What happens when you take an arid landscape with faulting and high relief, add in some loosely consolidated poorly sorted sedimentary and volcanic rocks, and add yearly monsoon flashfloods? You get the
Kasha-Katuwe Tent Rocks, a national monument located in New Mexico on the Cochiti Pueblo at the western edge of the Rio Grande Rift. This was the location of
yesterday's mini-quiz, as noted by Chas in the comments.
Yesterday's post was all about looking at erosion from the bottom as we walked through a slot canyon on our way to the upper part of the mesa. Today we take a look at the upper reaches, to see what happens when erosion eats away at slopes composed of random boulders and cobbles in a matrix of finer fragments. The results are fascinating to see...
First are the pedestal rocks. Two formations are exposed at Kasha-Katuwe, the Peralta tuff (6.5-7.0 million years), and the Cochiti formation (2-6 million years). The Peralta was produced by a series of explosive rhyolite ash eruptions from the vicinity of Jemez (Valles) Caldera. The Cochiti has a mixture of sand and gravel with larger boulders. As the boulders are exposed by erosion, they protect the softer rocks underneath from erosion, forming the pedestals.
Our hike takes us through the narrows that we explored in yesterday's post...
...and we emerge into the upper canyon where some tall hoodoos can be seen high above. The trail starts climbing in earnest.
And then there is the really weird stuff...the "tent rocks". I think of a tent is being dome-shaped, but it starts becoming apparent that the tents in this case were more like the tepees of the Plains Indians. These originated as pedestal rocks, but the boulders have rolled off, leaving the softer rocks to erode into the bizarre cones.
There are dozens of these odd tent rocks in the upper canyon. World travelers note the similarity to the rocks of Cappadocia in Turkey. The origin of the rocks is quite similar. In the case of Kasha-Katuwe, no one is living inside the cones as some people do in Turkey. The park area has been historically utilized by the people of the Cochiti Pueblo for hunting and plant gathering. The park was established by the Bureau of Land Management in cooperation with the Pueblo.
The trail ends at a marvelous lookout that provides views of the Rio Grande Rift and the distant Sandia Mountains, as well as a view into the canyons below.
But mostly it's the "tents" that latch onto your attention. I couldn't stop staring at the odd and bizarre shapes. I headed down the trail. It was time to head to our camp outside Santa Fe, as we got ready to two pueblos whose histories went in opposite directions, Pecos and Taos.
For the reasons for my odd title for this series, check out
http://geotripper.blogspot.com/2012/06/abandoned-landsa-journey-through.html
Sunday's mystery photo produced a pretty good response of hypotheses, both of the scientific sort, but also of the fanciful sort (and sometimes the boundary gets blurred!). The rock in question is known as Globe Rock, and it can be seen along the Sierra Vista Scenic Byway, an 85-mile long loop road in the high forest country between Yosemite and Kings Canyon National Parks. The area is drained by the San Joaquin River. The elevation is about 7,000 feet.
As noted on the geologic map by N. King Huber of the Shuteye Peak quadrangle (USGS Geological Quadrangle GQ-728; get all of the central Sierra Nevada geologic maps here), the rock underlying the Globe is the Mt. Givens granodiorite, a medium to coarse-grained hornblende-biotite granodiorite and granite that is locally porphyritic with tabular K-spar phenocrysts with a U-Pb age of about 90 million years (my casual non-geologists readers are forgiven for not following this, but my students had better understand it; there's a midterm Tuesday on this stuff!). The rock is labeled on the geologic map, near the top and to the left.
A lot of the answers to my challenge on Sunday noted the possibility that this was a glacial erratic sitting on a pedestal. This is the most likely answer based on my cursory examination, although the possibility must be considered that it may also be an unusually weathered exfoliation slab (in other words, the rock formed in place, not transported from elsewhere). There are glacial deposits shown on Huber's map only a mile or so away, and the larger pre-Tahoe glaciers may have covered much of the region. In either case, the pillar would have developed because the large boulder protected the underlying rock from weathering, while the surrounding rock was eroded away. In this setting, the amount of erosion suggests that the boulder has been there for a very long time. The odd spherical shape of the boulder itself would also be an indicator of age; corners and edges tend to be weathered away first.
Rocks on pedestals are quite interesting for a number of reasons, not the least of which is their contribution to the scenery. If they can be tied to a particular glacial episode, they can be a clue to the rate of erosion in a region. If they are truly precarious, they provide some data about the likelihood of severe shaking during earthquakes. If they haven't toppled in the thousands of years they have existed in a particular spot, it would suggest a limited degree of ground movement (although I have to wonder...this rock is close to major faults in the eastern Sierra Nevada).
