Tuesday, September 6, 2011

A visit to Arches National Park benefits from a knowledge of the history of the geological oddities that have formed in the area over the last 150 million years. This way, when you're looking at a prime example of a fin arch like Skyline Arch, you can also realize that you're standing on top of what was once an ocean, then a 2 mile tall pillar of salt. You're also standing under the space were 5000 additional feet of sediment had settled before being washed away to reveal the 140 million year old entrada sandstone that forms the arch you now see.

The mechanisms that generated Arches National Park were set in motion at the bottom of an ocean 300 million years ago. The paradox basin was a deep bucket with its lip at sea level. As a result, Southern Utah was frequently submerged under an inland sea during interglacial periods when ice thawed and raised sea levels. During each ice age, sea levels would drop as water was collected into glaciers and the inland sea would be cut off from the retreating ocean. As a result, massive volumes of salt and other minerals were collected in the paradox basin as the trapped stagnant seas evaporated.

As this period of flowing and ebbing seas subsided, the great well of salt was being buried beneath sediment washed down from the Uncompahgre uplift to the Northeast. Squeezed beneath thousands of feet of sand and mud, the salt became liquid and flowed southwest in subterranean waves. These saline swells migrated southwest until they crashed into 6000 foot sheer cliffs that had resulted from northwest trending faults in southeast Utah. Their progress halted by these underground walls of stone, the salt began to push upward through the surrounding rock. At the same time, the red entrada sandstone that you see in Arches national park was solidifying at the surface. The intrusive strips of salt bulged and cracked the surface sandstone like a baking loaf of bread, parallel to the underlying fault lines.

These cracks allowed surface water to flow down to the rising salt to dissolve and wash it away to the ocean. This continued until less soluble gypsum left behind as the salt dissolved, formed a cap over the salt pillar and the sandstone collapsed into the void left by the departed salt. The cracks in the sandstone remained access points for water to enter and weather the sandstone. These cracks widened as freezing water broke them apart and wind cleaned them out, eventually leaving the long fins seen in The Devils Garden. Fins are crucial to the formation of arches, because it is the careful demolition of these fins by weathering that allows for holes in the fins to widen into great arches before falling completely.

The other crucial factor in the formation of arches is the composition of the fins. This is another aspect in which the Moab area is exceptional. The hard Entrada sandstone in underlain by the fragile siltstone of the Dewey Bridge member. As you look at the formations in Arches national park, you'll notice that most arches form at the junction between the Entrada and Dewey Bridge layers. The Dewey Bridge stone wears away much more quickly than the entrada and thus the entrada is undercut in many places. After being under such immense pressure for so many years, the newly exposed base of the Entrada layer begins to break apart as a way of decompressing. The pattern of weathering extends radially out from the division of the two strata and eventually widens into an arch. Water accomplishes this weathering by seeping into the cracks in the sandstone and expanding as it freezes, literally pushing out and breaking the stone apart from inside. These holes continue to grow through weathering until the fin becomes unstable and collapses.

Arches in all stages of life can be found in the park. The most recent arch to collapse, Wall Arch, is very close to Landscape Arch, the longest in the park. Landscape Arch is an arch near the end of this life cycle as well. By luck, none of the chunks of stone littering the area around the arch were vital enough to its stability to bring it down when they fell out. Large chunks have fallen out as recently as the 90s and it would appear that there aren't many more pieces that can be removed before this arch falls. At the same time, new arches are growing throughout the park. This means that the Arches National Park that people see 1000 years from now will look quite different than the one we see today. The spectacle of arches can remind us that while orogeny may progress at a ponderously slow pace, noticeable changes to the earth occur within a human lifetime.

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