Vai al contenuto

Geology is the Way

Come la geologia di Walnut Canyon ha plasmato questi insediamenti pre-Colombiani

Today we venture in one of the most spectacular – yet lesser-known – places of Arizona: Walnut Canyon. Walnut canyon is a marvel of nature and one of the nicest examples of how ancient civilizations exploited the geology of their territory to their advantage.

The canyon entrance is spectacular! You reach the canyon from the visitor center to the north, not too far away from Flagstaff, entering a meander of the Walnut Creek. The canyon rim stands 2040 (6690 ft) meters above sea level and about 105 meters (350 ft) over the canyon floor.

The entrance of Walnut Canyon from the visitor center.

The geologic formations exposed in the area are the same Permian rocks that constitute the top of the Grand Canyon rim. The oldest formation in the canyon is the Coconino Sandstone, a 275 million years old sandstone that witness an ancient desert environment. Above, the Toroweap Formation is a sequence of gypsum-bearing shales and sandstones that formed about 273 million years ago in a shallow sea that invaded the desert of the Coconino Sandstone. Finally, the top of the canyon is marked by cliffs of 270 million years old Kaibab Limestone, a sequence of carbonate rocks interbedded with siliciclastic rocks formed in a shallow sea.

The Kaibab Limestone is an erosion-resistant formation that produces the prominent cliffs at the top of the canyon. The formation shows parallel beds of sandy limestone and dolomite interlayered with thin beds of shales. Shales are rich in clay minerals and are easier to erode than limestone, so the canyon walls show a series of ‘notches’ where the clay-rich beds are.

Prominent cliffs of Kaibab Limestone surround the canyon from all sides.

The underlying Toroweap Formation consist largely of shales that form gentle slopes covered by shrubs. Immediately below, there is another erosion-resistant formation, the Coconino Sandstone, forming the lowermost cliffs in the canyon, right above the creek.

The Toroweap Formation (above) and the Coconino Sandstone (below) are easily recognizable for their different resistance to erosion.

I have to say that the Coconino Sandstone is certainly my favorite formation in the canyon! It is a beautiful bedded sandstone with impressive cross-bedding that testifies the movement of ancient sand dunes. Of course, the direction of the dominant winds changed more than one time during the geologic history of this ancient desert and now we can see complex erosional surfaces separating layers produced by dunes that migrated in different directions. I love it.

Cross-bedding comes in all forms and shapes in the Coconino Sandstone, as the cliffs of the canyon intersect at variable angle with the cross-bedded sequences.

So, why did I say that early humans, the Sinagua people, learned to exploit the geology of the canyon to their advantage? It all has to do, precisely, with the presence of these three different formations in the canyon and their different resistance to erosion. In fact, if you look at the canyon walls, the Sinagua dwellings all occur at the same elevation, along the same limestone layers.

The cliffs of Walnut Canyon. Do you see the settlements at the base of the limestone?
Close-up of the cliffs showing Sinagua settlements at the contact between the Kaibab Limestone and the Toroweap Formation.

But why there? As I have mentioned before, the Kaibab Limestone is very resistant to erosion. By contrast, the underlying Toroweap Formation, consisting of shales, weathers very easily, producing gentle, muddy slopes. The progressive removal of the shales undermines the base of the limestones, which eventually fall along the slopes.

The base of the Kaibab Limestone must have looked like a cozy place to the Sinagua!
The Kaibab Limestone is prone to rock falls as its roots are bared by erosion. This produces a lot of limestone blocks that the Sinagua used as building material for their shelters.

As you can see, this process produces a natural ledge and, at the same time, a locally flat area that is easily accessible if you walk at the contact between the Toroweap and Kaibab Limestone formations. The ledge provided low-cost housing and the Sinagua just had to build a couple of walls to shelter themselves from the weather. The top of the Toroweap and its gentle slopes allowed them to move laterally from house to house, while eventual attackers were discouraged by the prominent and difficult to climb cliffs of Kaibab Limestone, bounding the canyon. A further level of protection was added by the fact of living in a canyon, invisible to others for miles and miles. It surely must have been a solitary way of living but it also must have felt like being in touch with nature.

The boards in the canyon are interesting and really informative.

I actually found Walnut Canyon by chance when visiting Arizona 7 years ago. It was not in my travel guide, as it is off the beaten path for international travelers compared to places like the Grand Canyon, the Monument Valley, or the Meteor Crater. This place was a hidden gem that left an unforgettable mark on me. I hope this post might help you consider this place for tour next trip to the Colorado Plateau! Here are some bonus pics of Walnut Canyon.

Suggested Readings


Lascia un commento

Il tuo indirizzo email non sarà pubblicato. I campi obbligatori sono contrassegnati *