The Abandoned Lands...A Journey Through the Colorado Plateau: A day in the canyonlands, Part 2

In the last post, we saw the sun rise over the Canyonlands region of the Colorado Plateau, an area that includes Arches and Canyonlands National Parks and Dead Horse Point State Park. We saw all three on this one extraordinary day.

We woke up and Devils Garden Campground in Arches, and in the relative cool of the morning we followed the trail out of Devils Garden to see the largest natural span in existence: Landscape Arch. At 290 feet, the span is as long as a football field.

If you haven't ever seen Landscape Arch, you should bump it up to the top of your bucket list, because I don't think it is going to persist much beyond our collective life times. Since 1991, slabs measuring 30, 47 and 70 feet have dropped off the thinnest part of the arch. The trail under the arch was closed years ago for fear of rocks falling on visitors. The folks at Wikipedia think that it might mean there is less weight being supported by the arch, but I think that the math is increasingly working against the continued existence of the sandstone span.

Some of the students who were on the trip might not remember the moon lurking underneath the arch that day. I'm cheating just a little, pulling up my favorite shots from the 2008 trip, since I actually walked a different trail that morning. I was headed to Pine Tree Arch, which is on a short spur trail off the walk to Landscape.

Because of the unique geological circumstances of the Paradox Basin, the park contains the highest concentration of arches on the planet. Most of them occur in the Jurassic Entrada Sandstone, a relatively uniform dune sand that formed in a tropical or equatorial coastal region. The Entrada lies above salt layers that developed in the Paradox basin (a large isolated bay complex that occasionally was cut off from the ocean and dried up) in late Paleozoic time. Salt deforms easily under pressure, and has pushed the overlying rocks into a series of anticlines (upward pointing folds in the rock). When the salt began to interact with groundwater, the salt dissolved and the top of the anticline collapsed inwards, stressing the overlying Entrada, and causing a series of parallel joints to form, like those in the picture above, at the start of the Landscape Arch Trail.

The raven apparently supports an alternate hypothesis of arch formation.

Once the fins develop, precipitation is directed preferentially into the sand-filled joints and fractures, and the constant soaking of the sandstone at the base of the fin causes the solid rock to lose cement at a higher rate than the higher parts of the fin. Eventually a small opening occurs, and slabs of rock fall out, enlarging the arch over time. The arches grow until they collapse, an event that is fairly common, even in human times of reference. Several dozen arches have collapsed since 1971 according to some web sources, but I can't find any original confirmation for the number (that's out of 2,000 arches in the park). It's not that we are running out of arches, though; new ones are forming too.

Pine Tree Arch is an attractive little span with a nice view through the opening. One thing about Landscape Arch is the difficulty of getting a good angle on the sky. There is a cliff behind Landscape, and the closed trail prevents access for views upward. I found Pine Tree far less crowded and more intimate, even though it was only a few hundred feet from the main trail.

We finished our hike and returned to the vans. We headed to the Windows Section of the park, and the heat...well the heat descended on us like a blanket. Just look how the gang fought for the pitiful amount of shade at our lecture stop!

Next, an Indiana Jones adventure! This is a continuing series about what I've called the Abandoned Lands of the Colorado Plateau. It's kind of hard to be describing a very crowded national park as an abandoned place, but it was abandoned already by the Native Americans, the original ranchers, the uranium miners, and when the price of oil becomes prohibitively expensive, I worry it will be abandoned by our society as well. In the meantime, it is a place of beauty and inspiration.

Geo-Picture-a-Day Week: Moon, Sky and Rock

It's day six of Geo-Picture-a-Day Week and today we go to basics: stone, sky and cosmos. It is an unusual framing of a familiar arch at Arches National Park in Utah: Landscape Arch, which is one of the widest spans in nature, the length of a football field. It's so thin in some sections that it is a wonder that it still stands (a big chunk fell out of it in the 1990's). So many geological processes are incredibly slow, but I will not be surprised if this arch collapses during our lifetimes.

Arches may seem like a deserted landscape, but what a contrast to a place like the moon, where essentially nothing happens aside from the occasional meteorite strike. A distant lonely place...but thinking back to the 1960's and my youthful dreams, I wish I had had a chance to walk upon it...

The Real Jurassic Parks: Arches National Park, continued. And an Eminent Threat.

I didn't really mean to leave everyone hanging in the Jurassic during our months-long tour of the geology of the Colorado Plateau, but I am easily distracted by things like work and politics. I want to at least make it out of the Mesozoic era before Thanksgiving, so I am picking up the thread where we left off on Oct. 17 with a visit to Arches National Park.

The stratigraphy of the park is relatively straightforward: the visible scenery mainly includes the Navajo Sandstone, the Dewey Bridge member of the Entrada Sandstone, and the Slickrock member of the Entrada. The Navajo Sandstone has a huge role in the scenery of other parks on the Plateau (see discussions here), but is mostly muted here. It is familiar to park tourists as the "petrified sand dunes" that can seen along the road in the south part of the park, and the very dramatic white rock along the park entrance road. The Dewey Bridge member is a "crinkly" layer of sand, silt and gypsum at the base of many of the arches. The reason for the deformed layers is unclear. One intriguing proposal is that a nearby asteroid impact distorted the layers (Upheaval Dome in Canyonlands National Park is the proposed impact site). Most of the arches occur in the Slickrock member, a cliff-forming layer that is obvious throughout the park.

The origin of the park's famous arches (there are hundreds) is tied to the salts of the Paradox Basin discussed earlier in this post. The salt beds are thousands of feet thick, and when placed under pressure, can slowly flow upwards through the overlying rock, forming domes and anticlines (upward pointing folds). Over millions of years, erosion removed thousands of feet of overlying sediment, and the salt was close enough to the surface to be affected by groundwater: it was dissolved away. The tops of the folds collapsed inwards, and the Entrada Sandstone was fractured (jointing) into a series of parallel fins. A large northwest trending anticline crosses the park; most of the arches occur along the flanks of the anticline.

The top of the fins are exposed to the arid desert climate, where the rock dries quickly following the infrequent storms. At the base of the fins, where the rock is buried by soil and loose sand, the rock may stay moist much longer. This leads to the solution of the cement holding the sandstone together, and the fins start to weather from below. Eventually a small window may open up, and falling slabs of rock enlarge the opening. The largest arch, Landscape, has an opening the length of a football field. Unfortunately, it is so thin and fractured that it may not last our lifetimes (it is in the third picture of the day). A large chunk fell about fifteen years ago. Wall Arch, another arch in the same area, just collapsed last summer.

Today's first photograph shows a small arch in the vicinity of Delicate Arch (a corner of Delicate is visible on the left side). The foreground includes tilted layers of the Entrada where they collapsed into the salt anticline. The La Sal Mountains can be seen in the distance.

The second photo shows my nomination for one of the most dramatic trails in existence, the one that leads to Delicate Arch. This is definitely a case where the end of the trail is not the main attraction; the whole trail is a treat, from one end to the other.

Unfortunately, politics and politicians don't stop politicking, elections or not. Arches, like the Grand Canyon is being threatened by the possibility of gas drilling just outside the park boundaries. The outgoing administration seems to be trying to run a fire sale on leases before they leave office in January. It is especially irritating that the Bureau of Land Management seems to be trying to fly under the public radar in their announcements. More information on the issue (and maps of the proposed leases) can be found here.

Time Beyond Imagining

Still decompressing after a "long" trip of two weeks through geologic time on the Colorado Plateau. It is always difficult to comprehend the contrast between the hours and days of one's human existence with the millions and billions of years that characterize the geologic history of our planet and Universe. I expect to follow this theme in my blogs for a few days as I sift through the pictures of our journey.

The unusual view above comes courtesy of astronomical lunar cycles and Landscape Arch in Arches National Park. The arch has an opening nearly as long as a football field, and is only 6 feet or so across at the thinnest point. It is rather a wonder that the span still stands, and the future prognosis of the arch is probably measureable in human, not geologic, terms. A large chunk fell from the span around 1991.

The arches in the park are formed in the Slickrock Member of the Jurassic Entrada Sandstone. The unit has been pushed upwards by bodies of mobile salt from the late Paleozoic Paradox Basin. The upward pressure in the salt anticline has fractured and jointed the sandstone into a series of "fins", long narrow walls of rock. The base of the fins lie in the Dewey Bridge member of the Entrada (or Carmel), which is easily eroded, allowing a small window to form at the base of the fin. Chunks of rock fall from the window, until an arch develops. There are hundreds in the park.