Blogging A Bit of Rock Art...

Took a brief foray into the Franciscan Complex and Salinian Block this last week...normal people call the region Big Sur, and there can hardly be a more spectacular coastal stretch anywhere (I know there is a lot of competition, though; I'd love to see your favorite photos). I came across a delicate bit of coastal rock art I thought I'd share.

I thought of providing a bit of artistic critiquing, but mostly I simply enjoyed the fragile balancing act, and I pass it on to you. The larger balanced boulder is a chunk of Salinian granite sitting on a much larger boulder of Franciscan chert. The small disk-shaped topper may be a little chunk of graywacke. I appreciated the artist's juxtaposition of the major elements of the local geological relationships.

The Real Jurassic Parks: Capitol Reef (Oh, and Sheep Blogging)

Capitol Reef National Park was mentioned in an earlier post discussing the Wingate Sandstone, but the Navajo Sandstone is also a major contributor to the scenery of the region. The Capitol Dome is an eroded outcrop of Navajo that looks, well, kinda like the capitol dome. These distinctive knobby peaks are a characteristic erosional pattern of the Navajo Sandstone, and can be appreciated during a stroll through Capitol Gorge.

Prior to the construction of the present-day paved highway 24 along the Fremont River (in the 1960's), the only way through the "Reef" was the dirt and gravel road through the narrows of the Capitol Gorge. Like the Virgin Narrows at Zion, canyons carved only in Navajo tend to be deep and narrow. When the river erodes to the level of the underlying Kayenta Formation, the easily eroded shale layers cause slope retreat and rockfalls that widen the canyon. The road was narrow and winding, and was flooded constantly; the streambed and the road were one and the same. I have no doubt that cars and people were constantly trapped and carried away before the new highway was built. Today, the narrows is a pleasant scenic stroll.

The Fremont People were the first inhabitants of the region, and their petroglyphs are a constant reminder of their former presence. At Capitol Gorge, a badly vandalized panel still preserves some of the hallucinogenic figures. The urge to write on stone did not end with the rise of modern civilization. The rocks of the gorge contain hundreds of inscriptions from travelers from the 1880's to the present day (unfortunate, in the latter case). Some of the carvings are on apparently inaccessible cliffs; I wonder if it was worth it to hang from a rope to leave a sign of your existence for future generations. Some people certainly thought so (I guess I am blogging, instead of carving).

I have wondered at times why bighorn sheep seem to be the predominate choice of petroglyphs when animals are the subject matter. I rarely see deer or bear or lions, but there always seem to be sheep. Are the horns a "guy" thing? In any case, I sent my students down the gorge in 2005 and took off to see the trail to the Capitol Dome. I rounded a corner in an alcove cut into the Kayenta shales, and came face to face with a big ram. He seemed supremely unconcerned with my presence, and allowed me to take quite a few telephoto images. So, I can't help but notice that I am posting a picture of a bighorn, but not one of a deer or a jackrabbit. Maybe I understand the Fremont better than I think...

Zion: A Real Jurassic Park, Part 2

Zion National Park is a real Jurassic Park, a place where the scenery is dominated by a single formation, the Navajo Sandstone. Today's photographs include the Angels Landing ridge and a view from the upper part of the trail towards the Narrows of the Virgin River. I consider the hike one of the most spectacular treks in the American west. The middle section of the trail is cut through a sheer cliff, and climbs through a narrow cleft called Refrigerator Canyon (for obvious reasons on a hot day). Then the fun parts: Walter's Wiggles, a series of tight steep switchbacks, and then a vertigo-inducing climb up the spine of Angel's Landing itself (top photo). The climber is assisted by a series of chains placed in particular steep sections. The payoff is a wonderful falcon-eye's view of Zion Canyon (peregrine falcons nest in the region). There is nothing quite like looking over a sheer 1,000 foot cliff.



The sand dune environment is not conducive to the preservation of fossil bones, but dinosaurs did wander over the dunes, leaving behind numerous trackways. Here and there, fresh-water oases left behind deposits of limestone that contain a fossil record of crocodiles, small dinosaurs, and other smaller creatures.



The sandstone of the Navajo is quite porous and the underlying Kayenta Formation is not. Water percolates through the sandstone, and when the Kayenta/Navajo contact zone is exposed, springs often flow. The much visited Weeping Rock, with its hanging gardens of ferns and flowers is one of these springs.

Zion is a beautiful place...

An added thought in terms of perspective. The incredibly steep and tall looking Angel's Landing in the photo at the top can also be seen in my last post, from Observation Point, which is on the rim of Zion. In that first picture, Angels Landing is the flat bench-like cliff in the center right of the photo. The cliff has the shadows on it. Observation Point is a tough hike, almost twice the climb of Angel's Landing, although there are no chains on top of vertical cliffs to give one that feeling of vertigo...

Zion: A Real Jurassic Park

Zion National Park is one of the crown jewels of the national park system. Millions include it on a tour of Grand Canyon and Bryce Canyon (on their way to the air-conditioned casinos in Las Vegas). It is sometimes described as a Yosemite Valley without the granite and the glaciers, and the description is apt, but for a unique reason: like Yosemite, probably 90% of the visitors spend their time on the valley floor, and few take the time to explore the remainder of the park, including the spectacular Kolob Fingers section and the extensive backcountry.

The park is the ultimate expression of scenery produced by the Navajo Sandstone. The canyon walls rise 3,000 feet or more above the valley floor, and 2,000 feet of that are cliffs of Navajo. The canyon was carved by the Virgin River, a tributary of the Colorado River. In the lower reaches, the canyon floor is relatively wide and flat (and thus the site of camps and lodges and other developments), but upstream beyond the end of the road, the canyon forms a series of narrow gorges a few yards wide, but thousands of feet deep (the Narrows of the Virgin River).

The difference in the nature of the upper and lower canyon is striking, and is explained by the different ways that sedimentary rock is eroded. The cliff-forming Navajo Sandstone lies atop the Kayenta Formation, which is easily eroded and forms ledges and slopes. When the water carves through only Navajo, the canyon is narrow, but when the Kayenta is exposed, it erodes quickly and undercuts the cliffs of Navajo, which then collapse by way of mass-wasting and slope retreat. Rockfalls are a serious hazard in parts of the canyon. During my visits to the park (perhaps 25 times), I have seen two rockfalls, one of which filled the whole gorge with dust.

What to do at Zion? If you have only a short time (shame!), a hike up the Narrows (an easy stroll on a paved trail, and then a wet walk up the middle of the river), and a hike to Angels Landing (quite a bit harder, with 5 miles and a climb of 1,500 feet or so) should be a priority. With more time, get out to the Kolob Fingers, and take a longer hike. Quite a few miles in is the Kolob Arch, which lays claim to being one of the largest arches in the world. A hike to Observation Point is incredible (see the top two pictures), but also exhausting. What is your favorite secret at Zion?

50 Minerals one must see....a geologic meme (and me too!)

Jumping on the bandwagon, here are the 50 minerals from Chuck's list that I have seen (See Lounge of the Lab Lemming and others here, here and here). Like Dave, I would have added more of my own, but I am at work after all. On the other hand, California is a dream site for finding strange and exotic minerals, and I am adding three to my list: Serpentine (yes, chrysotile, but I like the greasy form), because it is the California State Rock (legislature was supposed to call it serpentinite, but didn't know any better); gold, because it is our state mineral for obvious reasons, and seeing it in the wild is always a memorable experience; and benitoite, because it is our state gemstone, and it is found in gem quality specimens pretty much at a single mine in in San Benito County. It is also one of the first minerals discovered that crystallizes in the ditrigonal dipyramidal class, making for some beautiful triangular crystals. It is my picture of the day...

As Chuck says:

Use bold to indicate minerals you’ve seen in the wild. Italics is for those seen in laboratories, museums, stores, or other non field locations. Ex pet nerds may use underlining to indicate those that they’ve grown with their own two hands. And I won’t bother with stuff you intend on seeing- if you didn’t want to see all these minerals yourself, you’d be spending your precious lunch hour on a physics or biomedical blog.

Andalucite

Apatite

Barite

Beryl

Biotite

Chromite

Chrysotile

Cordierite

Corundum

Diamond

Dolomite

Florencite

Galena

Garnet

Graphite

Gypsum

Halite

Hematite

Hornblende

Illite

Illmenite

Kaolinite

Kyanite

Lepidolite

Limonite

Magnetite

Molybdenite

Monazite

Nepheline

Olivine

Omphacite

Opal

Perovskite

Plagioclase

Pyrite

Quartz

Rutile

Sanidine

Sillimanite

Silver (native)

Sphalerite

Staurolite

Sulphur (native)

Talc

Tourmaline

Tremolite

Turquoise

Vermiculite

Willemite

Zeolite

Zircon

Antelope Canyon, A Real Jurassic Park

Following up on the previous post, I am adding a few close-ups of the Navajo Sandstone in Antelope Canyon near Page, Arizona. The beautiful slot canyon lies within the Navajo Nation, and is a tribal park. The upper canyon is known by the Navajo name Tse' bighanilini, which means "the place where water runs through rocks", while the lower canyon is referred to as Hasdestwazi, or "spiral rock arches."

Crossbedding occurs in dunes as sand is carried up and over the crest and cascades down the lee slope (the "slipface"). The layers form with a distinct slope, at variance with the usual horizontal nature of sediments. The size of the sand grains varies with windspeed, and erosion brings out the those variations, as seen in the pictures above.

The Navajo Sandstone is a beautiful rock...

The Real Jurassic Parks: Navajo!

If the question arises as to what formation is responsible for the most dramatic scenery in all the Colorado Plateau, the Navajo Sandstone would have to be near the top of everyone's list (anyone out there have opposing arguments?). It is hard to imagine a rock unit that can erode into more beautiful colors, forms and structures (and I am being totally scientifically objective here....).



200 million years ago, a vast desert had spread across what is now Colorado, Arizona, Utah and Nevada and north into Wyoming. The idea that sand dunes represent a typical desert is a widespread misconception (typical American deserts are no more than 5% sand dunes), but in this case it was true: huge dunes crept across the landscape, in places burying the underlying rocks to a depth of 2,000 feet or more. The source of the sand? Early assumptions said the last remnants of the Ancestral Rockies, but some recent work suggests that the sand was derived from vast river systems with headwaters in the Appalachian-Caledonian mountain system.

Map from Ron Blakey

The uniqueness of the Navajo lies in the structure of the rock itself, the crossbedding, and the somewhat unique range of color: in different parks the rock is white, orange, red, yellow and vermilion. The variations are apparently due to a bleaching effect of once-present hydrocarbons that dissolved, and then reprecipitated, iron oxides in structures like domes and thrust faults. The iron oxide minerals hematite, goethite and limonite provide the intense color, and in places have formed the unique "moqui marbles", spherical concretions of hematite that are popular with rockhounds.

I want to explore a couple of the Navajo Sandstone parks, and I start today with a lesser-known park, by name and visitation, but very familiar because pictures of this canyon are a staple in art galleries. I am referring to Antelope Canyon, outside of Page, Arizona. Antelope Canyon is on the Navajo Reservation, and has been established as a Tribal Park (like Monument Valley). The canyon is a small tributary to the Colorado River, and from above, you would barely know it is there: you can literally jump across the opening. Underneath is a slot canyon, more than 100 feet deep...

Over hundreds of thousands of years rushing waters have ripped away at the sandstone, but because the well-cemented nature of the rock, mass wasting is ineffective at widening the gorge. Beneath the surface is a fascinating labrinth of twisting passages and curving walls. The canyon is both a challenge and a Mecca for photographers. The play of reflected light ranges across the brightness spectrum, and at the right times the rock seems to glow internally.

Slot canyons carry their own dangers, and flash floods are a real concern here. In 1997, eleven tourists were killed when a distant storm sent waters slashing through the canyon. The Navajo Nation now carefully monitors visitation when flooding is a possibility. Still, despite entry fees and charges, the canyon is a delight to visit: there are two sections, an upper canyon with a level floor, and a lower canyon that requires some climbing and squeezing, although ladders and steps are now in place to allow fast evacuation if floods come. It is a truly beautiful place. I visited in 2007 during a day I had to drive 750 miles, but I spent a long time in the canyon, snapping more than a hundred pictures. It was magical....