I've been to Arches National Park a great many times, but almost always with a large group of students. A group necessitates some serious planning, and our trips usually include the same three places: a hike to Delicate Arch for the sunset, a hike to Double Arch, and a hike to Landscape Arch, the largest in the park (and probably in the world). On this particular day, our new experience was stopping and sitting at one of the less iconic viewpoints. Literally sitting, for a rather long time. We had a picnic dinner, and just experienced the desert while the sun set behind us.
Before settling in, we made a quick circuit past the La Sal Mountains overlook and the Windows Section. The La Sal Mountains are outside of Arches National Park, but provide a spectacular backdrop to arches and fins that make up most of the scenery in the park. The mountains have an unusual origin. They are not quite volcanic in origin (although volcanoes almost surely existed here, but have been eroded away), but instead are formed of intrusive rock that reached shallow depths in the crust, wedging and bulging up the sedimentary layers that lay above the molten rock. The intrusions are called laccoliths. The mountains rise to 12,000 feet, and are often snow-covered, even in early summer.
Upheaval Dome. The speculation is controversial, but the explanation appeals to me simply on the scale of grandeur and in thinking outside the box.
Time Beyond Imagining. Here is a short excerpt:
"The origin of the park's famous arches (there are hundreds) is tied to the salts of the Paradox Basin. 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."