Small Rockfall at Grand Canyon Witnessed by Grand Canyon Sememster Students

This semester at Northern Arizona University, I will teach an Honors class (HON 343) as part of a program called Grand Canyon Semester. Thirteen students from around the US have arrived on campus and will undertake an intensive 16-week course focused on science and issues relating to the Grand Canyon region.

During their Orientation Week, they camped at the South Rim for four nights. I led them on a hike down the South Kaibab Trail to introduce them to some aspects of the geology of the canyon. On Thursday, August 23, storm clouds began to develop as they listened to a talk at Hopi Point. A few claps of thunder could be heard away in the distance. Then, one was heard that just kept rumbling and rumbling and never stopped. The students quickly looked into the canyon and saw a large billow of red dust and rock floating up from the south side of Shiva Temple. A rockfall was underway! This image was captured by NPS ranger Jacob Fillion.

The rockfall appears to have initiated in the upper portions of the Redwall Limestone, deposited during the Mississippian time period about 340 Ma (million years ago). Although the amount of material likely let loose in this fall would dwarf anything human, it is quite small compared to the overall dimensions of the canyon. Notice that evidence for other recent rockfalls is absent in this view and in most views of the Grand Canyon. It is likely that the canyon is not currently experiencing active widening or deepening in modern times and that it may undergo periods where it just sits there with nothing much happening. It is similar in many ways to the biological concept of punctuated equilibrium, whereby species undergo long periods of stasis interspersed with shorter periods of rapid change. Grand Canyon may also undergo a sort of punctuated equilibrium with regard to its formation. The agents that produce the punctuations in this case would be climate change, runoff amount in the river and uplift of the rocks. These are what initiate canyon cutting and widening.

In any event, the fact that our students got to experience a brief moment of canyon widening bodes well for their semester here in northern Arizona. We wish them well on their journey of discovery!

Photo courtesy of Jacob Fillion, NPS

The plume of dust and rock captured in this image is nearly 1,000 feet high from bottom to top. Compared to the overall dimensions of the canyon, this is likely a rather small event with a "flake" of rock only being dislodged. Yet anything directly below the fall having experienced a rather traumatic and violent crushing by the material that was let loose. The canyon challenges us to see our world on many different scales with respect to time and space, large and small. We are moved by its immense size and the lessons in earth history it provides to us.

Colorado's Royal Gorge and the Shelf Road

In July, I traveled to some places in central Colorado that I had never been to before. Here is an account of my trip to the Royal Gorge of the Arkansas River and a scenic geologic drive north of Canon City called the Shelf Road.

This was the second place Jack and I visited that is in the footsteps of Lt. Zebulon Pike (see the previous posting of an account to the Great Sands Dunes National Park). Pike was the first American to see the Royal Gorge of the Arkansas. I wonder why this expedition is not as famous as the Lewis and Clark expedition farther north? His orders were to do the same - explore the new boundary of the United States after the Louisiana Purchase. Hmm?

This is the bridge over the Arkansas River looking to the north. It has wooden planks on it so it rattles quite a bit when a vehicle drives over it.

The bridge over the gorge and the land surrounding it is owned by the municipality of Canon City (prounounced as both cannon and canyon). They operate the area as a kind of amusement park replete with all kinds of American kitsch, clowns, magic shows, and sky rides. A bit disheartening to see so much fluff in an otherwise fantastic landform.

For me the big attraction was the gorge itself. Here is a view to the northwest and the land beyond the gorge. This is a spectacular landform worthy of a visit but we did not find even one sign relating to the natural history of the area. A shame really and it explains why people like myself usually avoid visiting the area. In fact, as I thought about it, I know of very few people who have been here.

Here is one of the sky rides crossing the gorge

And a view of the Arkansas River below with the railroad tracks paralleling the river on river left. The story of how this fantastic rail line was built is told in a book called, "Rival Rails" by Walter R. Borneman. You can read about that book here. I found it to be a fascinating story as the ATSF and the DRG vied for rights to build through the gorge. The DRG eventually won that battle.

A view of the Royal Gorge in the upstream direction with the river and tracks at the bottom. At its deepest point the gorge is 1,250 feet deep. The rather flat surface on top of the gorge is curious. Perhaps this is an erosion surface from the Laramide (approximately 70 million years ago) or it could be a surface formed in the Precambrian that elsewhere in the west is called the Great Unconformity. For a fantastic and comprehensive explanation of the Great Unconformity, see a blog post by Jack Share here.

You can easily see the sheer depth of the gorge in this view. The rock is called the Pike's Peak Granite, although there are places where gneiss and other metamorphic rocks are also present. Formed just over 1,000 million years ago, the granite is an interesting rock to behold (see below).

A laerger view of the lip of the canyon and the Pike's Peak Granite

Close-up of river and railroad

The Pike's Peak Granite up close. A granite with crystals this large is technically called a pegmatite and these form when water in a magma facilitates the growth of large crystals.

A large felspar crystal in the granite can be seen, as well as the perthitic texture within it (lines)

After leaving the Royal Gorge, we entered Canon City and then headed north on a dirt road towards Cripple Creek. We soon found that this road followed Four Mile Creek which brought us to the Garden Park Fossil Area. It was here that the famous "Bone Wars" of the 1870's was played out as paleontologists Edward Cope and Othniel Marsh battled each other for the most significant dinosaur find. Read about it here.

We hiked a short distance to one of the old time quarries, set above the cliff in the red sediments. Note the obvious channel feature within the Morrison Formation at this locality. The dinosaurs would have found the riparian environments that coursed through this area to their liking. For a map of the Morrison paleogeography, see the map below from my book, "Ancient Landscapes of the Colorado Plateau".

Paleogeographic map of the Morrison Formation, Brushy Basin Member

The Shelf Road continues north. We had heard that the road was quite narrow and not to be taken in wet weather (it was wet). However, we found it to be no problem and continued onward.

There was an excellent exposure of the Great Unconformity in the canyon. In fact, the "shelf" of the Shelf Road was built utilizing the contact between the Ordovician Manitou Formation (upper strata) and the Precambrian crystalline rocks below. I had never before seen the unconformtiy as expressed in Ordovician rocks as Arizona has absolutely none of this age.

View of the Shelf Road and its rocks

Jack touching the unconformity at a road cut along the Shelf Road.

Nice photo of the unconformity with a dike trending vertically through the crystalline rocks.

Foliated and very friable schist in the Precembrian rocks along the Shelf Road

As we neared Cripple Creek, we found that we were entering one of the West's great Gold belts. The nearby town on Victor still has a working gold mine.

More Colorado Geology - Great Sand Dunes National Park

One of the hidden geologic gems located just east of the Colorado Plateau and the Rio Grande Rift is the Great Sand Dunes National Park and Preserve. In July, I approached this interesting area with colleague Jack Share while traveling from Durango, to the Continental Divide at Wolf Creek Pass, and across the Alamosa Valley located in the floor of the rift. It is a beautiful and interesting place.

The Sangre de Cristo Mountains loom behind the dunes. This view is to the northeast and behind us to the southeast, the range terminates at Blanca Peak, elevation 14,345. It is only 50 feet lower than Mt. Whitney in the Sierra Nevada and is the third highest peak in Colorado. We could not photograph Blanca Peak because it was perpetually in clouds.

On the approach we could see the dunes had been recently drenched in rain.This was the time in July when the Rocky Mountain front range was ablaze with fire and these are the rains that quenched the blazes.

Sunset in the Sangre de Cristo Mountains, July 8. The low angle light on the feldspar-rich granites are what gave the mountains their name (blood of Christ).

Sunrise on the dunes, July 9

We took a walk up the dunes to observe modern eolian processes. Here is a trackway of an insect that has been active the night before.

Some areas of the dunes had a blackish tinge and closer inspection showed that this was due to iron-rich minerals being winnowed from the quartz sand into discreet pockets. Density differences between these two mineral types causes this winnowing, much the same as gold-panning will winnow gold to the edge of the pan. It is the inclusion of these iron-rich minerals in the white quartz sand that could eventually turn this future sandstone red - when the whole package is buried, groundwater can infiltrate the sand and dissolve the iron component, whose solutions then coat each sand grain with a red coating.

The iron-rich minerals are derived from the granites in the Sangre de Cristo Mountains. But they do not simply travel from the mountain front to the dunes. First they must be washed to the west into the Alamosa Valley, where huge piles of sediment have filled the Rio Grande Rift. During the Ice Age, this valley contained Lake Alamosa, which would repeated fill and dry out. Then the wind picked up the dried sediment as sand and blew it to the east where it encountered the high range causing the wind to slow down and dump its load of sand here. What a journey the sand makes!

The recent rains made walking easier for us

And the vistas were superb. We both felt as if we were walking in some Jurassic dune field on the Colorado Plateau (although those dunes were much more Sahara-like and not so Alpine.

Low-angle light easily differentiates the windward and leeward sides of these wind ripples - the wind in this instance is blowing towards the person

The sand is trying to climb up into the range but this is about as far as it gets. Zebulon Pike came through this area in January, 1806 while surveying the new international boundary between the United States (Louisiana Purchase) and New Spain.

Looking north across the dunes to the northernmost part of the Sangre de Cristo range

As we left the park, we could see that another day of rain was building up

In a nine-day trip across southern and central Colorado, this was the biggest surprise (out of many). Here, the iron-rich grains are easily highlighted in the dunes.