Wednesday, February 23, 2011

Searching the Origins of the Esplanade Platform in Grand Canyon

Far away from the main tourist areas in Grand Canyon lies a huge wilderness of stone and space. It is silent beyond belief and seldom visited. Within this huge expanse lies the Esplanade Platform, a stunning landscape feature that is found only in the central and western portions of the canyon. The Esplanade forms a broad terrace positioned about a fourth of the way down in the canyon, where the Hermit Formation overlies the Esplanade Sandstone. The Esplanade thus creates a canyon within a canyon. Geologists have long been intrigued by the presence of the Esplanade Platform in Grand Canyon and many theories have been proposed to explain its origin. Did the Colorado River carve it during a period of erosional quiescence, as some say? Or did it form in response to the canyon's variable stratigraphy? I explored these questions on a recent trip to the Esplanade. From February 10 to 16 I was privileged to backpack with two other friends here. This is our story.

The trailhead is located in the Grand Canyon. A poor road leads to the boundary of the Kanab Creek Wilderness, administered by the BLM. Here we parked our car to begin our journey. You can see tumbleweeds piled against the boundary fence of the wilderness area on the right side of this photo. We had no idea at this point what wonders awaited us.

This is the canyon we walked down to access the wilderness. The lower red slopes are cut into the Hermit Formation, a rock unit that plays an important role in the development of the Esplanade.

At the start of our hike we immediately became aware of the great space we would be experiencing on this trip. Usually a backpack into the Grand Canyon is a trip into the "womb of Mother Earth," with the high cliffs closing in tightly all around. But travel on the Esplanade is different. You can see far and the promenade makes for some exhilarating views.

But make no mistake about it - this is very far away from everything human. Any misstep or fall that would lead to an injury would require someone to backtrack out again many days walk to the trailhead, then to fetch help as quickly as possible. A trip out here is to place yourself in the hands of fate, come what may.

Here Frank R. walks up a slope in the Esplanade Sandstone. You can see the cross-bedding evident in the many shadowed lines. The Esplanade is the first deposit in the canyon of Permian age and was laid down in a dune field about 290 Ma (million years ago). I include a paleogeographic map of the Esplanade Ss. below to show the geography of the Grand Canyon region at this time.

Esplanade Sandstone time in the American Southwest at 290 Ma. Note the faint outlines of the Four Corner states in this image. The yellowish pattern denotes sand dunes in the ancient landscape and the small red line in northern Arizona represents the 55 miles that we walked in the seven days of backpacking.

Even though we had bright sunshine for all but the last day of our hike, ice was still present in the many side drainages we passed that never receive sun this time of the year. They formed some spectacular icefalls on the Esplanade.

Frank taking notes on the Esplanade with Jump Up Point visible in the far distance.

The Esplanade forms many beautiful overhangs and we walked past many of them in our seven days. Note the pile of gray rubble on the floor of this one, which signifies a place where ancestral people once roasted agave. Although it felt like we were the only people in all of Grand Canyon on this hike, evidence like this reminded us that we were not the first ones here.

Some of the overhangs were extreme and formed cave like entrances that framed the canyon beyond.

And we utilized many overhangs for sleeping, as the temperatures at night dropped into the 20's. Sleeping in an overhang could make a 10 degree difference on the warm side!

In this wilderness adventure, our only source of water was from potholes that had been replenished from the late December snow storms that swept across northern Arizona. We estimated that 90% of the potholes were already dry from the extremely dry conditions that persisted in January after these storms. We had to hunt occasionally for potholes with water.

So, what about the Esplanade? Why is there such a broad platform within the canyon at this level? Look at this view of the upper 1/4th of the canyon and note that the Hermit Formation is barely visible, being covered in talus or colluvium from above. This strongly suggests that it is erosional retreat of the Hermit Formation that undercuts the overlying rocks, causing them to collapse and bury the Hermit in the collapsed debris. Note the more recent rivulets that have been etched into this debris apron (center) and a lot more of this recent erosion into the apron on the far right side of the slope. We can imagine that the apron of debris was once more extensive and "smoother" across this view.

In this view you can also see what used to be a continuous apron of talus covering the Hermit Formation, until it was partially eroded by runoff off of the upper cliff. It is likely that the aprons of talus formed during the Ice Age (pre-10,000 years ago) when conditions were much wetter in northern Arizona and the American Southwest. The dissection and partial removal of the talus aprons most likely has commenced in the last 10,000 years. So another question presents itself - what drives the retreat of the Hermit Formation (and the generation of talus) and what causes the non-retreat of the Hermit Fm. and the partial destruction of the talus aprons?

First, the episodic nature of talus creation and removal can be seen in the many talus remnants that are perched today as isolated remnants on the Esplanade. Here, Bryan B. stands in front of one huge remnant which is seen in the middle distance as a whitish-capped surface that slopes to the right, well below the canyon rim. This isolated remnant of upper cliff material used to be connected to a talus slope that once covered the Hermit Fm., well before it was stripped back to its present position.

And here, I am standing in front of another smaller remnant in the right background. These old remnants definitely show that the creation of talus and its ultimate destruction is episodic in nature.

In modern Grand Canyon, many springs are developed at the contact between the red Hermit Fm. and overlying white Coconino Sandstone, marvelously visible here where the talus cone has been recently eroded. We can imagine that during the wetter conditions of the Ice Age, spring discharge was much greater at this contact. This increase in discharge is likely what caused the Hermit Fm. to retreat rather rapidly, and thus undercut the cliff above it, thereby causing the creation of an extensive talus apron. When the Ice Age ended, the springs dried up, the retreat of the Hermit Fm. was arrested, and the talus cones began to be eroded like we see in the photo above. Thus, the remnant talus deposits that are curiously scattered across the Esplanade may show the rather recent development of the Esplanade Platform. In my observations, the Colorado River in ancient times had nothing to do with forming the Esplanade, it developed "locally" through fits and starts during Ice Age glacials and the intervening interglacials.

Within the talus debris that litters the Esplanade, we observed many great fossils in boulders of the Kaibab Limestone. Here are some well preserved crinoids we found one day on our hike.

And here is a fossil sponge also preserved in the a block of the Kaibab.

Walking on the Esplanade. There was very little trail in this stretch of the hike and only well-experienced Grand Canyon hikers should attempt to enter these remote locations. The possibility to get lost or sustain an injury is great.

But the scenery was fantastic. Here Bryan and Frank ponder a rock sentinel.

Our route took on a platform that is extremely untraveled in the Grand Canyon, the top of the Redwall Limestone. This surface is not well-developed in most parts of the canyon and does not have a name at all like the Tonto or Esplanade platforms.

But we enjoyed this tramp across the surface of the huge Redwall cliff.

Back on top of the Esplanade, I noticed some well-developed channels that are cut into the Esplanade Sandstone and filled with deposits of the Hermit Formation. These channels would have been features on earth;s surface some 285 Ma, when the lower part of the Hermit was laid down across the lithified dunes of the Esplanade Sandstone.

Here is the same photo with some guides to show you these very subtle features. It is likely that the Esplanade Ss. was already a rock when the channel was cut into its top surface. Then fluvial mud was washed into the channel.

Here is a close-up of the same channel above.

And a look at one other channel I took a photo of. This one might be a bit easier to see to the untrained eye.

Our hike was a glorious success. No one was injured, we found water when and where we needed it, and we lucked out beating a huge snow storm that blew in just as we were leaving. But more than that, we experienced the Grand Canyon on a very personal level. Bryan and Frank were excellent backpacking companions and I am lucky to have friends so dear and willing to undertake such an adventure. Geology was also my constant companion on this hike and I cannot imagine not being aware of its overriding presence everywhere upon this landscape. If you would like to experience the marvels of the Esplanade Platform in Grand Canyon, you can take a much more accessible backpack on the South Bass Trail, or drive out to Toroweap Overlook on the North Rim.


Gaelyn said...

Toroweap was awesome. I'd like to experience more of the Esplanade.

Johnny Montezuma said...

A riveting tale well told! Thank You for putting such Heart, Passion, Skill, Perception, Professionalism and SPIRIT into your blog posts, Wayne. This one is beyond awesome! MUY KUDOS to you for being "out there!"

Dr. Jack Share said...

A great post, Wayne! As always, your vivid descriptions in the field make me feel as if I'm there. The Esplanade has always intrigued me.

Donna said...

I enjoyed reading of your adventures and all the insight you shared with your readers about the geologic forces and presence in the beauty of the Grand Canyon!

Russell O said...

Great post Wayne. Perfect timing as well as we will be on the esplanade(south side) in 5 weeks! Thx!

Beamer said...

Wayne ~ I am really looking forwad to our visit to Toroweap next month with you after reading this great post. The Esplanade Platform has always intrigued me. I hope we get a chance to tramp about on it. As always, your photos and wonderful descriptions make you feel like one is there. Frank, best to you, too. Steve

BRIAN JOHN said...

Wayne -- I'm intrigued by that fantastic photo showing the exposure of red material where parts of the talus apron have been eroded away. But look at the bedding of the red sandstone exposed in the sides of the new cuts -- they show that what looks like a thick talus apron isn't thick at all. It's a very thin veneer of talus material, on a red rock slope in which the original sandstone stratification can still be seen. So we are looking at a bedrock slope with a thin veneer of white or buff coloured talus on it, from the higher parts of the cliff. If these were thick talus fans we would see stratification parallel with the current ground surface...... so there hasn't neen a huge buildup of broken rock material at all -- most of the debris has been effectively transported away, somehow or other. Any thoughts?

Wayne Ranney said...

Brian - There is no doubt that the talus was deposited on a slope cut into the stratified “bedrock.” And that the talus is variable in thickness from one amphitheater to another. Some places it is thin and other places it is thicker. There are some places where the talus is thick and one of the problems of these photographs is that you lose the scale of everything in the Grand Canyon. Granted that the talus is not hundreds of feet thick but still, the amount of talus cannot be easily discerned in these photographs. Thank you for your comment!

Wayne Ranney

BRIAN JOHN said...

Hi Wayne

Thanks for the response. This is fascinating! In the photo the thickest talus over the red rock base seems to be about 10m thick (am I in the right ballpark here?) in the exposure on the far right of the photo. Elsewhere it seems much thinner. I suppose it might be thicker in some old bedrock gullies. How important is frost action in providing the debris from the upper part of the cliff?

Wayne Ranney said...

Brian - about 10 meters is a ballpark thickness although 15 to 18 might be a better guess. Frost action is huge in this process but fracturing and gravity are the two most important parameters.