One of the things about hiking with kids is that you have to find a hike with the right balance of interesting-ness*. If a trail isn’t very interesting- say just walking along through a forest for example- kids will whine and complain. If on the other hand the hike is something fun-filled, mysterious and/or exciting- say a slot canyon for example- the kids will be continually entertained and not whine at all. But if the hike is too interesting, with too many intriguing feature and nooks and crannies and a ramps and alcoves and what-not to explore- say a virtual “garden” of dozens of massive Entrada sandstone fins, for example- then you’ll never make any distance because the kids will stop to play on and scramble around everything, and then after a couple of miles of course they’ll be exhausted**…
*Yes, I’m making up another word. But I feel justified in doing so, as interesting-ness and interest really are not the same thing.
**Know who/what else does this? Dogs, especially youthful, inexperienced dogs. They run back and forth each section of trail like 6 times, and after 5 miles of repeated sprinting and halting they’re destroyed.
Catching up after my shrub-stop, I managed to corral the Trifecta and keep them more or less on track as we followed the trail between and over Entrada fins from one arch to the next. Landscape Arch (pic left) is the most improbable looking, but Navajo, Wall, Partition and Double O Arches are also all incredible.
All About Arches
The last arch we looked at in this blog- Conglomerate Arch in Tuckup Canyon- was specifically* a natural bridge, in that a current of water runs/ran under it and was the primary agent in its formation. The arches in Devils Garden- and in Arches NP in general- are mostly just “arch” arches, and not natural bridges. Natural arches form by weathering processes, not stream erosion, although as we will see in a moment, water can be one of the agents in their formation.
*Some sources say natural bridges and natural arches are 2 different things. Others say that natural bridges are a specific type of natural arch. I don’t have a dog in the fight**; I just carefully worded that sentence so as to not tick off an adherent of either camp.
**I love that phrase. I get these phrase-crushes from time to time. In the spring of 2008 I was just enthralled with the phrase “... can’t swing a dead cat without hitting a…” Now it’s the “…no dog in the fight…” Why is that expression still around? Until the whole Michael Vick*** thing I didn’t even know they had dog fights. Anyway, I’m using it all the time. At work, someone asks for my opinion? I don’t have a dog in the fight. Awesome Wife asks which color upholstery I prefer? No dog in that fight either.
***What I really hate about these way-over-covered celebrity-gone-bad stories is that they so often inform me of some really unappealing aspect of the world of which I had been previously quite happily unaware. The Michael Vick-dogfighting thing was one distasteful example. Another was the whole Senator Larry Craig thing. Ever since then I’ve found myself making mental excuses to avoid using the stalls in airport restrooms… (“Aw, my flight takes off in just 40 minutes anyway- I’ll just poop on the plane…”)
Extra Detail: There’s actually a third category of arch-like-thing, which is not technically either a natural bridge or a natural arch, in which a span of rock forms as a result of karst processes, which is the dissolution of carbonate rocks, such as limestone. Karst processes are the common cause of most sinkholes. Angels Window along the North Rim of the Grand Canyon is an example of a karst-arch.
Two big requirements for arch formation are a narrow wall of rock, and a type of rock strong enough to support to support the roof of the arch. The Entrada fins of Devil’s garden meet both these requirements. A third requirement is that the rock below the top of the fin erodes faster than the “roof” and there are a couple of reasons why this can happen.
The Entrada formation around Moab consists of 3 primary members (from top to bottom)- Moab Tongue, the Slickrock member and the Dewey Bridge member. The Dewey Bridge member is much softer and weaker than the overlying Slickrock member, and erodes faster. This is the mechanism at work in the roadside formation of Balanced Rock; the rock itself is of the Slickrock member, while the supporting pillar is the weaker Dewey Bridge.
The exact same differential erosion process is at work in several of the park’s major arches, particularly in the close-by Window section. Turret Arch, North Window Arch and South Window Arch are all Slickrock member on top, Dewey bridge below.
Extra Detail: Delicate Arch BTW, though it is the emblematic arch of the park, is actually somewhat atypical geologically. The “roof” of Delicate Arch is the Moab Tongue member, and the lower pillars and base are of the slickrock member. Separating the two is a minor red siltstone layer that you can pick out at the arch’s thinnest and weakest-looking point.
Another kind of arch BTW is a pothole arch, formed when the water in a pothole gradually dissolves the lattice of the sandstone, eventually eroding through the sandstone altogether. Pothole arches generally only form from potholes right on or close by cliff edges. Gold Bar and Pritchett Arches in the Moab area are example of pothole arches, as is Window Rock down in Arizona. I mention this because there’s a neat un-named* pothole arch (pic right) just a couple hundred yards from Delicate Arch. On the final stretch of trail to the arch, just before the trail climbs, narrows and passes by a small rock “window” on the (climber’s right), the trail crosses an open slickrock area. Leave the trail here**, veering to the due East, about 45 degrees from the trail, and follow the slickrock around the head of the canyon to the South/right. Follow the rim around and to the South and watch the rim- you’ll be there in 5-7 minutes.
*Actually it probably does have a name. Something like 500+ arches in the park are named. It just doesn’t have a sign, and I don’t know its name.
**Yes, you’re off-trail, but you can stay on slickrock the whole way, so you can get there just fine without trampling any cryptobiotic soils, which I explained in this post. Man it is like I have a post for everything.
But in Devil’s Garden, many of the arches are entirely carved out of the Entrada Slickrock member, both above and below (pic left), and part of what causes arch formation up here is the close positioning and alignment of the fins. Winds blow sand in between the fins, and these sands hold moisture from rain and snowmelt against the rock, which in turn concentrates the erosive force of water at a particular level of the fin. As the surface erodes an alcove* forms, which in turn increases the surface area exposed to erosion and frost wedging.
Extra Detail: The big “erosive force” in this case is carbonic acid, H2CO3, which is formed from water and atmospheric CO2. Carbonic acid is one of 2 ways in which raindrops erode rocks to form sand and silt (the other being the sheer force of impact.) Frost wedging refers to the process of water droplet working their way into tiny cracks in rock surfaces, increasing in volume when they freeze and cracking bits of the rock lattice around them- the same basic process that causes front heaves in paved roads.
So the arches in Devils Garden form from the Entrada fins because there are Entrada fins, which seems sort of circular, but in any case begs the question of why there are fins in the first place, and why are they all aligned parallel to one another?
What About The Fins?
The real geologic action of Arches, Canyonlands, and this whole chunk of the Colorado Plateau isn’t up at the level of slickrock and spires and arches; that’s just the bling. The real action is far, far below, way, way under the Entrada and Navajo formations, below the Chinle and Moenkopi, down below layers we haven’t even talked about yet. The big action is down in the depths of the ancient Paradox Formation.
Some 300 million years ago, what is now Southeast Utah lay more or less astride the equator and was covered by a largely inland sea- the Paradox Sea- which was sometimes connected to the greater ocean (like the modern-day Mediterranean or Black Seas), and other times isolated from it (like the modern-day Caspian or Aral Seas.) When isolated it would sometimes dry up completely, leaving behind a thick layer of (primarily) salt. The Paradox Sea did this- drying up completely- at least 29 times over a 15 million year period, leaving salt deposits as much as 6,000 feet thick.
Around 25 million years later the rising of the ancestral Rocky Mountains sort of compressionally “scrunched” the Earth’s crust in this region, and this scrunching deformed the underlying Paradox formation. Salt is solid, but is somewhat plastic in that it can also flow like a liquid. The scrunching thickened the salt layer in some places, causing it to push the overlying rock layers upward in elevated areas called anticlines. As the compressional-scrunching ran generally Northeast (from the Rockies) to Southwest, the scrunched-up crustal folds/anticlines ran perpendicular to the scrunching, from Northwest to Southeast.
Fast forward to the last 10-15 million years, as the Colorado Plateau is gradually uplifted several thousand feet. As it rises, the weight of the last ~280 million years of rock atop the paradox causes further plastic deformation, which called brittle upper layers- such as the Entrada- to buckle and crack.
In many places, the dissolution of Paradox formation salts caused huge areas of land to essentially collapse, and this is the origin of modern-day Moab valley as well as other nearby valleys, including Castle and Fisher valleys to the Northeast. These valleys all run Northwest to Southeast, along the axis of the now-partially-dissolved underlying salt anticlines.
In Devils Garden, the underlying collapse caused buckling in the Entrada along the same Northwest-to-Southeast axis as Moab and Castle valleys. These “axial” joint fissures allowed water to penetrate, which in turn widened the fissures, permitting more water, snow and sand access to the now exposed vertical surfaces, widening the cracks over time, and leading to the profusion of parallel fins you see today.
Side Note: The same process BTW, has created the fins in the nearby Fiery Furnace section of the park, but the fin-development there is younger and so the cracks between the fins much narrower.
After a snack-break in a sunny, windless depression by Double O Arch (pic left), we set out on our return hike via the so-called “primitive” trail, which isn’t all that primitive; it’s just not as clearly traveled or marked as the main trail, sees less traffic, and feels a bit more wild, with fun little scrambles, angled ramps that are sometimes run up (before friction breaks) or butt-slid down. The trail passes through the heart of the fins, providing a birds-eye view of the erosional processes at work.
About half-way back we were interrupted by a sudden flurry of fluttering and chirping; a flock of about a dozen or so small birds alighted in some branches just ahead of us. On a Spring or Fall day we likely wouldn’t even have noticed them; in the bitter cold we’d hardly seen any moving creatures, barring a couple of ravens close to the trailhead.
So the birds caught my eye, and as I checked them out, I noticed that they were completely unlike any birds I’d seen previously around here. Now that in and of itself isn’t surprising; I’m a way amateur birder, and I’m constantly seeing little brown or gray-ish birds I don’t quite recognize.
But I usually have some idea what general kind of bird I’m looking at, and in any case these birds weren’t brownish or grayish, but visually quite striking, black with distinct gray “cap” and outer wings lined in lighter color feather with just a hint or red, or maybe “rose”. We stopped, watched them for bit and snapped these photos before continuing on.
It wasn’t till we returned home the following evening that I ID’d them in the Kaufman guide: Black Rosy Finches, Leucosticte atrata. They’re closely-related to the more wide-ranging (and migratory) Gray-Crowned Rosy Finch, L. tephrocotis as well as the Brown-capped Rosy Finch*, L. australis and for a long time all 3 were considered the same species. But the BR Finch has a distinctly different habitat and lifestyle than its Gray-crowned cousin.
*The Brown-Capped Finch, a bird of the high Southern Rockies, has the most limited range of the 3 species.
The Leucosticte species (there are 7 total) are known as the Mountain Finches, and are part of the large family of True Finches, Fringillidae. We’ve looked at a number of True Finches previously right here in my back yard, including House Finches and Pine Siskins, from whose ancestors the Mountain Finches parted ways some 10.5 million years ago.
The BR Finch is a bird of high altitudes, nesting above timberline in largely inaccessible nooks and crannies. Much about its lifecycle is still a mystery; as of 2002, only 3 nests had ever been located. One interesting thing about the bird is its concept of a mobile territory, which a male patrols and enforces around his mate, and is not attached to any specific piece of space or land, but rather moves with her. While not really migrators in the traditional long-distance sense, in Winter BR Finches tend to form noisy flocks and descend in altitude, which was why we were seeing them here at just ~5,000 feet.
I first assumed they’d descended from the nearby 12,000-foot La Sal Mountains*, but studying the range map more closely it doesn’t seem there’s anytime summertime record of them in that range, nor any closer to Moab than the Uintas. BR Finches do migrate as much as a couple hundred miles South (in addition to lower) in Winter, so this flock was probably further from home than I initially thought. Our frigid hike had provided an expected reward: a bird species we’d normally never see except several thousand feet higher, and only onesy-twosy in number. Sometimes it’s worth tramping around in the cold.
*The La Sals BTW, are an absolutely awesome range, one I’d hoped to blog about but likely will not get to prior to completion of the project. I can’t recommend them highly enough. Mt. Peale, the high point is a high bang-for-the-buck climb, only 90- 120 minutes of very reasonable ascent if you’re in decent shape. The trail network around Moonlight Meadows makes for wonderful summertime mountain-biking, like a piece of the Wasatch just picked up and moved down to the middle of the desert.
The shadows growing longer, we quickened our pace, our thoughts turning to turkey, pie and hotel jacuzzis, as we strode across the frozen desert.
Note about sources: My best geologic source for this post was the absolutely wonderful book Geology Underfoot in Southern Utah, Richard L. Orndoff, Robert W. Wieder and David G. Futey. I just can’t recommend this book highly enough for its content, context and easy readability. Additional geo-info came from Halka Chronic’s classic Roadside Geology of Utah and Canyonlands Geology – A Visual Toolkit*, Jerry Shue, Neal Herbert and Barbara Webb.
*Best 3 bucks I ever spent.
Info on the Black Rosy-Finch came from Kaufman Field Guide to Birds of North America, Kenn Kaufman, the Cornell Lab of Ornithology All About Birds website, and Genetic Relationships of North American Cardueline Finches, Jill A. Marten et al.
The photos of Delicate Arch, the “unnamed” pothole arch and Balanced Rock were taken on a separate trip in May 2009.