Work has kept me from getting outside much lately, so I thought I’d take some time to talk about a past trip. I’m heading to Utah this summer so I was looking back at photos from a geology field trip there from March of 2010. One memorable stop was at Arches National Park near Moab in the eastern part of the state.
The contrasting dull green vegetation, red rocks of the Entrada Sandstone, and snow-capped peaks of the La Sal Mountains made for some amazing views. As the name suggests, however, this park’s claim to fame lies in the abundance of natural rock arches. Here are some of the big ones:
These arches along with over 2000 others have been slowly carved out of the sandstone thanks to an unstable foundation and erosion.
The thick salt beds of the Paradox Formation underlie this area. This salt was deposited over millions of years around 300 million years ago. Shallow seas repeatedly invaded this area and then evaporated, leaving behind the salt. Subsequent burial by other sediments over the next tens of millions of years compressed the salt into rock. The beach and dune sands of the Entrada Sandstone in particular were deposited here in the Jurassic Period around 150-160 million years ago. The weight of overlying sediments lead to a shifting of the salt beds below. As the softer salt layers slowly flowed the harder overlying rock beds were subjected to uplift and collapse, causing them to crack and buckle.
Erosion has since stripped away younger rocks, exposing the cracked Entrada Sandstone and working away at its weaknesses. The slow but powerful forces of rainwater and ice widened the cracks over time. The rock eroded into thin walls (fins), and then holes collapsed in the sides to eventually form the arches. This diagram from one of the interpretive signs helps illustrate the process:
In many places you can see new arches being formed:
The arches aren’t the only interesting features here. The weathered sandstone has also left behind unique features like these spires and fins…
…as well as the fascinating Balanced Rock:
The same processes that continually form these arches also work to continually destroy them. About forty of the arches have collapsed in my lifetime, with Wall Arch being one of the more recent.
It’s amazing what stunning panoramic views and cool geologic features are packed into this relatively small national park. It was definitely one of my favorites.