning light as our group enjoys breakfast by the river. We spent the entire day meandering around this isolated spur in the heart of the Grand Canyon.

To explain the previous caption and photo, I include this section of the geologic map of Grand Canyon. The yellow arrow denotes the direction of the previous photo and the two circles denote the two camps. Note how the river makes huge, sweeping loops around the Powell Plateau. One night we looked west to the east side of the Plateau; the next we looked east to its west side! The river flows 28 miles in this stretch.

With the river flowing south here near River Mile 114, it moves around the eastern side of the Powell Plateau.

At last! The Great Unconformity is attained at River Mile 121 in Blacktail Canyon. Here Liz's hand is on the Tapeats Sandstone with her foot near the Prescambrian crystalline rocks.

Another guest, Ann, has her finger right on the contact. 1,200 million years of the rock record is missing at this contact. However, something can be known about what happened during this long span of geologic history (more than 1/4 of all Earth history). That is because isolated remnants of a thick stack of sedimentary rocks escaped complete erosion and destruction in other parts of the Grand Canyon. Because of that fortuitous preservation we know what happened during large portion of this history.

Real men (well, at least the older ones) wear sarongs on these river trips. It helps keeps the intense sun off of ones legs.

Entering "the Ice Box", a section of the river trip where limestone and sandstone walls rise nearly 3,000 feet vertically to block out much of the sun in this stretch. This is a downstream view.

And an upstream view where Laura and Lis converse about the river. Or our schedule. Or the beauty of the Grand Canyon.

Within the Ice Box, there are many seeps and springs that harbor endemic species of plants and animals. This one was at our Ledge Camp at River Mile 152.

One of the joys I have experienced in leading these trips is running into other guides whom I know from guide gatherings and lectures. It is wonderful to meet friends in the middle of nowhere.

Barry has found a comfortable seat in an eroded patch of travertine in Havasu Canyon.

Trip leader Laura Fallon giving one of her readings during an especially quiet stretch of the river.

On the horizon in the distance is Toroweap Point on the North Rim. I will be leading two trips to Toroweap in October traveling overland across the famous 62 mile-long road.

The Red Slide is a curiosity at River Mile 175. Huge pillars of loose rocky debris are capped by boulders all seemingly derived from the Supai Formation cliff above. There doesn't appear to be much (if any) Redwall debris in the mix - although a Redwall cliff is also exposed upslope. I have noticed there may be two different deposits here of different ages, with one sitting on top of the other. Someone will have to figure out the sequence of events here one day.

Vulcan's Anvil signals our arrival to the 'recent' lava section of the river trip. This remnant is interpreted as the site of a volcanic vent - right in the river corridor! A nearby dike and sill suggest that the volcano that grew within the channel of the river may have been quite high. It boggles the mind!

Downstream from Lava Falls, a side canyon in the south wall is filled with hardened basalt rock. Note the new canyon being carved to the right of the black fill. If the Colorado River had a motto, it would be, "No obstacles allowed!".

I snapped this picture of our group (sans guides, who were busy with camp chores) with Patrick's camera. Thank you all for being such wonderful river mates! And thank you for supporting the Grand Canyon Association! Thanks also to AzRA and Laura and Lis for giving us an excellent trip! Write me at wayneranney17@gmail.com if you are interested in any of my upcoming river trips or visit www.wayneranney.com.

1 comment:

Unknown said...

Great summary of our trip Wayne. Well done, thanks.