The La Sal Mountains, a backdrop for Arches National Park
The Arches National Park brochure (You get one at every national park) goes into great detail about how, 300 million years ago, this region was covered with a great sea that eventually evaporated and left a salt bed thousands of feet thick. This was followed by residue of oceans and floods that "came and went." The residue turned into rock, "maybe" more than a mile thick. The salt, with all this pressure, became unstable then "shifted and buckled, liquefied, and repositioned itself thrusting rock layers upward into domes." Faults deep within the earth contributed to this instability.
Over time (eons) fissures in the rock developed, water entered, freezing and thawing, eroding and washing, then wind blew out loose particles. Voila! "A series of free-standing fins remained."
As we drove into the park, not yet having read the brochure, Mary exclaimed that these rock towers look very thin! They are indeed thin... like fins!
The Brochure continues . . .
"Many damaged fins collapsed. Others, with the right degree of hardness and balance, survived despite missing sections. These became the famous arches. Pothole arches form by chemical weathering as water collects in natural depressions and eventually cuts through to the layer below. This is the geologic story of arches -- probably. The evidence is largely circumstantial."
Yet, some things just don't quite add up: Potholes are horizontal, but these arches are vertical. Whatever really happened, the result is simply marvelous!
National Parks Service; U.S. Department of the Interior