Arches National Park

The park lies atop an underground salt bed that is responsible for the arches, spires, balanced rocks, sandstone fins, and eroded monoliths [...]. Thousands of feet thick in places, this salt bed was deposited across the Colorado Plateau 300 million years ago when a sea flowed into the region and eventually evaporated. Over millions of years, residue from floods, winds, and the oceans that came and went blanketed the salt bed. The debris was compressed as rock, at one time possibly a mile thick.

Salt under pressure is unstable, and the salt bed lying below Arches was no match for the weight of this thick cover of rock. The salt layer shifted, buckled, liquefied, and repositioned itself, thrusting the rock layers upward as domes, and whole sections fell into the cavities [left when the salt moved].

Faults deep in the Earth made the surface even more unstable. [...] Fault-caused vertical cracks later contributed to the development of arches. As the salt’s subsurface shifting shaped the Earth, surface erosion stripped off younger rock layers. Except for isolated remnants, today’s major formations are salmon-colored Entrada Sandstone, in which most arches form, and buff-colored Navajo Sandstone. They stand like a layer cake over most of the park. Over time water seeped into superficial cracks, joints, and folds. Ice formed in the fissures, expanding and pressuring the rock, breaking off bits and pieces. Wind later cleaned out the loose particles, leaving a series of free-standing fins. Wind and water then attacked these fins until the cementing material in some gave way and chunks of rock tumbled out. Many of these damaged fins collapsed. Others, hard enough and balanced, survived despite missing sections. These became the famous arches. Pothole arches are formed by chemical weathering as water collects in natural depressions and then eventually cuts through to the geologic layer below. This is the geologic story of Arches National Park—probably. The evidence is largely circumstantial.

The map at the left shows the three parks we visited and the town of Moab, where we stayed. It takes in a wide area so that you can see how the Green River comes down from the north and joins the Colorado inside Canyonlands, which the rivers divide into three sections.

If you click the map, you’ll see just a part of it expanded—the part that shows the routes we took from Moab to get to each of the parks. It shows where we entered each park, but not where we went once we were inside. The expanded version doesn’t give you the full overview, but at least the labels on the map are big enough to read.

As the map shows, the entrance to the Arches National Park is just a few miles north of Moab. We followed guidebook advice to get there early in the morning—kind of early, anyway. There were plenty of other visitors, but not so many as to interfere seriously with our picture-taking. The map in the NPS brochure would probably have served us very well, but in Moab we had bought a more detailed National Geographic map of the park, so we followed that, going everywhere that we could drive.